proc.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import (
	"internal/cpu"
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
)

var buildVersion = sys.TheVersion

// Goroutine scheduler
// The scheduler's job is to distribute ready-to-run goroutines over worker threads.
//
// The main concepts are:
// G - goroutine.
// M - worker thread, or machine.
// P - processor, a resource that is required to execute Go code.
//     M must have an associated P to execute Go code, however it can be
//     blocked or in a syscall w/o an associated P.
//
// Design doc at https://golang.org/s/go11sched.

// Worker thread parking/unparking.
// We need to balance between keeping enough running worker threads to utilize
// available hardware parallelism and parking excessive running worker threads
// to conserve CPU resources and power. This is not simple for two reasons:
// (1) scheduler state is intentionally distributed (in particular, per-P work
// queues), so it is not possible to compute global predicates on fast paths;
// (2) for optimal thread management we would need to know the future (don't park
// a worker thread when a new goroutine will be readied in near future).
//
// Three rejected approaches that would work badly:
// 1. Centralize all scheduler state (would inhibit scalability).
// 2. Direct goroutine handoff. That is, when we ready a new goroutine and there
//    is a spare P, unpark a thread and handoff it the thread and the goroutine.
//    This would lead to thread state thrashing, as the thread that readied the
//    goroutine can be out of work the very next moment, we will need to park it.
//    Also, it would destroy locality of computation as we want to preserve
//    dependent goroutines on the same thread; and introduce additional latency.
// 3. Unpark an additional thread whenever we ready a goroutine and there is an
//    idle P, but don't do handoff. This would lead to excessive thread parking/
//    unparking as the additional threads will instantly park without discovering
//    any work to do.
//
// The current approach:
// We unpark an additional thread when we ready a goroutine if (1) there is an
// idle P and there are no "spinning" worker threads. A worker thread is considered
// spinning if it is out of local work and did not find work in global run queue/
// netpoller; the spinning state is denoted in m.spinning and in sched.nmspinning.
// Threads unparked this way are also considered spinning; we don't do goroutine
// handoff so such threads are out of work initially. Spinning threads do some
// spinning looking for work in per-P run queues before parking. If a spinning
// thread finds work it takes itself out of the spinning state and proceeds to
// execution. If it does not find work it takes itself out of the spinning state
// and then parks.
// If there is at least one spinning thread (sched.nmspinning>1), we don't unpark
// new threads when readying goroutines. To compensate for that, if the last spinning
// thread finds work and stops spinning, it must unpark a new spinning thread.
// This approach smooths out unjustified spikes of thread unparking,
// but at the same time guarantees eventual maximal CPU parallelism utilization.
//
// The main implementation complication is that we need to be very careful during
// spinning->non-spinning thread transition. This transition can race with submission
// of a new goroutine, and either one part or another needs to unpark another worker
// thread. If they both fail to do that, we can end up with semi-persistent CPU
// underutilization. The general pattern for goroutine readying is: submit a goroutine
// to local work queue, #StoreLoad-style memory barrier, check sched.nmspinning.
// The general pattern for spinning->non-spinning transition is: decrement nmspinning,
// #StoreLoad-style memory barrier, check all per-P work queues for new work.
// Note that all this complexity does not apply to global run queue as we are not
// sloppy about thread unparking when submitting to global queue. Also see comments
// for nmspinning manipulation.

var (
	// 代表进程的主线程
	m0           m
	// m0 的 g0，即 m0.g0 = &g0
	g0           g
	raceprocctx0 uintptr
)

//go:linkname runtime_init runtime.init
func runtime_init()

//go:linkname main_init main.init
func main_init()

// main_init_done is a signal used by cgocallbackg that initialization
// has been completed. It is made before _cgo_notify_runtime_init_done,
// so all cgo calls can rely on it existing. When main_init is complete,
// it is closed, meaning cgocallbackg can reliably receive from it.
var main_init_done chan bool

//go:linkname main_main main.main
func main_main()

// mainStarted indicates that the main M has started.
var mainStarted bool

// runtimeInitTime is the nanotime() at which the runtime started.
var runtimeInitTime int64

// Value to use for signal mask for newly created M's.
// 用于新创建的M的信号屏蔽的值。
var initSigmask sigset

// The main goroutine.
// main协程创建过程和普通协程基本一致，也会参与系统调度，我们下节再详细了解创建协程过程。
// 作为系统中第一个执行的协程，它主要的还是执行一些初始化工作，我们先简单了解下：
// 1. 设置栈空间上限
// 2. 创建系统监控线程
// 3. 执行runtime包中的所有Init函数
// 4. 初始化GC相关协程
// 5. 执行main包中所有Init函数
// 6. 调用main.main，用户的所有代码在此处开始执行
func main() {
	// 获取当前g。g = main goroutine，不再是 g0 了
	g := getg()

	// Racectx of m0->g0 is used only as the parent of the main goroutine.
	// It must not be used for anything else.
	g.m.g0.racectx = 0

	// Max stack size is 1 GB on 64-bit, 250 MB on 32-bit.
	// Using decimal instead of binary GB and MB because
	// they look nicer in the stack overflow failure message.
	// 设置允许的最大栈空间，32位系统：256M，64位系统：1G
	if sys.PtrSize == 8 {
		maxstacksize = 1000000000
	} else {
		maxstacksize = 250000000
	}

	// Allow newproc to start new Ms.
	// main协程已启动，开始允许创建其他M
	mainStarted = true

	// 创建系统监控进程
	if GOARCH != "wasm" { // no threads on wasm yet, so no sysmon
		// 系统监控是一个独立的线程，入口函数为sysmon
		// 创建监控线程，该线程独立于调度器，不需要跟 p 关联即可运行
		systemstack(func() {
			newm(sysmon, nil)
		})
	}

	// Lock the main goroutine onto this, the main OS thread,
	// during initialization. Most programs won't care, but a few
	// do require certain calls to be made by the main thread.
	// Those can arrange for main.main to run in the main thread
	// by calling runtime.LockOSThread during initialization
	// to preserve the lock.
	// 锁定main协程和主线程，锁定期间，main协程只能在此线程上运行
	lockOSThread()

	if g.m != &m0 {
		throw("runtime.main not on m0")
	}

	// 执行runtime包中的所有Init函数
	// 调用 runtime 包的初始化函数，由编译器实现
	runtime_init() // must be before defer
	if nanotime() == 0 {
		throw("nanotime returning zero")
	}

	// Defer unlock so that runtime.Goexit during init does the unlock too.
	// 在defer中将main协程和主线程解锁
	needUnlock := true
	defer func() {
		if needUnlock {
			unlockOSThread()
		}
	}()

	// Record when the world started.
	// 记录系统启动时间
	runtimeInitTime = nanotime()

	// 初始化GC相关协程
	gcenable()

	main_init_done = make(chan bool)
	if iscgo {
		if _cgo_thread_start == nil {
			throw("_cgo_thread_start missing")
		}
		if GOOS != "windows" {
			if _cgo_setenv == nil {
				throw("_cgo_setenv missing")
			}
			if _cgo_unsetenv == nil {
				throw("_cgo_unsetenv missing")
			}
		}
		if _cgo_notify_runtime_init_done == nil {
			throw("_cgo_notify_runtime_init_done missing")
		}
		// Start the template thread in case we enter Go from
		// a C-created thread and need to create a new thread.
		startTemplateThread()
		cgocall(_cgo_notify_runtime_init_done, nil)
	}

	// 调用main.main, 用户的所有代码在此处开始执行
	// main 包的初始化，递归的调用我们 import 进来的包的初始化函数
	fn := main_init // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
	fn()
	close(main_init_done)

	needUnlock = false
	unlockOSThread()

	if isarchive || islibrary {
		// A program compiled with -buildmode=c-archive or c-shared
		// has a main, but it is not executed.
		return
	}
	// 调用 main.main 函数
	fn = main_main // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
	fn()
	if raceenabled {
		racefini()
	}

	// Make racy client program work: if panicking on
	// another goroutine at the same time as main returns,
	// let the other goroutine finish printing the panic trace.
	// Once it does, it will exit. See issues 3934 and 20018.
	if atomic.Load(&runningPanicDefers) != 0 {
		// Running deferred functions should not take long.
		for c := 0; c < 1000; c++ {
			if atomic.Load(&runningPanicDefers) == 0 {
				break
			}
			Gosched()
		}
	}
	if atomic.Load(&panicking) != 0 {
		gopark(nil, nil, waitReasonPanicWait, traceEvGoStop, 1)
	}

	// 退出， 如果main协程退出，也就是main.main函数执行完成，则系统会退出
	// 进入系统调用，退出进程，可以看出 main goroutine 并未返回，而是直接进入系统调用退出进程了
	exit(0)
	// 保护性代码，如果 exit 意外返回，下面的代码会让该进程 crash 死掉
	for {
		var x *int32
		*x = 0
	}
}

// os_beforeExit is called from os.Exit(0).
//go:linkname os_beforeExit os.runtime_beforeExit
func os_beforeExit() {
	if raceenabled {
		racefini()
	}
}

// start forcegc helper goroutine
func init() {
	go forcegchelper()
}

func forcegchelper() {
	forcegc.g = getg()
	for {
		lock(&forcegc.lock)
		if forcegc.idle != 0 {
			throw("forcegc: phase error")
		}
		atomic.Store(&forcegc.idle, 1)
		goparkunlock(&forcegc.lock, waitReasonForceGGIdle, traceEvGoBlock, 1)
		// this goroutine is explicitly resumed by sysmon
		if debug.gctrace > 0 {
			println("GC forced")
		}
		// Time-triggered, fully concurrent.
		// 时间触发，完全并发。
		gcStart(gcTrigger{kind: gcTriggerTime, now: nanotime()})
	}
}

//go:nosplit

// Gosched yields the processor, allowing other goroutines to run. It does not
// suspend the current goroutine, so execution resumes automatically.
// 协作式抢占与系统监控
// 某些协程可能长时间占用线程运行，其他协程得不到调度执行机会
// 在Go语言1.1以及以前版本中，程序只能依靠协程主动让出CPU资源才能触发调度。
// 函数runtime.Gosched实现了协程主动让出处理器资源，允许其他协程运行。
// 且方法不会将当前协程挂起，所以当前协程也可能会被立即调度执行。
func Gosched() {
	// 空函数，暂未知其用途
	checkTimeouts()
	// 切换到当前m到g0栈执行函数gosched_m
	mcall(gosched_m)
}

// goschedguarded yields the processor like gosched, but also checks
// for forbidden states and opts out of the yield in those cases.
//go:nosplit
func goschedguarded() {
	mcall(goschedguarded_m)
}

// Puts the current goroutine into a waiting state and calls unlockf.
// If unlockf returns false, the goroutine is resumed.
// unlockf must not access this G's stack, as it may be moved between
// the call to gopark and the call to unlockf.
// Reason explains why the goroutine has been parked.
// It is displayed in stack traces and heap dumps.
// Reasons should be unique and descriptive.
// Do not re-use reasons, add new ones.
// 挂起当前协程
func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceEv byte, traceskip int) {
	if reason != waitReasonSleep {
		checkTimeouts() // timeouts may expire while two goroutines keep the scheduler busy
	}
	mp := acquirem()
	gp := mp.curg
	status := readgstatus(gp)
	// 非运行状态的协程不需要挂起
	if status != _Grunning && status != _Gscanrunning {
		throw("gopark: bad g status")
	}
	mp.waitlock = lock
	mp.waitunlockf = *(*unsafe.Pointer)(unsafe.Pointer(&unlockf))
	gp.waitreason = reason
	mp.waittraceev = traceEv
	mp.waittraceskip = traceskip
	// 允许协程被抢占，如果当前正处于抢占状态，则重置stackguard0的值
	releasem(mp)
	// can't do anything that might move the G between Ms here.
	// 切换到g0栈，将当前m上的协程状态改为_Gwaiting，并和m解绑，然后开始新一轮的调度
	mcall(park_m)
}

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling goready(gp).
// 挂起当前协程
func goparkunlock(lock *mutex, reason waitReason, traceEv byte, traceskip int) {
	gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceEv, traceskip)
}

// 切换到g0协程进行调度
func goready(gp *g, traceskip int) {
	systemstack(func() {
		// 将协程gp状态更新为可运行(_Grunnable),并加入p的本地可运行g队列中
		ready(gp, traceskip, true)
	})
}

//go:nosplit
func acquireSudog() *sudog {
	// Delicate dance: the semaphore implementation calls
	// acquireSudog, acquireSudog calls new(sudog),
	// new calls malloc, malloc can call the garbage collector,
	// and the garbage collector calls the semaphore implementation
	// in stopTheWorld.
	// Break the cycle by doing acquirem/releasem around new(sudog).
	// The acquirem/releasem increments m.locks during new(sudog),
	// which keeps the garbage collector from being invoked.
	mp := acquirem()
	pp := mp.p.ptr()
	if len(pp.sudogcache) == 0 {
		lock(&sched.sudoglock)
		// First, try to grab a batch from central cache.
		for len(pp.sudogcache) < cap(pp.sudogcache)/2 && sched.sudogcache != nil {
			s := sched.sudogcache
			sched.sudogcache = s.next
			s.next = nil
			pp.sudogcache = append(pp.sudogcache, s)
		}
		unlock(&sched.sudoglock)
		// If the central cache is empty, allocate a new one.
		if len(pp.sudogcache) == 0 {
			pp.sudogcache = append(pp.sudogcache, new(sudog))
		}
	}
	n := len(pp.sudogcache)
	s := pp.sudogcache[n-1]
	pp.sudogcache[n-1] = nil
	pp.sudogcache = pp.sudogcache[:n-1]
	if s.elem != nil {
		throw("acquireSudog: found s.elem != nil in cache")
	}
	releasem(mp)
	return s
}

//go:nosplit
func releaseSudog(s *sudog) {
	if s.elem != nil {
		throw("runtime: sudog with non-nil elem")
	}
	if s.isSelect {
		throw("runtime: sudog with non-false isSelect")
	}
	if s.next != nil {
		throw("runtime: sudog with non-nil next")
	}
	if s.prev != nil {
		throw("runtime: sudog with non-nil prev")
	}
	if s.waitlink != nil {
		throw("runtime: sudog with non-nil waitlink")
	}
	if s.c != nil {
		throw("runtime: sudog with non-nil c")
	}
	gp := getg()
	if gp.param != nil {
		throw("runtime: releaseSudog with non-nil gp.param")
	}
	mp := acquirem() // avoid rescheduling to another P
	pp := mp.p.ptr()
	if len(pp.sudogcache) == cap(pp.sudogcache) {
		// Transfer half of local cache to the central cache.
		var first, last *sudog
		for len(pp.sudogcache) > cap(pp.sudogcache)/2 {
			n := len(pp.sudogcache)
			p := pp.sudogcache[n-1]
			pp.sudogcache[n-1] = nil
			pp.sudogcache = pp.sudogcache[:n-1]
			if first == nil {
				first = p
			} else {
				last.next = p
			}
			last = p
		}
		lock(&sched.sudoglock)
		last.next = sched.sudogcache
		sched.sudogcache = first
		unlock(&sched.sudoglock)
	}
	pp.sudogcache = append(pp.sudogcache, s)
	releasem(mp)
}

// funcPC returns the entry PC of the function f.
// It assumes that f is a func value. Otherwise the behavior is undefined.
// CAREFUL: In programs with plugins, funcPC can return different values
// for the same function (because there are actually multiple copies of
// the same function in the address space). To be safe, don't use the
// results of this function in any == expression. It is only safe to
// use the result as an address at which to start executing code.
//go:nosplit
func funcPC(f interface{}) uintptr {
	return **(**uintptr)(add(unsafe.Pointer(&f), sys.PtrSize))
}

// called from assembly
func badmcall(fn func(*g)) {
	throw("runtime: mcall called on m->g0 stack")
}

func badmcall2(fn func(*g)) {
	throw("runtime: mcall function returned")
}

func badreflectcall() {
	panic(plainError("arg size to reflect.call more than 1GB"))
}

var badmorestackg0Msg = "fatal: morestack on g0\n"

//go:nosplit
//go:nowritebarrierrec
func badmorestackg0() {
	sp := stringStructOf(&badmorestackg0Msg)
	write(2, sp.str, int32(sp.len))
}

var badmorestackgsignalMsg = "fatal: morestack on gsignal\n"

//go:nosplit
//go:nowritebarrierrec
func badmorestackgsignal() {
	sp := stringStructOf(&badmorestackgsignalMsg)
	write(2, sp.str, int32(sp.len))
}

//go:nosplit
func badctxt() {
	throw("ctxt != 0")
}

func lockedOSThread() bool {
	gp := getg()
	return gp.lockedm != 0 && gp.m.lockedg != 0
}

var (
	// 保存所有的 g
	allgs    []*g
	allglock mutex
)

func allgadd(gp *g) {
	if readgstatus(gp) == _Gidle {
		throw("allgadd: bad status Gidle")
	}

	lock(&allglock)
	allgs = append(allgs, gp)
	allglen = uintptr(len(allgs))
	unlock(&allglock)
}

const (
	// Number of goroutine ids to grab from sched.goidgen to local per-P cache at once.
	// 16 seems to provide enough amortization, but other than that it's mostly arbitrary number.
	_GoidCacheBatch = 16
)

// cpuinit extracts the environment variable GODEBUG from the environment on
// Unix-like operating systems and calls internal/cpu.Initialize.
func cpuinit() {
	const prefix = "GODEBUG="
	var env string

	switch GOOS {
	case "aix", "darwin", "dragonfly", "freebsd", "netbsd", "openbsd", "solaris", "linux":
		cpu.DebugOptions = true

		// Similar to goenv_unix but extracts the environment value for
		// GODEBUG directly.
		// TODO(moehrmann): remove when general goenvs() can be called before cpuinit()
		n := int32(0)
		for argv_index(argv, argc+1+n) != nil {
			n++
		}

		for i := int32(0); i < n; i++ {
			p := argv_index(argv, argc+1+i)
			s := *(*string)(unsafe.Pointer(&stringStruct{unsafe.Pointer(p), findnull(p)}))

			if hasPrefix(s, prefix) {
				env = gostring(p)[len(prefix):]
				break
			}
		}
	}

	cpu.Initialize(env)

	// Support cpu feature variables are used in code generated by the compiler
	// to guard execution of instructions that can not be assumed to be always supported.
	x86HasPOPCNT = cpu.X86.HasPOPCNT
	x86HasSSE41 = cpu.X86.HasSSE41

	arm64HasATOMICS = cpu.ARM64.HasATOMICS
}

// The bootstrap sequence is:
//
//	call osinit
//	call schedinit
//	make & queue new G
//	call runtime·mstart
//
// The new G calls runtime·main.
// schedinit核心实现，其主要由一系列初始化方法和procresize方法组成。
func schedinit() {
	// raceinit must be the first call to race detector.
	// In particular, it must be done before mallocinit below calls racemapshadow.
	// 获取当前g(此刻为g0)，getg由编译器实现，源码中无具体实现，在编译过程中会被直接替换为从TLS获取g
	// get_tls(CX)
	// MOVQ g(CX), BX; BX存器里面现在放的是当前g结构体对象的地址
	_g_ := getg()
	if raceenabled {
		_g_.racectx, raceprocctx0 = raceinit()
	}

	// 设定M的最大个数
	sched.maxmcount = 10000

	// trace初始化
	tracebackinit()
	// 模块数据确认
	moduledataverify()
	// 栈初始化
	stackinit()
	// 内存分配初始化
	mallocinit()
	// m初始化，绑定当前线程号
	mcommoninit(_g_.m)
	// CPU初始化
	cpuinit()       // must run before alginit
	// 内存对齐初始化，在此之前map类型不可用
	alginit()       // maps must not be used before this call
	// 模块初始化
	modulesinit()   // provides activeModules
	// 类型 依赖map, activeModules
	typelinksinit() // uses maps, activeModules
	// 接口itab初始化，依赖 activeModules
	itabsinit()     // uses activeModules

	msigsave(_g_.m)
	initSigmask = _g_.m.sigmask

	// 解析程序参数
	goargs()
	// 解析环境变量
	goenvs()
	// 解析调试参数
	parsedebugvars()
	// 初始化垃圾回收
	gcinit()

	sched.lastpoll = uint64(nanotime())
	procs := ncpu
	if n, ok := atoi32(gogetenv("GOMAXPROCS")); ok && n > 0 {
		procs = n
	}
	// 创建并初始化系统所有P
	if procresize(procs) != nil {
		throw("unknown runnable goroutine during bootstrap")
	}

	// For cgocheck > 1, we turn on the write barrier at all times
	// and check all pointer writes. We can't do this until after
	// procresize because the write barrier needs a P.
	if debug.cgocheck > 1 {
		writeBarrier.cgo = true
		writeBarrier.enabled = true
		for _, p := range allp {
			p.wbBuf.reset()
		}
	}

	if buildVersion == "" {
		// Condition should never trigger. This code just serves
		// to ensure runtime·buildVersion is kept in the resulting binary.
		buildVersion = "unknown"
	}
}

func dumpgstatus(gp *g) {
	_g_ := getg()
	print("runtime: gp: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
	print("runtime:  g:  g=", _g_, ", goid=", _g_.goid, ",  g->atomicstatus=", readgstatus(_g_), "\n")
}

func checkmcount() {
	// sched lock is held
	if mcount() > sched.maxmcount {
		print("runtime: program exceeds ", sched.maxmcount, "-thread limit\n")
		throw("thread exhaustion")
	}
}

// 初始化 m
func mcommoninit(mp *m) {
	// 初始化过程中_g_ = g0
	_g_ := getg()

	// g0 stack won't make sense for user (and is not necessary unwindable).
	if _g_ != _g_.m.g0 {
		callers(1, mp.createstack[:])
	}

	lock(&sched.lock)
	if sched.mnext+1 < sched.mnext {
		throw("runtime: thread ID overflow")
	}
	// 设置 m 的 id
	mp.id = sched.mnext
	sched.mnext++
	// 检查已创建系统线程是否超过了数量限制（10000）
	checkmcount()

	mp.fastrand[0] = 1597334677 * uint32(mp.id)
	mp.fastrand[1] = uint32(cputicks())
	if mp.fastrand[0]|mp.fastrand[1] == 0 {
		mp.fastrand[1] = 1
	}

	mpreinit(mp)
	if mp.gsignal != nil {
		mp.gsignal.stackguard1 = mp.gsignal.stack.lo + _StackGuard
	}

	// Add to allm so garbage collector doesn't free g->m
	// when it is just in a register or thread-local storage.
	mp.alllink = allm

	// NumCgoCall() iterates over allm w/o schedlock,
	// so we need to publish it safely.
	atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
	unlock(&sched.lock)

	// Allocate memory to hold a cgo traceback if the cgo call crashes.
	if iscgo || GOOS == "solaris" || GOOS == "windows" {
		mp.cgoCallers = new(cgoCallers)
	}
}

// Mark gp ready to run.
func ready(gp *g, traceskip int, next bool) {
	if trace.enabled {
		traceGoUnpark(gp, traceskip)
	}

	status := readgstatus(gp)

	// Mark runnable.
	_g_ := getg()
	_g_.m.locks++ // disable preemption because it can be holding p in a local var
	if status&^_Gscan != _Gwaiting {
		dumpgstatus(gp)
		throw("bad g->status in ready")
	}

	// status is Gwaiting or Gscanwaiting, make Grunnable and put on runq
	casgstatus(gp, _Gwaiting, _Grunnable)
	runqput(_g_.m.p.ptr(), gp, next)
	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
		wakep()
	}
	_g_.m.locks--
	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in Case we've cleared it in newstack
		_g_.stackguard0 = stackPreempt
	}
}

// freezeStopWait is a large value that freezetheworld sets
// sched.stopwait to in order to request that all Gs permanently stop.
const freezeStopWait = 0x7fffffff

// freezing is set to non-zero if the runtime is trying to freeze the
// world.
var freezing uint32

// Similar to stopTheWorld but best-effort and can be called several times.
// There is no reverse operation, used during crashing.
// This function must not lock any mutexes.
func freezetheworld() {
	atomic.Store(&freezing, 1)
	// stopwait and preemption requests can be lost
	// due to races with concurrently executing threads,
	// so try several times
	for i := 0; i < 5; i++ {
		// this should tell the scheduler to not start any new goroutines
		sched.stopwait = freezeStopWait
		atomic.Store(&sched.gcwaiting, 1)
		// this should stop running goroutines
		if !preemptall() {
			break // no running goroutines
		}
		usleep(1000)
	}
	// to be sure
	usleep(1000)
	preemptall()
	usleep(1000)
}

func isscanstatus(status uint32) bool {
	if status == _Gscan {
		throw("isscanstatus: Bad status Gscan")
	}
	return status&_Gscan == _Gscan
}

// All reads and writes of g's status go through readgstatus, casgstatus
// castogscanstatus, casfrom_Gscanstatus.
//go:nosplit
func readgstatus(gp *g) uint32 {
	return atomic.Load(&gp.atomicstatus)
}

// Ownership of gcscanvalid:
//
// If gp is running (meaning status == _Grunning or _Grunning|_Gscan),
// then gp owns gp.gcscanvalid, and other goroutines must not modify it.
//
// Otherwise, a second goroutine can lock the scan state by setting _Gscan
// in the status bit and then modify gcscanvalid, and then unlock the scan state.
//
// Note that the first condition implies an exception to the second:
// if a second goroutine changes gp's status to _Grunning|_Gscan,
// that second goroutine still does not have the right to modify gcscanvalid.

// The Gscanstatuses are acting like locks and this releases them.
// If it proves to be a performance hit we should be able to make these
// simple atomic stores but for now we are going to throw if
// we see an inconsistent state.
func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
	success := false

	// Check that transition is valid.
	switch oldval {
	default:
		print("runtime: casfrom_Gscanstatus bad oldval gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
		dumpgstatus(gp)
		throw("casfrom_Gscanstatus:top gp->status is not in scan state")
	case _Gscanrunnable,
		_Gscanwaiting,
		_Gscanrunning,
		_Gscansyscall:
		if newval == oldval&^_Gscan {
			success = atomic.Cas(&gp.atomicstatus, oldval, newval)
		}
	}
	if !success {
		print("runtime: casfrom_Gscanstatus failed gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
		dumpgstatus(gp)
		throw("casfrom_Gscanstatus: gp->status is not in scan state")
	}
}

// This will return false if the gp is not in the expected status and the cas fails.
// This acts like a lock acquire while the casfromgstatus acts like a lock release.
func castogscanstatus(gp *g, oldval, newval uint32) bool {
	switch oldval {
	case _Grunnable,
		_Grunning,
		_Gwaiting,
		_Gsyscall:
		if newval == oldval|_Gscan {
			return atomic.Cas(&gp.atomicstatus, oldval, newval)
		}
	}
	print("runtime: castogscanstatus oldval=", hex(oldval), " newval=", hex(newval), "\n")
	throw("castogscanstatus")
	panic("not reached")
}

// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
// and casfrom_Gscanstatus instead.
// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that
// put it in the Gscan state is finished.
//go:nosplit
func casgstatus(gp *g, oldval, newval uint32) {
	if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval {
		systemstack(func() {
			print("runtime: casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
			throw("casgstatus: bad incoming values")
		})
	}

	if oldval == _Grunning && gp.gcscanvalid {
		// If oldvall == _Grunning, then the actual status must be
		// _Grunning or _Grunning|_Gscan; either way,
		// we own gp.gcscanvalid, so it's safe to read.
		// gp.gcscanvalid must not be true when we are running.
		systemstack(func() {
			print("runtime: casgstatus ", hex(oldval), "->", hex(newval), " gp.status=", hex(gp.atomicstatus), " gp.gcscanvalid=true\n")
			throw("casgstatus")
		})
	}

	// See https://golang.org/cl/21503 for justification of the yield delay.
	const yieldDelay = 5 * 1000
	var nextYield int64

	// loop if gp->atomicstatus is in a scan state giving
	// GC time to finish and change the state to oldval.
	for i := 0; !atomic.Cas(&gp.atomicstatus, oldval, newval); i++ {
		if oldval == _Gwaiting && gp.atomicstatus == _Grunnable {
			throw("casgstatus: waiting for Gwaiting but is Grunnable")
		}
		// Help GC if needed.
		// if gp.preemptscan && !gp.gcworkdone && (oldval == _Grunning || oldval == _Gsyscall) {
		// 	gp.preemptscan = false
		// 	systemstack(func() {
		// 		gcphasework(gp)
		// 	})
		// }
		// But meanwhile just yield.
		if i == 0 {
			nextYield = nanotime() + yieldDelay
		}
		if nanotime() < nextYield {
			for x := 0; x < 10 && gp.atomicstatus != oldval; x++ {
				procyield(1)
			}
		} else {
			osyield()
			nextYield = nanotime() + yieldDelay/2
		}
	}
	if newval == _Grunning {
		gp.gcscanvalid = false
	}
}

// casgstatus(gp, oldstatus, Gcopystack), assuming oldstatus is Gwaiting or Grunnable.
// Returns old status. Cannot call casgstatus directly, because we are racing with an
// async wakeup that might come in from netpoll. If we see Gwaiting from the readgstatus,
// it might have become Grunnable by the time we get to the cas. If we called casgstatus,
// it would loop waiting for the status to go back to Gwaiting, which it never will.
//go:nosplit
func casgcopystack(gp *g) uint32 {
	for {
		oldstatus := readgstatus(gp) &^ _Gscan
		if oldstatus != _Gwaiting && oldstatus != _Grunnable {
			throw("copystack: bad status, not Gwaiting or Grunnable")
		}
		if atomic.Cas(&gp.atomicstatus, oldstatus, _Gcopystack) {
			return oldstatus
		}
	}
}

// scang blocks until gp's stack has been scanned.
// It might be scanned by scang or it might be scanned by the goroutine itself.
// Either way, the stack scan has completed when scang returns.
// 函数负责扫描G的栈
func scang(gp *g, gcw *gcWork) {
	// Invariant; we (the caller, markroot for a specific goroutine) own gp.gcscandone.
	// Nothing is racing with us now, but gcscandone might be set to true left over
	// from an earlier round of stack scanning (we scan twice per GC).
	// We use gcscandone to record whether the scan has been done during this round.
	// 标记扫描未完成
	gp.gcscandone = false

	// See https://golang.org/cl/21503 for justification of the yield delay.
	const yieldDelay = 10 * 1000
	var nextYield int64

	// Endeavor to get gcscandone set to true,
	// either by doing the stack scan ourselves or by coercing gp to scan itself.
	// gp.gcscandone can transition from false to true when we're not looking
	// (if we asked for preemption), so any time we lock the status using
	// castogscanstatus we have to double-check that the scan is still not done.
	// 循环直到扫描完成
loop:
	for i := 0; !gp.gcscandone; i++ {
		// 判断G的当前状态
		switch s := readgstatus(gp); s {
		default:
			dumpgstatus(gp)
			throw("stopg: invalid status")

		// G已中止, 不需要扫描它
		case _Gdead:
			// No stack.
			gp.gcscandone = true
			break loop

		// G的栈正在扩展, 下一轮重试
		case _Gcopystack:
		// Stack being switched. Go around again.

		// G不是运行中, 首先需要防止它运行
		case _Grunnable, _Gsyscall, _Gwaiting:
			// Claim goroutine by setting scan bit.
			// Racing with execution or readying of gp.
			// The scan bit keeps them from running
			// the goroutine until we're done.
			if castogscanstatus(gp, s, s|_Gscan) {
				// 原子切换状态成功时扫描它的栈
				if !gp.gcscandone {
					scanstack(gp, gcw)
					gp.gcscandone = true
				}
				// 恢复G的状态, 并跳出循环
				restartg(gp)
				break loop
			}

		// G正在扫描它自己, 等待扫描完毕
		case _Gscanwaiting:
		// newstack is doing a scan for us right now. Wait.

		// G正在运行
		case _Grunning:
			// Goroutine running. Try to preempt execution so it can scan itself.
			// The preemption handler (in newstack) does the actual scan.

			// Optimization: if there is already a pending preemption request
			// (from the previous loop iteration), don't bother with the atomics.
			// 如果已经有抢占请求, 则抢占成功时会帮我们处理
			if gp.preemptscan && gp.preempt && gp.stackguard0 == stackPreempt {
				break
			}

			// Ask for preemption and self scan.
			// 抢占G, 抢占成功时G会扫描它自己
			if castogscanstatus(gp, _Grunning, _Gscanrunning) {
				if !gp.gcscandone {
					gp.preemptscan = true
					gp.preempt = true
					gp.stackguard0 = stackPreempt
				}
				casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
			}
		}

		// 第一轮休眠10毫秒, 第二轮休眠5毫秒
		if i == 0 {
			nextYield = nanotime() + yieldDelay
		}
		if nanotime() < nextYield {
			procyield(10)
		} else {
			osyield()
			nextYield = nanotime() + yieldDelay/2
		}
	}

	// 扫描完成, 取消抢占扫描的请求
	// 设置preemptscan后, 在抢占G成功时会调用scanstack扫描它自己的栈.
	gp.preemptscan = false // cancel scan request if no longer needed
}

// The GC requests that this routine be moved from a scanmumble state to a mumble state.
func restartg(gp *g) {
	s := readgstatus(gp)
	switch s {
	default:
		dumpgstatus(gp)
		throw("restartg: unexpected status")

	case _Gdead:
	// ok

	case _Gscanrunnable,
		_Gscanwaiting,
		_Gscansyscall:
		casfrom_Gscanstatus(gp, s, s&^_Gscan)
	}
}

// stopTheWorld stops all P's from executing goroutines, interrupting
// all goroutines at GC safe points and records reason as the reason
// for the stop. On return, only the current goroutine's P is running.
// stopTheWorld must not be called from a system stack and the caller
// must not hold worldsema. The caller must call startTheWorld when
// other P's should resume execution.
//
// stopTheWorld is safe for multiple goroutines to call at the
// same time. Each will execute its own stop, and the stops will
// be serialized.
//
// This is also used by routines that do stack dumps. If the system is
// in panic or being exited, this may not reliably stop all
// goroutines.
func stopTheWorld(reason string) {
	semacquire(&worldsema)
	getg().m.preemptoff = reason
	systemstack(stopTheWorldWithSema)
}

// startTheWorld undoes the effects of stopTheWorld.
func startTheWorld() {
	systemstack(func() { startTheWorldWithSema(false) })
	// worldsema must be held over startTheWorldWithSema to ensure
	// gomaxprocs cannot change while worldsema is held.
	semrelease(&worldsema)
	getg().m.preemptoff = ""
}

// Holding worldsema grants an M the right to try to stop the world
// and prevents gomaxprocs from changing concurrently.
var worldsema uint32 = 1

// stopTheWorldWithSema is the core implementation of stopTheWorld.
// The caller is responsible for acquiring worldsema and disabling
// preemption first and then should stopTheWorldWithSema on the system
// stack:
//
//	semacquire(&worldsema, 0)
//	m.preemptoff = "reason"
//	systemstack(stopTheWorldWithSema)
//
// When finished, the caller must either call startTheWorld or undo
// these three operations separately:
//
//	m.preemptoff = ""
//	systemstack(startTheWorldWithSema)
//	semrelease(&worldsema)
//
// It is allowed to acquire worldsema once and then execute multiple
// startTheWorldWithSema/stopTheWorldWithSema pairs.
// Other P's are able to execute between successive calls to
// startTheWorldWithSema and stopTheWorldWithSema.
// Holding worldsema causes any other goroutines invoking
// stopTheWorld to block.
// 停止整个世界, 这个函数必须在g0中运行
func stopTheWorldWithSema() {
	_g_ := getg()

	// If we hold a lock, then we won't be able to stop another M
	// that is blocked trying to acquire the lock.
	if _g_.m.locks > 0 {
		throw("stopTheWorld: holding locks")
	}

	lock(&sched.lock)

	// 需要停止的P数量
	sched.stopwait = gomaxprocs
	// 设置gc等待标记, 调度时看见此标记会进入等待
	atomic.Store(&sched.gcwaiting, 1)
	// 抢占所有运行中的G
	preemptall()
	// stop current P 停止当前的P
	_g_.m.p.ptr().status = _Pgcstop //
	// 减少需要停止的P数量(当前的P算一个)
	sched.stopwait--
	// try to retake all P's in Psyscall status
	// 抢占所有在Psyscall状态的P, 防止它们重新参与调度
	for _, p := range allp {
		s := p.status
		if s == _Psyscall && atomic.Cas(&p.status, s, _Pgcstop) {
			if trace.enabled {
				traceGoSysBlock(p)
				traceProcStop(p)
			}
			p.syscalltick++
			sched.stopwait--
		}
	}
	// stop idle P's
	// 防止所有空闲的P重新参与调度
	for {
		p := pidleget()
		if p == nil {
			break
		}
		p.status = _Pgcstop
		sched.stopwait--
	}
	wait := sched.stopwait > 0
	unlock(&sched.lock)

	// wait for remaining P's to stop voluntarily
	// 如果仍有需要停止的P, 则等待它们停止
	if wait {
		for {
			// wait for 100us, then try to re-preempt in case of any races
			// 循环等待 + 抢占所有运行中的G
			if notetsleep(&sched.stopnote, 100*1000) {
				noteclear(&sched.stopnote)
				break
			}
			preemptall()
		}
	}

	// sanity checks
	// 逻辑正确性检查
	bad := ""
	if sched.stopwait != 0 {
		bad = "stopTheWorld: not stopped (stopwait != 0)"
	} else {
		for _, p := range allp {
			if p.status != _Pgcstop {
				bad = "stopTheWorld: not stopped (status != _Pgcstop)"
			}
		}
	}
	if atomic.Load(&freezing) != 0 {
		// Some other thread is panicking. This can cause the
		// sanity checks above to fail if the panic happens in
		// the signal handler on a stopped thread. Either way,
		// we should halt this thread.
		lock(&deadlock)
		lock(&deadlock)
	}
	if bad != "" {
		throw(bad)
	}

	// 到这里所有运行中的G都会变为待运行, 并且所有的P都不能被M获取
	// 也就是说所有的go代码(除了当前的)都会停止运行, 并且不能运行新的go代码
}

/*
重启世界后各个M会重新开始调度, 调度时会优先使用上面提到的findRunnableGCWorker函数查找任务, 之后就有大约25%的P运行后台标记任务.
*/
// 函数会重新启动世界
func startTheWorldWithSema(emitTraceEvent bool) int64 {
	_g_ := getg()

	// 禁止G被抢占
	_g_.m.locks++ // disable preemption because it can be holding p in a local var
	// 判断收到的网络事件(fd可读可写或错误)并添加对应的G到待运行队列
	if netpollinited() {
		list := netpoll(false) // non-blocking
		injectglist(&list)
	}
	// 判断是否要启动gc helper
	lock(&sched.lock)

	// 如果要求改变gomaxprocs则调整P的数量
	// procresize会返回有可运行任务的P的链表
	procs := gomaxprocs
	if newprocs != 0 {
		procs = newprocs
		newprocs = 0
	}
	p1 := procresize(procs)
	// 取消GC等待标记
	sched.gcwaiting = 0
	// 如果sysmon在等待则唤醒它
	if sched.sysmonwait != 0 {
		sched.sysmonwait = 0
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)

	// 唤醒有可运行任务的P
	for p1 != nil {
		p := p1
		p1 = p1.link.ptr()
		if p.m != 0 {
			mp := p.m.ptr()
			p.m = 0
			if mp.nextp != 0 {
				throw("startTheWorld: inconsistent mp->nextp")
			}
			mp.nextp.set(p)
			notewakeup(&mp.park)
		} else {
			// Start M to run P.  Do not start another M below.
			newm(nil, p)
		}
	}

	// Capture start-the-world time before doing clean-up tasks.
	startTime := nanotime()
	if emitTraceEvent {
		traceGCSTWDone()
	}

	// Wakeup an additional proc in case we have excessive runnable goroutines
	// in local queues or in the global queue. If we don't, the proc will park itself.
	// If we have lots of excessive work, resetspinning will unpark additional procs as necessary.
	// 如果有空闲的P，并且没有自旋中的M则唤醒或者创建一个M
	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
		wakep()
	}

	// 允许G被抢占
	_g_.m.locks--
	// 如果当前G要求被抢占则重新尝试
	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
		_g_.stackguard0 = stackPreempt
	}

	return startTime
}

// Called to start an M.
//
// This must not split the stack because we may not even have stack
// bounds set up yet.
//
// May run during STW (because it doesn't have a P yet), so write
// barriers are not allowed.
//
//go:nosplit
//go:nowritebarrierrec
// 函数mstart首先按需初始化m.g0的栈空间，然后调用mstart1启动调度循环。
// 参数fn指向新线程执行的函数，我们看代码中为&mstart，也就是说mstart是新创建的M的入口方法。
func mstart() {
	_g_ := getg()

	// 如果stack.lo为空，则初始化g0的栈空间
	osStack := _g_.stack.lo == 0
	if osStack {
		// Initialize stack bounds from system stack.
		// Cgo may have left stack size in stack.hi.
		// minit may update the stack bounds.
		// 从系统堆栈初始化堆栈边界。
		// Cgo可能在堆栈中保留了堆栈大小。
		// minit可能会更新堆栈边界。
		size := _g_.stack.hi
		if size == 0 {
			size = 8192 * sys.StackGuardMultiplier
		}
		_g_.stack.hi = uintptr(noescape(unsafe.Pointer(&size)))
		_g_.stack.lo = _g_.stack.hi - size + 1024
	}
	// Initialize stack guards so that we can start calling
	// both Go and C functions with stack growth prologues.
	// 初始化`stackguard0`和`stackguard1`
	_g_.stackguard0 = _g_.stack.lo + _StackGuard
	_g_.stackguard1 = _g_.stackguard0

	// 启动调度循环
	mstart1()

	// Exit this thread.
	if GOOS == "windows" || GOOS == "solaris" || GOOS == "plan9" || GOOS == "darwin" || GOOS == "aix" {
		// Window, Solaris, Darwin, AIX and Plan 9 always system-allocate
		// the stack, but put it in _g_.stack before mstart,
		// so the logic above hasn't set osStack yet.
		osStack = true
	}
	// 一般来说，Go中的线程永不退出
	mexit(osStack)
}

// 函数mstart1首先进行一些检测工作以及初始化工作，然后执行m的入口函数，接下来通过函数acquirep绑定p，
// 最终通过函数schedule启动调度循环。
func mstart1() {
	// 启动过程时 _g_ = m0.g0
	_g_ := getg()

	// 调度循环只能在m.g0协程上执行
	if _g_ != _g_.m.g0 {
		throw("bad runtime·mstart")
	}

	// Record the caller for use as the top of stack in mcall and
	// for terminating the thread.
	// We're never coming back to mstart1 after we call schedule,
	// so other calls can reuse the current frame.
	// 由于不再回到mstart1函数，保存栈帧：
	// 1. mcall中使用
	// 2. 结束线程
	// 一旦调用 schedule() 函数，永不返回。所以栈帧可以被复用
	save(getcallerpc(), getcallersp())
	// 空方法
	asminit()
	// 初始化线程信号量， 绑定线程ID
	minit()

	// Install signal handlers; after minit so that minit can
	// prepare the thread to be able to handle the signals.
	// 初始化信号
	if _g_.m == &m0 {
		mstartm0()
	}

	// 调用每个M的入口函数，可为空。执行启动函数。初始化过程中，fn == nil
	if fn := _g_.m.mstartfn; fn != nil {
		fn()
	}

	// 绑定p
	if _g_.m != &m0 {
		acquirep(_g_.m.nextp.ptr())
		_g_.m.nextp = 0
	}

	// 运行调度循环，永不返回
	schedule()
}

// mstartm0 implements part of mstart1 that only runs on the m0.
//
// Write barriers are allowed here because we know the GC can't be
// running yet, so they'll be no-ops.
//
//go:yeswritebarrierrec
func mstartm0() {
	// Create an extra M for callbacks on threads not created by Go.
	// An extra M is also needed on Windows for callbacks created by
	// syscall.NewCallback. See issue #6751 for details.
	if (iscgo || GOOS == "windows") && !cgoHasExtraM {
		cgoHasExtraM = true
		newextram()
	}
	initsig(false)
}

// mexit tears down and exits the current thread.
//
// Don't call this directly to exit the thread, since it must run at
// the top of the thread stack. Instead, use gogo(&_g_.m.g0.sched) to
// unwind the stack to the point that exits the thread.
//
// It is entered with m.p != nil, so write barriers are allowed. It
// will release the P before exiting.
//
//go:yeswritebarrierrec
func mexit(osStack bool) {
	g := getg()
	m := g.m

	if m == &m0 {
		// This is the main thread. Just wedge it.
		//
		// On Linux, exiting the main thread puts the process
		// into a non-waitable zombie state. On Plan 9,
		// exiting the main thread unblocks wait even though
		// other threads are still running. On Solaris we can
		// neither exitThread nor return from mstart. Other
		// bad things probably happen on other platforms.
		//
		// We could try to clean up this M more before wedging
		// it, but that complicates signal handling.
		handoffp(releasep())
		lock(&sched.lock)
		sched.nmfreed++
		checkdead()
		unlock(&sched.lock)
		notesleep(&m.park)
		throw("locked m0 woke up")
	}

	sigblock()
	unminit()

	// Free the gsignal stack.
	if m.gsignal != nil {
		stackfree(m.gsignal.stack)
	}

	// Remove m from allm.
	lock(&sched.lock)
	for pprev := &allm; *pprev != nil; pprev = &(*pprev).alllink {
		if *pprev == m {
			*pprev = m.alllink
			goto found
		}
	}
	throw("m not found in allm")
found:
	if !osStack {
		// Delay reaping m until it's done with the stack.
		//
		// If this is using an OS stack, the OS will free it
		// so there's no need for reaping.
		atomic.Store(&m.freeWait, 1)
		// Put m on the free list, though it will not be reaped until
		// freeWait is 0. Note that the free list must not be linked
		// through alllink because some functions walk allm without
		// locking, so may be using alllink.
		m.freelink = sched.freem
		sched.freem = m
	}
	unlock(&sched.lock)

	// Release the P.
	handoffp(releasep())
	// After this point we must not have write barriers.

	// Invoke the deadlock detector. This must happen after
	// handoffp because it may have started a new M to take our
	// P's work.
	lock(&sched.lock)
	sched.nmfreed++
	checkdead()
	unlock(&sched.lock)

	if osStack {
		// Return from mstart and let the system thread
		// library free the g0 stack and terminate the thread.
		return
	}

	// mstart is the thread's entry point, so there's nothing to
	// return to. Exit the thread directly. exitThread will clear
	// m.freeWait when it's done with the stack and the m can be
	// reaped.
	exitThread(&m.freeWait)
}

// forEachP calls fn(p) for every P p when p reaches a GC safe point.
// If a P is currently executing code, this will bring the P to a GC safe point and execute fn on that P.
// If the P is not executing code (it is idle or in a syscall), this will call fn(p) directly while preventing the P from exiting its state.
// This does not ensure that fn will run on every CPU executing Go code, but it acts as a global memory barrier.
// GC uses this as a "ragged barrier."
// The caller must hold worldsema.
//
//go:systemstack
//当p到达GC安全点时，forEachP为每个p调用fn（p）。
//如果P当前正在执行代码，这将使P到达GC安全点，并在该P上执行fn。
//如果P没有执行代码（它是空闲的或在系统调用中），这将直接调用fn（P），同时防止P退出其状态。
//这并不能确保fn将在每个执行Go代码的CPU上运行，但它充当了一个全局内存屏障。
//GC将其用作“粗糙的屏障”
//调用方必须持有worldsema。
func forEachP(fn func(*p)) {
	mp := acquirem()
	_p_ := getg().m.p.ptr()

	lock(&sched.lock)
	if sched.safePointWait != 0 {
		throw("forEachP: sched.safePointWait != 0")
	}
	sched.safePointWait = gomaxprocs - 1
	sched.safePointFn = fn

	// Ask all Ps to run the safe point function.
	// 要求所有P运行安全指针方法。
	for _, p := range allp {
		if p != _p_ {
			atomic.Store(&p.runSafePointFn, 1)
		}
	}
	// 设置G可以被抢占
	preemptall()

	// Any P entering _Pidle or _Psyscall from now on will observe
	// p.runSafePointFn == 1 and will call runSafePointFn when
	// changing its status to _Pidle/_Psyscall.

	// Run safe point function for all idle Ps. sched.pidle will
	// not change because we hold sched.lock.
	for p := sched.pidle.ptr(); p != nil; p = p.link.ptr() {
		if atomic.Cas(&p.runSafePointFn, 1, 0) {
			fn(p)
			sched.safePointWait--
		}
	}

	wait := sched.safePointWait > 0
	unlock(&sched.lock)

	// Run fn for the current P.
	// fn函数执行当前P
	fn(_p_)

	// Force Ps currently in _Psyscall into _Pidle and hand them
	// off to induce safe point function execution.
	for _, p := range allp {
		s := p.status
		if s == _Psyscall && p.runSafePointFn == 1 && atomic.Cas(&p.status, s, _Pidle) {
			if trace.enabled {
				traceGoSysBlock(p)
				traceProcStop(p)
			}
			p.syscalltick++
			handoffp(p)
		}
	}

	// Wait for remaining Ps to run fn.
	if wait {
		for {
			// Wait for 100us, then try to re-preempt in case of any races.
			// 等待100秒，然后在任何情况中尝试重新抢先。
			// Requires system stack.
			if notetsleep(&sched.safePointNote, 100*1000) {
				noteclear(&sched.safePointNote)
				break
			}
			preemptall()
		}
	}
	if sched.safePointWait != 0 {
		throw("forEachP: not done")
	}
	for _, p := range allp {
		if p.runSafePointFn != 0 {
			throw("forEachP: P did not run fn")
		}
	}

	lock(&sched.lock)
	sched.safePointFn = nil
	unlock(&sched.lock)
	releasem(mp)
}

// runSafePointFn runs the safe point function, if any, for this P.
// This should be called like
//
//     if getg().m.p.runSafePointFn != 0 {
//         runSafePointFn()
//     }
//
// runSafePointFn must be checked on any transition in to _Pidle or
// _Psyscall to avoid a race where forEachP sees that the P is running
// just before the P goes into _Pidle/_Psyscall and neither forEachP
// nor the P run the safe-point function.
func runSafePointFn() {
	p := getg().m.p.ptr()
	// Resolve the race between forEachP running the safe-point
	// function on this P's behalf and this P running the
	// safe-point function directly.
	if !atomic.Cas(&p.runSafePointFn, 1, 0) {
		return
	}
	sched.safePointFn(p)
	lock(&sched.lock)
	sched.safePointWait--
	if sched.safePointWait == 0 {
		notewakeup(&sched.safePointNote)
	}
	unlock(&sched.lock)
}

// When running with cgo, we call _cgo_thread_start
// to start threads for us so that we can play nicely with
// foreign code.
var cgoThreadStart unsafe.Pointer

type cgothreadstart struct {
	g   guintptr
	tls *uint64
	fn  unsafe.Pointer
}

// Allocate a new m unassociated with any thread.
// Can use p for allocation context if needed.
// fn is recorded as the new m's m.mstartfn.
//
// This function is allowed to have write barriers even if the caller
// isn't because it borrows _p_.
//
//go:yeswritebarrierrec
func allocm(_p_ *p, fn func()) *m {
	_g_ := getg()
	_g_.m.locks++ // disable GC because it can be called from sysmon
	if _g_.m.p == 0 {
		acquirep(_p_) // temporarily borrow p for mallocs in this function
	}

	// Release the free M list. We need to do this somewhere and
	// this may free up a stack we can use.
	if sched.freem != nil {
		lock(&sched.lock)
		var newList *m
		for freem := sched.freem; freem != nil; {
			if freem.freeWait != 0 {
				next := freem.freelink
				freem.freelink = newList
				newList = freem
				freem = next
				continue
			}
			stackfree(freem.g0.stack)
			freem = freem.freelink
		}
		sched.freem = newList
		unlock(&sched.lock)
	}

	mp := new(m)
	mp.mstartfn = fn
	mcommoninit(mp)

	// In case of cgo or Solaris or Darwin, pthread_create will make us a stack.
	// Windows and Plan 9 will layout sched stack on OS stack.
	if iscgo || GOOS == "solaris" || GOOS == "windows" || GOOS == "plan9" || GOOS == "darwin" {
		mp.g0 = malg(-1)
	} else {
		mp.g0 = malg(8192 * sys.StackGuardMultiplier)
	}
	mp.g0.m = mp

	if _p_ == _g_.m.p.ptr() {
		releasep()
	}
	_g_.m.locks--
	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
		_g_.stackguard0 = stackPreempt
	}

	return mp
}

// needm is called when a cgo callback happens on a
// thread without an m (a thread not created by Go).
// In this case, needm is expected to find an m to use
// and return with m, g initialized correctly.
// Since m and g are not set now (likely nil, but see below)
// needm is limited in what routines it can call. In particular
// it can only call nosplit functions (textflag 7) and cannot
// do any scheduling that requires an m.
//
// In order to avoid needing heavy lifting here, we adopt
// the following strategy: there is a stack of available m's
// that can be stolen. Using compare-and-swap
// to pop from the stack has ABA races, so we simulate
// a lock by doing an exchange (via Casuintptr) to steal the stack
// head and replace the top pointer with MLOCKED (1).
// This serves as a simple spin lock that we can use even
// without an m. The thread that locks the stack in this way
// unlocks the stack by storing a valid stack head pointer.
//
// In order to make sure that there is always an m structure
// available to be stolen, we maintain the invariant that there
// is always one more than needed. At the beginning of the
// program (if cgo is in use) the list is seeded with a single m.
// If needm finds that it has taken the last m off the list, its job
// is - once it has installed its own m so that it can do things like
// allocate memory - to create a spare m and put it on the list.
//
// Each of these extra m's also has a g0 and a curg that are
// pressed into service as the scheduling stack and current
// goroutine for the duration of the cgo callback.
//
// When the callback is done with the m, it calls dropm to
// put the m back on the list.
//go:nosplit
func needm(x byte) {
	if (iscgo || GOOS == "windows") && !cgoHasExtraM {
		// Can happen if C/C++ code calls Go from a global ctor.
		// Can also happen on Windows if a global ctor uses a
		// callback created by syscall.NewCallback. See issue #6751
		// for details.
		//
		// Can not throw, because scheduler is not initialized yet.
		write(2, unsafe.Pointer(&earlycgocallback[0]), int32(len(earlycgocallback)))
		exit(1)
	}

	// Lock extra list, take head, unlock popped list.
	// nilokay=false is safe here because of the invariant above,
	// that the extra list always contains or will soon contain
	// at least one m.
	mp := lockextra(false)

	// Set needextram when we've just emptied the list,
	// so that the eventual call into cgocallbackg will
	// allocate a new m for the extra list. We delay the
	// allocation until then so that it can be done
	// after exitsyscall makes sure it is okay to be
	// running at all (that is, there's no garbage collection
	// running right now).
	mp.needextram = mp.schedlink == 0
	extraMCount--
	unlockextra(mp.schedlink.ptr())

	// Save and block signals before installing g.
	// Once g is installed, any incoming signals will try to execute,
	// but we won't have the sigaltstack settings and other data
	// set up appropriately until the end of minit, which will
	// unblock the signals. This is the same dance as when
	// starting a new m to run Go code via newosproc.
	msigsave(mp)
	sigblock()

	// Install g (= m->g0) and set the stack bounds
	// to match the current stack. We don't actually know
	// how big the stack is, like we don't know how big any
	// scheduling stack is, but we assume there's at least 32 kB,
	// which is more than enough for us.
	setg(mp.g0)
	_g_ := getg()
	_g_.stack.hi = uintptr(noescape(unsafe.Pointer(&x))) + 1024
	_g_.stack.lo = uintptr(noescape(unsafe.Pointer(&x))) - 32*1024
	_g_.stackguard0 = _g_.stack.lo + _StackGuard

	// Initialize this thread to use the m.
	asminit()
	minit()

	// mp.curg is now a real goroutine.
	casgstatus(mp.curg, _Gdead, _Gsyscall)
	atomic.Xadd(&sched.ngsys, -1)
}

var earlycgocallback = []byte("fatal error: cgo callback before cgo call\n")

// newextram allocates m's and puts them on the extra list.
// It is called with a working local m, so that it can do things
// like call schedlock and allocate.
func newextram() {
	c := atomic.Xchg(&extraMWaiters, 0)
	if c > 0 {
		for i := uint32(0); i < c; i++ {
			oneNewExtraM()
		}
	} else {
		// Make sure there is at least one extra M.
		mp := lockextra(true)
		unlockextra(mp)
		if mp == nil {
			oneNewExtraM()
		}
	}
}

// oneNewExtraM allocates an m and puts it on the extra list.
func oneNewExtraM() {
	// Create extra goroutine locked to extra m.
	// The goroutine is the context in which the cgo callback will run.
	// The sched.pc will never be returned to, but setting it to
	// goexit makes clear to the traceback routines where
	// the goroutine stack ends.
	mp := allocm(nil, nil)
	gp := malg(4096)
	gp.sched.pc = funcPC(goexit) + sys.PCQuantum
	gp.sched.sp = gp.stack.hi
	gp.sched.sp -= 4 * sys.RegSize // extra space in case of reads slightly beyond frame
	gp.sched.lr = 0
	gp.sched.g = guintptr(unsafe.Pointer(gp))
	gp.syscallpc = gp.sched.pc
	gp.syscallsp = gp.sched.sp
	gp.stktopsp = gp.sched.sp
	gp.gcscanvalid = true
	gp.gcscandone = true
	// malg returns status as _Gidle. Change to _Gdead before
	// adding to allg where GC can see it. We use _Gdead to hide
	// this from tracebacks and stack scans since it isn't a
	// "real" goroutine until needm grabs it.
	casgstatus(gp, _Gidle, _Gdead)
	gp.m = mp
	mp.curg = gp
	mp.lockedInt++
	mp.lockedg.set(gp)
	gp.lockedm.set(mp)
	gp.goid = int64(atomic.Xadd64(&sched.goidgen, 1))
	if raceenabled {
		gp.racectx = racegostart(funcPC(newextram) + sys.PCQuantum)
	}
	// put on allg for garbage collector
	allgadd(gp)

	// gp is now on the allg list, but we don't want it to be
	// counted by gcount. It would be more "proper" to increment
	// sched.ngfree, but that requires locking. Incrementing ngsys
	// has the same effect.
	atomic.Xadd(&sched.ngsys, +1)

	// Add m to the extra list.
	mnext := lockextra(true)
	mp.schedlink.set(mnext)
	extraMCount++
	unlockextra(mp)
}

// dropm is called when a cgo callback has called needm but is now
// done with the callback and returning back into the non-Go thread.
// It puts the current m back onto the extra list.
//
// The main expense here is the call to signalstack to release the
// m's signal stack, and then the call to needm on the next callback
// from this thread. It is tempting to try to save the m for next time,
// which would eliminate both these costs, but there might not be
// a next time: the current thread (which Go does not control) might exit.
// If we saved the m for that thread, there would be an m leak each time
// such a thread exited. Instead, we acquire and release an m on each
// call. These should typically not be scheduling operations, just a few
// atomics, so the cost should be small.
//
// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread
// variable using pthread_key_create. Unlike the pthread keys we already use
// on OS X, this dummy key would never be read by Go code. It would exist
// only so that we could register at thread-exit-time destructor.
// That destructor would put the m back onto the extra list.
// This is purely a performance optimization. The current version,
// in which dropm happens on each cgo call, is still correct too.
// We may have to keep the current version on systems with cgo
// but without pthreads, like Windows.
func dropm() {
	// Clear m and g, and return m to the extra list.
	// After the call to setg we can only call nosplit functions
	// with no pointer manipulation.
	mp := getg().m

	// Return mp.curg to dead state.
	casgstatus(mp.curg, _Gsyscall, _Gdead)
	atomic.Xadd(&sched.ngsys, +1)

	// Block signals before unminit.
	// Unminit unregisters the signal handling stack (but needs g on some systems).
	// Setg(nil) clears g, which is the signal handler's cue not to run Go handlers.
	// It's important not to try to handle a signal between those two steps.
	sigmask := mp.sigmask
	sigblock()
	unminit()

	mnext := lockextra(true)
	extraMCount++
	mp.schedlink.set(mnext)

	setg(nil)

	// Commit the release of mp.
	unlockextra(mp)

	msigrestore(sigmask)
}

// A helper function for EnsureDropM.
func getm() uintptr {
	return uintptr(unsafe.Pointer(getg().m))
}

var extram uintptr
var extraMCount uint32 // Protected by lockextra
var extraMWaiters uint32

// lockextra locks the extra list and returns the list head.
// The caller must unlock the list by storing a new list head
// to extram. If nilokay is true, then lockextra will
// return a nil list head if that's what it finds. If nilokay is false,
// lockextra will keep waiting until the list head is no longer nil.
//go:nosplit
func lockextra(nilokay bool) *m {
	const locked = 1

	incr := false
	for {
		old := atomic.Loaduintptr(&extram)
		if old == locked {
			yield := osyield
			yield()
			continue
		}
		if old == 0 && !nilokay {
			if !incr {
				// Add 1 to the number of threads
				// waiting for an M.
				// This is cleared by newextram.
				atomic.Xadd(&extraMWaiters, 1)
				incr = true
			}
			usleep(1)
			continue
		}
		if atomic.Casuintptr(&extram, old, locked) {
			return (*m)(unsafe.Pointer(old))
		}
		yield := osyield
		yield()
		continue
	}
}

//go:nosplit
func unlockextra(mp *m) {
	atomic.Storeuintptr(&extram, uintptr(unsafe.Pointer(mp)))
}

// execLock serializes exec and clone to avoid bugs or unspecified behaviour
// around exec'ing while creating/destroying threads.  See issue #19546.
var execLock rwmutex

// newmHandoff contains a list of m structures that need new OS threads.
// This is used by newm in situations where newm itself can't safely
// start an OS thread.
var newmHandoff struct {
	lock mutex

	// newm points to a list of M structures that need new OS
	// threads. The list is linked through m.schedlink.
	newm muintptr

	// waiting indicates that wake needs to be notified when an m
	// is put on the list.
	waiting bool
	wake    note

	// haveTemplateThread indicates that the templateThread has
	// been started. This is not protected by lock. Use cas to set
	// to 1.
	haveTemplateThread uint32
}

// Create a new m. It will start off with a call to fn, or else the scheduler.
// fn needs to be static and not a heap allocated closure.
// May run with m.p==nil, so write barriers are not allowed.
//go:nowritebarrierrec
// 接下来我们看下M的创建过程
// 参数fn表示M创建并运行后首先需要执行的方法，该参数可为空，如果为空或者是执行完成，则M会紧接着执行调度循环。
// 参数_p_表示M创建后优先绑定的P，可为空。
// 函数newm首先通过allocm方法创建一个新的M结构，然后通过函数newm1创建关联的系统线程.
func newm(fn func(), _p_ *p) {
	// 创建新的M结构
	mp := allocm(_p_, fn)
	// 暂存 m
	mp.nextp.set(_p_)
	mp.sigmask = initSigmask
	if gp := getg(); gp != nil && gp.m != nil && (gp.m.lockedExt != 0 || gp.m.incgo) && GOOS != "plan9" {
		// We're on a locked M or a thread that may have been
		// started by C. The kernel state of this thread may
		// be strange (the user may have locked it for that
		// purpose). We don't want to clone that into another
		// thread. Instead, ask a known-good thread to create
		// the thread for us.
		//
		// This is disabled on Plan 9. See golang.org/issue/22227.
		//
		// TODO: This may be unnecessary on Windows, which
		// doesn't model thread creation off fork.
		lock(&newmHandoff.lock)
		if newmHandoff.haveTemplateThread == 0 {
			throw("on a locked thread with no template thread")
		}
		mp.schedlink = newmHandoff.newm
		newmHandoff.newm.set(mp)
		if newmHandoff.waiting {
			newmHandoff.waiting = false
			notewakeup(&newmHandoff.wake)
		}
		unlock(&newmHandoff.lock)
		return
	}
	// 创建操作系统线程并运行
	newm1(mp)
}

// 创建系统线程
func newm1(mp *m) {
	if iscgo {
		var ts cgothreadstart
		if _cgo_thread_start == nil {
			throw("_cgo_thread_start missing")
		}
		ts.g.set(mp.g0)
		ts.tls = (*uint64)(unsafe.Pointer(&mp.tls[0]))
		ts.fn = unsafe.Pointer(funcPC(mstart))
		if msanenabled {
			msanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
		}
		execLock.rlock() // Prevent process clone.
		asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
		execLock.runlock()
		return
	}
	execLock.rlock() // Prevent process clone.
	// 创建系统线程
	newosproc(mp)
	execLock.runlock()
}

// startTemplateThread starts the template thread if it is not already
// running.
//
// The calling thread must itself be in a known-good state.
func startTemplateThread() {
	if GOARCH == "wasm" { // no threads on wasm yet
		return
	}
	if !atomic.Cas(&newmHandoff.haveTemplateThread, 0, 1) {
		return
	}
	newm(templateThread, nil)
}

// templateThread is a thread in a known-good state that exists solely
// to start new threads in known-good states when the calling thread
// may not be in a good state.
//
// Many programs never need this, so templateThread is started lazily
// when we first enter a state that might lead to running on a thread
// in an unknown state.
//
// templateThread runs on an M without a P, so it must not have write
// barriers.
//
//go:nowritebarrierrec
func templateThread() {
	lock(&sched.lock)
	sched.nmsys++
	checkdead()
	unlock(&sched.lock)

	for {
		lock(&newmHandoff.lock)
		for newmHandoff.newm != 0 {
			newm := newmHandoff.newm.ptr()
			newmHandoff.newm = 0
			unlock(&newmHandoff.lock)
			for newm != nil {
				next := newm.schedlink.ptr()
				newm.schedlink = 0
				newm1(newm)
				newm = next
			}
			lock(&newmHandoff.lock)
		}
		newmHandoff.waiting = true
		noteclear(&newmHandoff.wake)
		unlock(&newmHandoff.lock)
		notesleep(&newmHandoff.wake)
	}
}

// Stops execution of the current m until new work is available.
// Returns with acquired P.
// 带着获得的P返回。
// 休眠，停止执行工作，直到有新的工作需要做为止
func stopm() {
	// 当前 goroutine，g0
	_g_ := getg()

	if _g_.m.locks != 0 {
		throw("stopm holding locks")
	}
	if _g_.m.p != 0 {
		throw("stopm holding p")
	}
	if _g_.m.spinning {
		throw("stopm spinning")
	}

	lock(&sched.lock)
	// 将 m 放到全局空闲链表里去
	mput(_g_.m)
	unlock(&sched.lock)
	// 进入睡眠状态
	notesleep(&_g_.m.park)
	// 这里被其他工作线程唤醒
	noteclear(&_g_.m.park)
	acquirep(_g_.m.nextp.ptr())
	_g_.m.nextp = 0
}

func mspinning() {
	// startm's caller incremented nmspinning. Set the new M's spinning.
	getg().m.spinning = true
}

// Schedules some M to run the p (creates an M if necessary).
// If p==nil, tries to get an idle P, if no idle P's does nothing.
// May run with m.p==nil, so write barriers are not allowed.
// If spinning is set, the caller has incremented nmspinning and startm will
// either decrement nmspinning or set m.spinning in the newly started M.
//go:nowritebarrierrec
// 函数wakep会直接调用startm唤醒或创建新的线程去执行用户代码。
// 1. 检查是否确实存在空闲的P
// 2. 尝试获取休眠可用的M
// 3. 如果不存在休眠可用的M，则创建新的M
// 4. 存在休眠可用的M，则唤醒该M，并设置当前空闲的P为M启动后优先绑定的P
// 调用 m 来绑定 p，如果没有 m，那就新建一个
// 如果 p 为空，那就尝试获取一个处于空闲状态的 p，如果找到 p，那就什么都不做
func startm(_p_ *p, spinning bool) {
	lock(&sched.lock)
	if _p_ == nil {
		// 检查是否确实存在空闲的P，否则执行返回即可
		// 没有指定 p 则需要从全局空闲队列中获取一个 p
		_p_ = pidleget()
		if _p_ == nil {
			unlock(&sched.lock)
			if spinning {
				// The caller incremented nmspinning, but there are no idle Ps,
				// so it's okay to just undo the increment and give up.
				// 如果找到 p，放弃。还原全局处于自旋状态的 m 的数量
				if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
					throw("startm: negative nmspinning")
				}
			}
			// 没有空闲的 p，直接返回
			return
		}
	}
	// 获取一个休眠中并且空闲可用的M
	// 获取空闲m的过程比较简单。需要注意的是mget方法操作全局变量，需要加锁。
	// 从 m 空闲队列中获取正处于睡眠之中的工作线程，
	// 所有处于睡眠状态的 m 都在此队列中
	mp := mget()
	unlock(&sched.lock)
	if mp == nil {
		// 如果没有找到 m
		var fn func()
		if spinning {
			// The caller incremented nmspinning, so set m.spinning in the new M.
			// 调用者增加了nmspinning，所以在新的M中设置m.spinning。
			fn = mspinning
		}
		// 如果不存在复用的M，则创建新的M
		newm(fn, _p_)
		return
	}
	if mp.spinning {
		throw("startm: m is spinning")
	}
	if mp.nextp != 0 {
		throw("startm: m has p")
	}
	if spinning && !runqempty(_p_) {
		throw("startm: p has runnable gs")
	}
	// The caller incremented nmspinning, so set m.spinning in the new M.
	mp.spinning = spinning
	// 设置M唤醒后优先使用的P
	mp.nextp.set(_p_)
	// 唤醒已空闲并休眠的M
	notewakeup(&mp.park)
}

// Hands off P from syscall or locked M.
// Always runs without a P, so write barriers are not allowed.
// 将P从系统调用中移开或锁定M。
// 始终在没有P的情况下运行，因此不允许使用写屏障。
//go:nowritebarrierrec
func handoffp(_p_ *p) {
	// handoffp must start an M in any situation where findrunnable would return a G to run on _p_.
	// 在findrunnable将返回一个G以运行的任何情况下，切换都必须启动一个M。

	// if it has local work, start it straight away
	// 如果它有本地工作，请立即开始
	// 如果 p 本地有工作或者全局有工作，需要绑定一个 m
	if !runqempty(_p_) || sched.runqsize != 0 {
		startm(_p_, false)
		return
	}
	// if it has GC work, start it straight away
	// 如果有GC工作，马上开始
	if gcBlackenEnabled != 0 && gcMarkWorkAvailable(_p_) {
		startm(_p_, false)
		return
	}
	// no local work, check that there are no spinning/idle M's,
	// otherwise our help is not required
	// 所有其它 p 都在运行 goroutine，说明系统比较忙，需要启动 m
	if atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) == 0 && atomic.Cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic
		// p 没有本地工作，启动一个自旋 m 来找工作
		startm(_p_, true)
		return
	}
	lock(&sched.lock)
	if sched.gcwaiting != 0 {
		_p_.status = _Pgcstop
		sched.stopwait--
		if sched.stopwait == 0 {
			notewakeup(&sched.stopnote)
		}
		unlock(&sched.lock)
		return
	}
	if _p_.runSafePointFn != 0 && atomic.Cas(&_p_.runSafePointFn, 1, 0) {
		sched.safePointFn(_p_)
		sched.safePointWait--
		if sched.safePointWait == 0 {
			notewakeup(&sched.safePointNote)
		}
	}
	// 全局队列有工作
	if sched.runqsize != 0 {
		unlock(&sched.lock)
		startm(_p_, false)
		return
	}
	// If this is the last running P and nobody is polling network,
	// need to wakeup another M to poll network.
	if sched.npidle == uint32(gomaxprocs-1) && atomic.Load64(&sched.lastpoll) != 0 {
		unlock(&sched.lock)
		startm(_p_, false)
		return
	}
	// 没有工作要处理，把 p 放入全局空闲队列
	pidleput(_p_)
	unlock(&sched.lock)
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
// 线程启动
// 在协程创建的最后阶段，也就是函数newproc1的最后阶段，会检测是否存在空闲的P,
// 如果存在空闲的P则表示系统有资源执行新创建的协程，然后调用方法wakep。
// 在我们先前提到的理论中，直观上感觉此时应该创建一个新的M去执行用户新创建的协程，我们看下wakep是如何实现的
func wakep() {
	// be conservative about spinning threads
	if !atomic.Cas(&sched.nmspinning, 0, 1) {
		return
	}
	startm(nil, true)
}

// Stops execution of the current m that is locked to a g until the g is runnable again.
// Returns with acquired P.
// 停止执行被锁定在一个g上的当前m，直到这个g再次可以运行。
// 返回时获得P。
func stoplockedm() {
	// 获取当前g（m.g0）
	_g_ := getg()

	if _g_.m.lockedg == 0 || _g_.m.lockedg.ptr().lockedm.ptr() != _g_.m {
		throw("stoplockedm: inconsistent locking")
	}
	// 剥离当前P，留待其他M使用
	if _g_.m.p != 0 {
		// Schedule another M to run this p.
		_p_ := releasep()
		handoffp(_p_)
	}
	incidlelocked(1)
	// Wait until another thread schedules lockedg again.
	// 当前线程陷入休眠，直到锁定的g被调度执行时再次唤醒
	notesleep(&_g_.m.park)
	// 重置同步信号量值
	noteclear(&_g_.m.park)
	// 检查g状态是否满足运行条件
	status := readgstatus(_g_.m.lockedg.ptr())
	if status&^_Gscan != _Grunnable {
		print("runtime:stoplockedm: g is not Grunnable or Gscanrunnable\n")
		dumpgstatus(_g_)
		throw("stoplockedm: not runnable")
	}
	// 将P和当前M绑定
	acquirep(_g_.m.nextp.ptr())
	_g_.m.nextp = 0
}

// Schedules the locked m to run the locked gp.
// May run during STW, so write barriers are not allowed.
// 安排锁定的m运行锁定的gp。
// 可能在STW期间运行，所以不允许有写障碍。
//go:nowritebarrierrec
func startlockedm(gp *g) {
	_g_ := getg()

	mp := gp.lockedm.ptr()
	if mp == _g_.m {
		throw("startlockedm: locked to me")
	}
	if mp.nextp != 0 {
		throw("startlockedm: m has p")
	}
	// directly handoff current P to the locked m
	// 剥离当前P，交由g.lockedm使用
	incidlelocked(-1)
	_p_ := releasep()
	mp.nextp.set(_p_)
	// 唤醒g锁定的m
	notewakeup(&mp.park)
	// 当前m已无p可用，陷入休眠，等待被复用
	stopm()
}

// Stops the current m for stopTheWorld.
// Returns when the world is restarted.
// 停止当前m的stopTheWorld。
// 当世界被重新启动时返回。
func gcstopm() {
	_g_ := getg()

	if sched.gcwaiting == 0 {
		throw("gcstopm: not waiting for gc")
	}
	if _g_.m.spinning {
		_g_.m.spinning = false
		// OK to just drop nmspinning here,
		// startTheWorld will unpark threads as necessary.
		// 可以在这里直接放弃nmspinning。
		// startTheWorld会在必要时取消停放线程。
		if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
			throw("gcstopm: negative nmspinning")
		}
	}
	_p_ := releasep()
	lock(&sched.lock)
	_p_.status = _Pgcstop
	sched.stopwait--
	if sched.stopwait == 0 {
		notewakeup(&sched.stopnote)
	}
	unlock(&sched.lock)
	stopm()
}

// Schedules gp to run on the current M.
// If inheritTime is true, gp inherits the remaining time in the
// current time slice. Otherwise, it starts a new time slice.
// Never returns.
//
// Write barriers are allowed because this is called immediately after
// acquiring a P in several places.
//
//go:yeswritebarrierrec
// 在schedule函数的最后会调用execute方法来执行一个可运行的g。
// 首先设置Goroutine状态为运行中，然后设置不可抢占，将m和g进行绑定，最终调用函数gogo执行g
func execute(gp *g, inheritTime bool) {
	// 获取当前g，应为g0，调度循环在g0上运行
	_g_ := getg()

	// 切换需要运行gp的状态为运行中
	casgstatus(gp, _Grunnable, _Grunning)
	gp.waitsince = 0
	// 不可抢占
	gp.preempt = false
	gp.stackguard0 = gp.stack.lo + _StackGuard
	// 不继承时间片
	if !inheritTime {
		// 调度器调度次数增加 1
		_g_.m.p.ptr().schedtick++
	}
	// m记录需要运行的g。将 gp 和 m 关联起来
	_g_.m.curg = gp
	gp.m = _g_.m

	// Check whether the profiler needs to be turned on or off.
	// 检查是否需要打开或关闭剖析器。
	hz := sched.profilehz
	if _g_.m.profilehz != hz {
		setThreadCPUProfiler(hz)
	}

	if trace.enabled {
		// GoSysExit has to happen when we have a P, but before GoStart.
		// So we emit it here.
		// GoSysExit必须在我们有一个P时发生，但在GoStart之前。
		// 所以我们在这里启动它。
		if gp.syscallsp != 0 && gp.sysblocktraced {
			traceGoSysExit(gp.sysexitticks)
		}
		traceGoStart()
	}

	// gogo 完成从 g0 到 gp 真正的切换
	// CPU 执行权的转让以及栈的切换
	// 执行流的切换从本质上来说就是 CPU 寄存器以及函数调用栈的切换，
	// 然而不管是 go 还是 c 这种高级语言都无法精确控制 CPU 寄存器的修改，
	// 因而高级语言在这里也就无能为力了，只能依靠汇编指令来达成目的
	gogo(&gp.sched)
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from global queue, poll network.
// 函数findrunnable会想尽一切办法获取可运行的g，本地队列、全局队列、偷取其他p本地队列、网络轮询器等。
// 如果获取不到，则当前M进行休眠。
// 从其他地方找 goroutine 来执行
func findrunnable() (gp *g, inheritTime bool) {
	_g_ := getg()

	// The conditions here and in handoffp must agree:
	// if findrunnable would return a G to run, handoffp must start an M.
	// 这里的条件和handoffp中的条件必须一致。
	// 如果findrunnable将返回一个G来运行，handoffp必须启动一个M。

top:
	_p_ := _g_.m.p.ptr()
	if sched.gcwaiting != 0 {
		gcstopm()
		goto top
	}
	if _p_.runSafePointFn != 0 {
		runSafePointFn()
	}
	if fingwait && fingwake {
		if gp := wakefing(); gp != nil {
			ready(gp, 0, true)
		}
	}
	if *cgo_yield != nil {
		asmcgocall(*cgo_yield, nil)
	}

	// local runq
	// 尝试获取p本地队列中的g
	if gp, inheritTime := runqget(_p_); gp != nil {
		return gp, inheritTime
	}

	// global runq
	// 尝试获取全局队列g
	if sched.runqsize != 0 {
		lock(&sched.lock)
		gp := globrunqget(_p_, 0)
		unlock(&sched.lock)
		if gp != nil {
			return gp, false
		}
	}

	// Poll network.
	// This netpoll is only an optimization before we resort to stealing.
	// We can safely skip it if there are no waiters or a thread is blocked
	// in netpoll already. If there is any kind of logical race with that
	// blocked thread (e.g. it has already returned from netpoll, but does
	// not set lastpoll yet), this thread will do blocking netpoll below
	// anyway.
	// 尝试异步获取网络轮询器中可运行g
	if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Load64(&sched.lastpoll) != 0 {
		if list := netpoll(false); !list.empty() { // non-blocking
			gp := list.pop()
			injectglist(&list)
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.enabled {
				traceGoUnpark(gp, 0)
			}
			return gp, false
		}
	}

	// Steal work from other P's.
	// 从其它的p本地队列偷g
	// 如果其他的 P 都处于空闲状态，那肯定没有其他工作要做
	procs := uint32(gomaxprocs)
	if atomic.Load(&sched.npidle) == procs-1 {
		// Either GOMAXPROCS=1 or everybody, except for us, is idle already.
		// New work can appear from returning syscall/cgocall, network or timers.
		// Neither of that submits to local run queues, so no point in stealing.
		goto stop
	}
	// If number of spinning M's >= number of busy P's, block.
	// This is necessary to prevent excessive CPU consumption
	// when GOMAXPROCS>>1 but the program parallelism is low.
	// 如果有很多工作线程在找工作，那我就停下休息。避免消耗太多 CPU
	if !_g_.m.spinning && 2*atomic.Load(&sched.nmspinning) >= procs-atomic.Load(&sched.npidle) {
		goto stop
	}
	if !_g_.m.spinning {
		// 设置自旋状态为 true
		_g_.m.spinning = true
		// 自旋状态数加 1
		atomic.Xadd(&sched.nmspinning, 1)
	}

	// 尝试偷取其他p本地队列
	// 总共执行4次偷取任务，为保证公平性，随机选择P
	// 前3次只偷取队列中g，第四次尝试偷取runnext
	for i := 0; i < 4; i++ {
		for enum := stealOrder.start(fastrand()); !enum.done(); enum.next() {
			if sched.gcwaiting != 0 {
				goto top
			}
			stealRunNextG := i > 2 // first look for ready queues with more than 1 g
			if gp := runqsteal(_p_, allp[enum.position()], stealRunNextG); gp != nil {
				return gp, false
			}
		}
	}

stop:

	// 找不到可执行任务，尝试协助GC ...

	// We have nothing to do. If we're in the GC mark phase, can
	// safely scan and blacken objects, and have work to do, run
	// idle-time marking rather than give up the P.
	if gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != 0 && gcMarkWorkAvailable(_p_) {
		_p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
		gp := _p_.gcBgMarkWorker.ptr()
		casgstatus(gp, _Gwaiting, _Grunnable)
		if trace.enabled {
			traceGoUnpark(gp, 0)
		}
		return gp, false
	}

	// wasm only:
	// If a callback returned and no other goroutine is awake,
	// then pause execution until a callback was triggered.
	if beforeIdle() {
		// At least one goroutine got woken.
		goto top
	}

	// Before we drop our P, make a snapshot of the allp slice,
	// which can change underfoot once we no longer block
	// safe-points. We don't need to snapshot the contents because
	// everything up to cap(allp) is immutable.
	// 记录下allp的快照
	allpSnapshot := allp

	// return P and block
	// 释放P
	lock(&sched.lock)
	if sched.gcwaiting != 0 || _p_.runSafePointFn != 0 {
		unlock(&sched.lock)
		goto top
	}
	if sched.runqsize != 0 {
		// 释放p之前再次尝试在全局队列中获取任务
		gp := globrunqget(_p_, 0)
		unlock(&sched.lock)
		return gp, false
	}
	// 释放p，m和p解绑
	// 当前工作线程解除与 p 之间的绑定，准备去休眠
	if releasep() != _p_ {
		throw("findrunnable: wrong p")
	}
	// p放入空闲队列中
	pidleput(_p_)
	unlock(&sched.lock)

	// Delicate dance: thread transitions from spinning to non-spinning state,
	// potentially concurrently with submission of new goroutines. We must
	// drop nmspinning first and then check all per-P queues again (with
	// #StoreLoad memory barrier in between). If we do it the other way around,
	// another thread can submit a goroutine after we've checked all run queues
	// but before we drop nmspinning; as the result nobody will unpark a thread
	// to run the goroutine.
	// If we discover new work below, we need to restore m.spinning as a signal
	// for resetspinning to unpark a new worker thread (because there can be more
	// than one starving goroutine). However, if after discovering new work
	// we also observe no idle Ps, it is OK to just park the current thread:
	// the system is fully loaded so no spinning threads are required.
	// Also see "Worker thread parking/unparking" comment at the top of the file.
	wasSpinning := _g_.m.spinning
	if _g_.m.spinning {
		// m 即将睡眠，不再处于自旋
		_g_.m.spinning = false
		if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
			throw("findrunnable: negative nmspinning")
		}
	}

	// check all runqueues once again
	// 再次检查所有p的本地运行队列，如果存在不空的本地队列，则重新绑定p并跳转到开始位置再次尝试获取任务
	// 休眠之前再检查一下所有的 p，看一下是否有工作要做
	for _, _p_ := range allpSnapshot {
		if !runqempty(_p_) {
			lock(&sched.lock)
			_p_ = pidleget()
			unlock(&sched.lock)
			if _p_ != nil {
				acquirep(_p_)
				if wasSpinning {
					_g_.m.spinning = true
					atomic.Xadd(&sched.nmspinning, 1)
				}
				goto top
			}
			break
		}
	}

	// Check for idle-priority GC work again.
	// 再次检查空闲优先级的GC工作。
	if gcBlackenEnabled != 0 && gcMarkWorkAvailable(nil) {
		lock(&sched.lock)
		_p_ = pidleget()
		if _p_ != nil && _p_.gcBgMarkWorker == 0 {
			pidleput(_p_)
			_p_ = nil
		}
		unlock(&sched.lock)
		if _p_ != nil {
			acquirep(_p_)
			if wasSpinning {
				_g_.m.spinning = true
				atomic.Xadd(&sched.nmspinning, 1)
			}
			// Go back to idle GC check.
			// 回到闲置的GC检查。
			goto stop
		}
	}

	// poll network
	// 阻塞模式检查网络轮询器是否包含可运行任务
	// 如果等到可执行任务，尝试获取P，如果取到，执行任务，如果没有空闲p，则将任务丢入全局队列
	if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Xchg64(&sched.lastpoll, 0) != 0 {
		if _g_.m.p != 0 {
			throw("findrunnable: netpoll with p")
		}
		if _g_.m.spinning {
			throw("findrunnable: netpoll with spinning")
		}
		list := netpoll(true) // block until new work is available
		atomic.Store64(&sched.lastpoll, uint64(nanotime()))
		if !list.empty() {
			lock(&sched.lock)
			_p_ = pidleget()
			unlock(&sched.lock)
			if _p_ != nil {
				acquirep(_p_)
				gp := list.pop()
				injectglist(&list)
				casgstatus(gp, _Gwaiting, _Grunnable)
				if trace.enabled {
					traceGoUnpark(gp, 0)
				}
				return gp, false
			}
			injectglist(&list)
		}
	}
	// 没有可运行任务，M休眠
	stopm()
	// 被唤醒，继续执行调度循环
	goto top
}

// pollWork reports whether there is non-background work this P could
// be doing. This is a fairly lightweight check to be used for
// background work loops, like idle GC. It checks a subset of the
// conditions checked by the actual scheduler.
func pollWork() bool {
	if sched.runqsize != 0 {
		return true
	}
	p := getg().m.p.ptr()
	if !runqempty(p) {
		return true
	}
	if netpollinited() && atomic.Load(&netpollWaiters) > 0 && sched.lastpoll != 0 {
		if list := netpoll(false); !list.empty() {
			injectglist(&list)
			return true
		}
	}
	return false
}

func resetspinning() {
	_g_ := getg()
	if !_g_.m.spinning {
		throw("resetspinning: not a spinning m")
	}
	_g_.m.spinning = false
	nmspinning := atomic.Xadd(&sched.nmspinning, -1)
	if int32(nmspinning) < 0 {
		throw("findrunnable: negative nmspinning")
	}
	// M wakeup policy is deliberately somewhat conservative, so check if we
	// need to wakeup another P here. See "Worker thread parking/unparking"
	// comment at the top of the file for details.
	// M的唤醒策略故意有些保守，所以检查我们是否需要在这里唤醒另一个P。参见 "工作线程停放/不停放" 详见文件顶部的注释。
	if nmspinning == 0 && atomic.Load(&sched.npidle) > 0 {
		wakep()
	}
}

// Injects the list of runnable G's into the scheduler and clears glist.
// Can run concurrently with GC.
// 将可运行G的列表注入调度器并清除glist。
// 可以与GC同时运行。
func injectglist(glist *gList) {
	if glist.empty() {
		return
	}
	if trace.enabled {
		for gp := glist.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
			traceGoUnpark(gp, 0)
		}
	}
	lock(&sched.lock)
	var n int
	for n = 0; !glist.empty(); n++ {
		gp := glist.pop()
		casgstatus(gp, _Gwaiting, _Grunnable)
		globrunqput(gp)
	}
	unlock(&sched.lock)
	for ; n != 0 && sched.npidle != 0; n-- {
		startm(nil, false)
	}
	*glist = gList{}
}

// One round of scheduler: find a runnable goroutine and execute it.
// Never returns.
// 在M启动的最后阶段，也就是函数mstart&mstart1的最后会调用函数schedule开启新的调度循环。
// 函数schedule是整个协程调度的核心：找到一个可运行的协程并执行它。
// 调度过程中主要通过globrunqget,runqget,findrunnable等方法寻找可运行等g，我们接下来依次看下这三个方法的实现
// 执行一轮调度器的工作：找到一个 runnable 的 goroutine，并且执行它。永不返回。
func schedule() {
	// _g_ = 每个工作线程 m 对应的 g0，初始化时是 m0 的 g0
	_g_ := getg()

	if _g_.m.locks != 0 {
		throw("schedule: holding locks")
	}

	// 如果当前m存在锁定的g，则当前m会进入休眠，等待锁定的g就绪可运行。
	if _g_.m.lockedg != 0 {
		stoplockedm()
		execute(_g_.m.lockedg.ptr(), false) // Never returns.
	}

	// We should not schedule away from a g that is executing a cgo call,
	// since the cgo call is using the m's g0 stack.
	if _g_.m.incgo {
		throw("schedule: in cgo")
	}

top:
	if sched.gcwaiting != 0 {
		gcstopm()
		goto top
	}
	if _g_.m.p.ptr().runSafePointFn != 0 {
		runSafePointFn()
	}

	var gp *g
	var inheritTime bool
	if trace.enabled || trace.shutdown {
		gp = traceReader()
		if gp != nil {
			casgstatus(gp, _Gwaiting, _Grunnable)
			traceGoUnpark(gp, 0)
		}
	}
	if gp == nil && gcBlackenEnabled != 0 {
		gp = gcController.findRunnableGCWorker(_g_.m.p.ptr())
	}
	// 每执行61次调度循环则尝试在全局待运行g队列获取任务
	if gp == nil {
		// Check the global runnable queue once in a while to ensure fairness.
		// Otherwise two goroutines can completely occupy the local runqueue
		// by constantly respawning each other.
		// 每执行61次调度循环则尝试在全局待运行g队列获取任务。保证公平性
		if _g_.m.p.ptr().schedtick%61 == 0 && sched.runqsize > 0 {
			lock(&sched.lock)
			// 获取全局可运行g队列内容。从全局队列最大获取 1 个 goroutine
			gp = globrunqget(_g_.m.p.ptr(), 1)
			unlock(&sched.lock)
		}
	}
	if gp == nil {
		// 尝试在本地可运行队列中获取g
		gp, inheritTime = runqget(_g_.m.p.ptr())
		if gp != nil && _g_.m.spinning {
			throw("schedule: spinning with local work")
		}
	}
	if gp == nil {
		// 阻塞式的寻找可运行的g
		// 其他P偷取，全局g队列，网络轮询器
		// 如果找不到可运行的g，则当前m则休眠挂起

		// 从本地运行队列和全局运行队列都没有找到需要运行的 goroutine，
		// 调用 findrunnable 函数从其它工作线程的运行队列中偷取，如果偷不到，则当前工作线程进入睡眠
		// 直到获取到 runnable goroutine 之后 findrunnable 函数才会返回。
		gp, inheritTime = findrunnable()
		// blocks until work is available
		// 阻塞，直到找到可运行的g
	}

	// This thread is going to run a goroutine and is not spinning anymore,
	// so if it was marked as spinning we need to reset it now and potentially
	// start a new spinning M.
	// 存在可运行g，当前m不再需要自旋。如果有需求，唤起新的m进行自旋
	if _g_.m.spinning {
		resetspinning()
	}

	if sched.disable.user && !schedEnabled(gp) {
		// Scheduling of this goroutine is disabled. Put it on
		// the list of pending runnable goroutines for when we
		// re-enable user scheduling and look again.
		lock(&sched.lock)
		if schedEnabled(gp) {
			// Something re-enabled scheduling while we
			// were acquiring the lock.
			unlock(&sched.lock)
		} else {
			sched.disable.runnable.pushBack(gp)
			sched.disable.n++
			unlock(&sched.lock)
			goto top
		}
	}

	// 如果当前g存在锁定的m，则唤起。
	// 当前m休眠，等待被唤起（一般来说此时应该没有可用的P）
	// 检测到当前g存在锁定的m，则唤醒锁定的m执行该g
	if gp.lockedm != 0 {
		// Hands off own p to the locked m,
		// then blocks waiting for a new p.
		startlockedm(gp)
		// 开启新的调度循环
		goto top
	}

	// 我们主要讨论了调度系统是如何获取一个可以运行的g的，当取到g后，下一步就是需要去执行它，此函数主要讨论g是如何调度执行的。
	// 执行 goroutine 任务函数
	// 当前运行的是 runtime 的代码，函数调用栈使用的是 g0 的栈空间
	// 调用 execute 切换到 gp 的代码和栈空间去运行
	execute(gp, inheritTime)
}

// dropg removes the association between m and the current goroutine m->curg (gp for short).
// Typically a caller sets gp's status away from Grunning and then
// immediately calls dropg to finish the job. The caller is also responsible
// for arranging that gp will be restarted using ready at an
// appropriate time. After calling dropg and arranging for gp to be
// readied later, the caller can do other work but eventually should
// call schedule to restart the scheduling of goroutines on this m.
func dropg() {
	_g_ := getg()

	setMNoWB(&_g_.m.curg.m, nil)
	setGNoWB(&_g_.m.curg, nil)
}

func parkunlock_c(gp *g, lock unsafe.Pointer) bool {
	unlock((*mutex)(lock))
	return true
}

// park continuation on g0.
func park_m(gp *g) {
	_g_ := getg()

	if trace.enabled {
		traceGoPark(_g_.m.waittraceev, _g_.m.waittraceskip)
	}

	// 修改协程gp状态为挂起
	casgstatus(gp, _Grunning, _Gwaiting)
	// 将g和当前m解绑
	dropg()

	if _g_.m.waitunlockf != nil {
		fn := *(*func(*g, unsafe.Pointer) bool)(unsafe.Pointer(&_g_.m.waitunlockf))
		ok := fn(gp, _g_.m.waitlock)
		_g_.m.waitunlockf = nil
		_g_.m.waitlock = nil
		if !ok {
			if trace.enabled {
				traceGoUnpark(gp, 2)
			}
			casgstatus(gp, _Gwaiting, _Grunnable)
			execute(gp, true) // Schedule it back, never returns.
		}
	}
	// 开始新一轮的调度循环
	schedule()
}

// 经过几次函数调用，最终goschedImpl实现了处理器的主动让出。
// 1. 首先将协程到状态更改为可运行，并将m和g解绑，并将g放入全局队列中。
// 2. 开启一轮新到调度循环
func goschedImpl(gp *g) {
	// 读取协程gp到状态
	status := readgstatus(gp)
	// 处于GC阶段栈扫描阶段到协程不可主动让出处理器
	if status&^_Gscan != _Grunning {
		dumpgstatus(gp)
		throw("bad g status")
	}
	// 切换协程运行状态为可运行状态
	casgstatus(gp, _Grunning, _Grunnable)
	// 将m和g解绑
	dropg()
	lock(&sched.lock)
	// 将g加入全局队列，等待被调度执行
	globrunqput(gp)
	unlock(&sched.lock)

	// 开启新一轮到调度
	schedule()
}

// Gosched continuation on g0.
// go上继续执行Gosched
func gosched_m(gp *g) {
	if trace.enabled {
		traceGoSched()
	}
	goschedImpl(gp)
}

// goschedguarded is a forbidden-states-avoided version of gosched_m
func goschedguarded_m(gp *g) {

	if gp.m.locks != 0 || gp.m.mallocing != 0 || gp.m.preemptoff != "" || gp.m.p.ptr().status != _Prunning {
		gogo(&gp.sched) // never return
	}

	if trace.enabled {
		traceGoSched()
	}
	goschedImpl(gp)
}

// 中间又处理了很多判断流程，再次判断 preempt 标志是 true 时，调用 gopreempt_m(gp) 将 gp 切换出去。
func gopreempt_m(gp *g) {
	if trace.enabled {
		traceGoPreempt()
	}
	// 放弃当前g的执行权，加入全局队列，开始新的调度循环。 具体可参考Gosched函数的解读
	goschedImpl(gp)
}

// Finishes execution of the current goroutine.
// 完成当前goroutine的执行。
func goexit1() {
	if raceenabled {
		racegoend()
	}
	if trace.enabled {
		traceGoEnd()
	}
	// 函数goexit，goexit1的实现都比较简单直接，最终是通过mcall方法实现了goexit0的调用。
	// 在上文说明systemstack和mcall的区别时有提到，mcall函数在执行完成后不会返回原来协程栈继续执行，
	// 这就要求函数goexit0永远不能返回。
	mcall(goexit0)
}

// goexit continuation on g0.
// 函数goexit，goexit1的实现都比较简单直接，最终是通过mcall方法实现了goexit0的调用。
// 在上文说明systemstack和mcall的区别时有提到，mcall函数在执行完成后不会返回原来协程栈继续执行，
// 这就要求函数goexit0永远不能返回。
// 协程的退出工作也不是特别复杂，将g的关键状态复位、解绑g和m，然后将g放入gfree列表中等待再次创建协程时复用。
// 在 g0 上执行
func goexit0(gp *g) {
	// 获取当前协程，m.g0，mcall方法已经完成了栈切换
	_g_ := getg()

	// 切换gp状态为死亡
	casgstatus(gp, _Grunning, _Gdead)
	if isSystemGoroutine(gp, false) {
		atomic.Xadd(&sched.ngsys, -1)
	}
	// 清空 gp 的一些字段
	// g和m解绑
	gp.m = nil
	locked := gp.lockedm != 0
	gp.lockedm = 0
	_g_.m.lockedg = 0
	gp.paniconfault = false
	// defer，panic，timer等关键变量全部置空值
	gp._defer = nil // should be true already but just in case.
	gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
	gp.writebuf = nil
	gp.waitreason = 0
	gp.param = nil
	gp.labels = nil
	gp.timer = nil

	if gcBlackenEnabled != 0 && gp.gcAssistBytes > 0 {
		// Flush assist credit to the global pool. This gives
		// better information to pacing if the application is
		// rapidly creating an exiting goroutines.
		scanCredit := int64(gcController.assistWorkPerByte * float64(gp.gcAssistBytes))
		atomic.Xaddint64(&gcController.bgScanCredit, scanCredit)
		gp.gcAssistBytes = 0
	}

	// Note that gp's stack scan is now "valid" because it has no
	// stack.
	gp.gcscanvalid = true
	// 移除g和m的绑定关系gp.m=nil&m.curg=nil
	// 解除 g 与 m 的关系
	dropg()

	if GOARCH == "wasm" { // no threads yet on wasm
		// 将g放入空闲列表中(gfree)，留待复用
		gfput(_g_.m.p.ptr(), gp)
		schedule() // never returns
	}

	if _g_.m.lockedInt != 0 {
		print("invalid m->lockedInt = ", _g_.m.lockedInt, "\n")
		throw("internal lockOSThread error")
	}
	// 将g放入空闲列表中(gfree)，留待复用
	// 将 g 放入 free 队列缓存起来
	gfput(_g_.m.p.ptr(), gp)
	if locked {
		// The goroutine may have locked this thread because it put it in an unusual kernel state.
		// Kill it rather than returning it to the thread pool.
		// Return to mstart, which will release the P and exit the thread.
		// 这个goroutine可能已经锁定了这个线程，因为它使它处于一个不寻常的内核状态。
		// 杀掉它，而不是把它返回到线程池中。
		// 返回到mstart，它将释放P并退出线程。
		if GOOS != "plan9" { // See golang.org/issue/22227.
			// 如果当前g存在锁定的m，则将对应的m(系统线程)销毁，不再复用
			gogo(&_g_.m.g0.sched)
		} else {
			// Clear lockedExt on plan9 since we may end up re-using
			// this thread.
			_g_.m.lockedExt = 0
		}
	}
	// 继续执行调度循环，永不返回
	schedule()
}

// save updates getg().sched to refer to pc and sp so that a following
// gogo will restore pc and sp.
//
// save must not have write barriers because invoking a write barrier
// can clobber getg().sched.
//
//go:nosplit
//go:nowritebarrierrec
func save(pc, sp uintptr) {
	_g_ := getg()

	_g_.sched.pc = pc
	_g_.sched.sp = sp
	_g_.sched.lr = 0
	_g_.sched.ret = 0
	_g_.sched.g = guintptr(unsafe.Pointer(_g_))
	// We need to ensure ctxt is zero, but can't have a write
	// barrier here. However, it should always already be zero.
	// Assert that.
	if _g_.sched.ctxt != nil {
		badctxt()
	}
}

// The goroutine g is about to enter a system call.
// Record that it's not using the cpu anymore.
// This is called only from the go syscall library and cgocall,
// not from the low-level system calls used by the runtime.
//
// Entersyscall cannot split the stack: the gosave must
// make g->sched refer to the caller's stack segment, because
// entersyscall is going to return immediately after.
//
// Nothing entersyscall calls can split the stack either.
// We cannot safely move the stack during an active call to syscall,
// because we do not know which of the uintptr arguments are
// really pointers (back into the stack).
// In practice, this means that we make the fast path run through
// entersyscall doing no-split things, and the slow path has to use systemstack
// to run bigger things on the system stack.
//
// reentersyscall is the entry point used by cgo callbacks, where explicitly
// saved SP and PC are restored. This is needed when exitsyscall will be called
// from a function further up in the call stack than the parent, as g->syscallsp
// must always point to a valid stack frame. entersyscall below is the normal
// entry point for syscalls, which obtains the SP and PC from the caller.
//
// Syscall tracing:
// At the start of a syscall we emit traceGoSysCall to capture the stack trace.
// If the syscall does not block, that is it, we do not emit any other events.
// If the syscall blocks (that is, P is retaken), retaker emits traceGoSysBlock;
// when syscall returns we emit traceGoSysExit and when the goroutine starts running
// (potentially instantly, if exitsyscallfast returns true) we emit traceGoStart.
// To ensure that traceGoSysExit is emitted strictly after traceGoSysBlock,
// we remember current value of syscalltick in m (_g_.m.syscalltick = _g_.m.p.ptr().syscalltick),
// whoever emits traceGoSysBlock increments p.syscalltick afterwards;
// and we wait for the increment before emitting traceGoSysExit.
// Note that the increment is done even if tracing is not enabled,
// because tracing can be enabled in the middle of syscall. We don't want the wait to hang.
//
//go:nosplit
func reentersyscall(pc, sp uintptr) {
	_g_ := getg()

	// Disable preemption because during this function g is in Gsyscall status,
	// but can have inconsistent g->sched, do not let GC observe it.
	// 禁用抢占
	_g_.m.locks++

	// Entersyscall must not call any function that might split/grow the stack.
	// (See details in comment above.)
	// Catch calls that might, by replacing the stack guard with something that
	// will trip any stack check and leaving a flag to tell newstack to die.
	// entersyscall方法中不允许调用栈分裂的代码
	_g_.stackguard0 = stackPreempt
	_g_.throwsplit = true

	// Leave SP around for GC and traceback.
	// 保存当前协程执行的各种现场：PC、SP等，
	// 系统调用结束后会使用这些数据恢复现场
	save(pc, sp)
	_g_.syscallsp = sp
	_g_.syscallpc = pc
	// 将当前协程状态切换为系统调用态
	casgstatus(_g_, _Grunning, _Gsyscall)
	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
		systemstack(func() {
			print("entersyscall inconsistent ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
			throw("entersyscall")
		})
	}

	if trace.enabled {
		systemstack(traceGoSysCall)
		// systemstack itself clobbers g.sched.{pc,sp} and we might
		// need them later when the G is genuinely blocked in a
		// syscall
		save(pc, sp)
	}

	if atomic.Load(&sched.sysmonwait) != 0 {
		systemstack(entersyscall_sysmon)
		save(pc, sp)
	}

	if _g_.m.p.ptr().runSafePointFn != 0 {
		// runSafePointFn may stack split if run on this stack
		systemstack(runSafePointFn)
		save(pc, sp)
	}

	// 将m和p的关联剥离，并将p记录在m的oldp字段上
	_g_.m.syscalltick = _g_.m.p.ptr().syscalltick
	_g_.sysblocktraced = true
	_g_.m.mcache = nil
	pp := _g_.m.p.ptr()
	pp.m = 0
	_g_.m.oldp.set(pp)
	_g_.m.p = 0
	// 更新p的状态为系统调用态
	atomic.Store(&pp.status, _Psyscall)
	if sched.gcwaiting != 0 {
		systemstack(entersyscall_gcwait)
		save(pc, sp)
	}

	// 恢复抢占功能
	_g_.m.locks--
}

// Standard syscall entry used by the go syscall library and normal cgo calls.
//go:nosplit

// 在正常的系统调用外，Go运行时通过runtime·entersyscall、runtime·exitsyscall这一对方法引入了协程调度的相关内容:

// 系统调用前的准备
func entersyscall() {
	reentersyscall(getcallerpc(), getcallersp())
}

func entersyscall_sysmon() {
	lock(&sched.lock)
	if atomic.Load(&sched.sysmonwait) != 0 {
		atomic.Store(&sched.sysmonwait, 0)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
}

func entersyscall_gcwait() {
	_g_ := getg()
	_p_ := _g_.m.oldp.ptr()

	lock(&sched.lock)
	if sched.stopwait > 0 && atomic.Cas(&_p_.status, _Psyscall, _Pgcstop) {
		if trace.enabled {
			traceGoSysBlock(_p_)
			traceProcStop(_p_)
		}
		_p_.syscalltick++
		if sched.stopwait--; sched.stopwait == 0 {
			notewakeup(&sched.stopnote)
		}
	}
	unlock(&sched.lock)
}

// The same as entersyscall(), but with a hint that the syscall is blocking.
//go:nosplit
func entersyscallblock() {
	_g_ := getg()

	_g_.m.locks++ // see comment in entersyscall
	_g_.throwsplit = true
	_g_.stackguard0 = stackPreempt // see comment in entersyscall
	_g_.m.syscalltick = _g_.m.p.ptr().syscalltick
	_g_.sysblocktraced = true
	_g_.m.p.ptr().syscalltick++

	// Leave SP around for GC and traceback.
	pc := getcallerpc()
	sp := getcallersp()
	save(pc, sp)
	_g_.syscallsp = _g_.sched.sp
	_g_.syscallpc = _g_.sched.pc
	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
		sp1 := sp
		sp2 := _g_.sched.sp
		sp3 := _g_.syscallsp
		systemstack(func() {
			print("entersyscallblock inconsistent ", hex(sp1), " ", hex(sp2), " ", hex(sp3), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
			throw("entersyscallblock")
		})
	}
	casgstatus(_g_, _Grunning, _Gsyscall)
	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
		systemstack(func() {
			print("entersyscallblock inconsistent ", hex(sp), " ", hex(_g_.sched.sp), " ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
			throw("entersyscallblock")
		})
	}

	systemstack(entersyscallblock_handoff)

	// Resave for traceback during blocked call.
	save(getcallerpc(), getcallersp())

	_g_.m.locks--
}

func entersyscallblock_handoff() {
	if trace.enabled {
		traceGoSysCall()
		traceGoSysBlock(getg().m.p.ptr())
	}
	handoffp(releasep())
}

// The goroutine g exited its system call.
// Arrange for it to run on a cpu again.
// This is called only from the go syscall library, not
// from the low-level system calls used by the runtime.
//
// Write barriers are not allowed because our P may have been stolen.
//
//go:nosplit
//go:nowritebarrierrec
// 系统调用后的收尾
func exitsyscall() {
	_g_ := getg()

	// 禁用抢占
	_g_.m.locks++ // see comment in entersyscall
	if getcallersp() > _g_.syscallsp {
		throw("exitsyscall: syscall frame is no longer valid")
	}

	_g_.waitsince = 0
	oldp := _g_.m.oldp.ptr()
	_g_.m.oldp = 0
	// 检查如果存在空闲的p，那么我们可以直接恢复运行
	if exitsyscallfast(oldp) {
		if _g_.m.mcache == nil {
			throw("lost mcache")
		}
		if trace.enabled {
			if oldp != _g_.m.p.ptr() || _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
				systemstack(traceGoStart)
			}
		}
		// There's a cpu for us, so we can run.
		_g_.m.p.ptr().syscalltick++
		// We need to cas the status and scan before resuming...
		// 协程g的状态切换为运行中
		casgstatus(_g_, _Gsyscall, _Grunning)

		// Garbage collector isn't running (since we are),
		// so okay to clear syscallsp.
		_g_.syscallsp = 0
		_g_.m.locks--
		if _g_.preempt {
			// restore the preemption request in case we've cleared it in newstack
			_g_.stackguard0 = stackPreempt
		} else {
			// otherwise restore the real _StackGuard, we've spoiled it in entersyscall/entersyscallblock
			_g_.stackguard0 = _g_.stack.lo + _StackGuard
		}
		_g_.throwsplit = false

		if sched.disable.user && !schedEnabled(_g_) {
			// Scheduling of this goroutine is disabled.
			Gosched()
		}

		return
	}

	_g_.sysexitticks = 0
	if trace.enabled {
		// Wait till traceGoSysBlock event is emitted.
		// This ensures consistency of the trace (the goroutine is started after it is blocked).
		for oldp != nil && oldp.syscalltick == _g_.m.syscalltick {
			osyield()
		}
		// We can't trace syscall exit right now because we don't have a P.
		// Tracing code can invoke write barriers that cannot run without a P.
		// So instead we remember the syscall exit time and emit the event
		// in execute when we have a P.
		_g_.sysexitticks = cputicks()
	}

	_g_.m.locks--

	// Call the scheduler.
	// 系统调用结束后不存在空闲的p，等待被调度
	mcall(exitsyscall0)

	if _g_.m.mcache == nil {
		throw("lost mcache")
	}

	// Scheduler returned, so we're allowed to run now.
	// Delete the syscallsp information that we left for
	// the garbage collector during the system call.
	// Must wait until now because until gosched returns
	// we don't know for sure that the garbage collector
	// is not running.
	_g_.syscallsp = 0
	_g_.m.p.ptr().syscalltick++
	_g_.throwsplit = false
}

//go:nosplit
// 获取可用P，恢复运行
func exitsyscallfast(oldp *p) bool {
	_g_ := getg()

	// Freezetheworld sets stopwait but does not retake P's.
	if sched.stopwait == freezeStopWait {
		return false
	}

	// Try to re-acquire the last P.
	// 检查系统调用前的P是否可用，优先使用
	if oldp != nil && oldp.status == _Psyscall && atomic.Cas(&oldp.status, _Psyscall, _Pidle) {
		// There's a cpu for us, so we can run.
		wirep(oldp)
		exitsyscallfast_reacquired()
		return true
	}

	// Try to get any other idle P.
	// 尝试获取其他空闲的P
	if sched.pidle != 0 {
		var ok bool
		systemstack(func() {
			ok = exitsyscallfast_pidle()
			if ok && trace.enabled {
				if oldp != nil {
					// Wait till traceGoSysBlock event is emitted.
					// This ensures consistency of the trace (the goroutine is started after it is blocked).
					for oldp.syscalltick == _g_.m.syscalltick {
						osyield()
					}
				}
				traceGoSysExit(0)
			}
		})
		if ok {
			return true
		}
	}
	return false
}

// exitsyscallfast_reacquired is the exitsyscall path on which this G
// has successfully reacquired the P it was running on before the
// syscall.
//
//go:nosplit
func exitsyscallfast_reacquired() {
	_g_ := getg()
	if _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
		if trace.enabled {
			// The p was retaken and then enter into syscall again (since _g_.m.syscalltick has changed).
			// traceGoSysBlock for this syscall was already emitted,
			// but here we effectively retake the p from the new syscall running on the same p.
			systemstack(func() {
				// Denote blocking of the new syscall.
				traceGoSysBlock(_g_.m.p.ptr())
				// Denote completion of the current syscall.
				traceGoSysExit(0)
			})
		}
		_g_.m.p.ptr().syscalltick++
	}
}

func exitsyscallfast_pidle() bool {
	lock(&sched.lock)
	_p_ := pidleget()
	if _p_ != nil && atomic.Load(&sched.sysmonwait) != 0 {
		atomic.Store(&sched.sysmonwait, 0)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
	if _p_ != nil {
		acquirep(_p_)
		return true
	}
	return false
}

// exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
//
//go:nowritebarrierrec
// 系统调用结束后，无空闲p，则将协程加入调度队列

/**
经过Go包装的系统调用加入了影响协程调度的流程:

1. 系统调用前（entersyscall）

	a. 保存协程g的运行现场，主要是更新`g.sched`的各值。将g的状态更新为`_Gsyscall`
	b. 将m与p解绑，设置p的状态为`_Psyscall`

2. 系统调用后（exitsyscall）

	a. 检查系统调用前绑定的p是否空闲，如果可用，则直接修改状态并和m进行绑定，然后继续运行用户代码
	b. 检查系统当前是否存在空闲的p，如果存在，则直接绑定，然后继续运行用户代码
	c. 修改协程状态为`_Grunnable`,然后将m和g解除绑定
	d. 再次尝试获取p，如果可以取到，则执行绑定p和m并运行用户代码
	e. 将g放入全局队列，等待调度
	d. 当前m休眠，等待被唤醒，然后执行新一轮的调度循环
 */
func exitsyscall0(gp *g) {
	_g_ := getg()

	// 修改协程状态为可运行，与当前M解绑定
	casgstatus(gp, _Gsyscall, _Grunnable)
	dropg()
	lock(&sched.lock)
	var _p_ *p
	if schedEnabled(_g_) {
		// 再次尝试获取P
		_p_ = pidleget()
	}
	if _p_ == nil {
		// 将g放入全局队列中
		globrunqput(gp)
	} else if atomic.Load(&sched.sysmonwait) != 0 {
		atomic.Store(&sched.sysmonwait, 0)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
	// 直接运行
	if _p_ != nil {
		acquirep(_p_)
		execute(gp, false) // Never returns.
	}
	// 如果m和特定g锁定，执行流程参考`章节6.1:G和M锁定`
	if _g_.m.lockedg != 0 {
		// Wait until another thread schedules gp and so m again.
		stoplockedm()
		execute(gp, false) // Never returns.
	}
	// 当前m休眠，等待被唤醒
	stopm()
	// m一旦被唤醒，则立即执行新一轮调度循环
	schedule() // Never returns.
}

func beforefork() {
	gp := getg().m.curg

	// Block signals during a fork, so that the child does not run
	// a signal handler before exec if a signal is sent to the process
	// group. See issue #18600.
	gp.m.locks++
	msigsave(gp.m)
	sigblock()

	// This function is called before fork in syscall package.
	// Code between fork and exec must not allocate memory nor even try to grow stack.
	// Here we spoil g->_StackGuard to reliably detect any attempts to grow stack.
	// runtime_AfterFork will undo this in parent process, but not in child.
	gp.stackguard0 = stackFork
}

// Called from syscall package before fork.
//go:linkname syscall_runtime_BeforeFork syscall.runtime_BeforeFork
//go:nosplit
func syscall_runtime_BeforeFork() {
	systemstack(beforefork)
}

func afterfork() {
	gp := getg().m.curg

	// See the comments in beforefork.
	gp.stackguard0 = gp.stack.lo + _StackGuard

	msigrestore(gp.m.sigmask)

	gp.m.locks--
}

// Called from syscall package after fork in parent.
//go:linkname syscall_runtime_AfterFork syscall.runtime_AfterFork
//go:nosplit
func syscall_runtime_AfterFork() {
	systemstack(afterfork)
}

// inForkedChild is true while manipulating signals in the child process.
// This is used to avoid calling libc functions in case we are using vfork.
var inForkedChild bool

// Called from syscall package after fork in child.
// It resets non-sigignored signals to the default handler, and
// restores the signal mask in preparation for the exec.
//
// Because this might be called during a vfork, and therefore may be
// temporarily sharing address space with the parent process, this must
// not change any global variables or calling into C code that may do so.
//
//go:linkname syscall_runtime_AfterForkInChild syscall.runtime_AfterForkInChild
//go:nosplit
//go:nowritebarrierrec
func syscall_runtime_AfterForkInChild() {
	// It's OK to change the global variable inForkedChild here
	// because we are going to change it back. There is no race here,
	// because if we are sharing address space with the parent process,
	// then the parent process can not be running concurrently.
	inForkedChild = true

	clearSignalHandlers()

	// When we are the child we are the only thread running,
	// so we know that nothing else has changed gp.m.sigmask.
	msigrestore(getg().m.sigmask)

	inForkedChild = false
}

// Called from syscall package before Exec.
//go:linkname syscall_runtime_BeforeExec syscall.runtime_BeforeExec
func syscall_runtime_BeforeExec() {
	// Prevent thread creation during exec.
	execLock.lock()
}

// Called from syscall package after Exec.
//go:linkname syscall_runtime_AfterExec syscall.runtime_AfterExec
func syscall_runtime_AfterExec() {
	execLock.unlock()
}

// Allocate a new g, with a stack big enough for stacksize bytes.
func malg(stacksize int32) *g {
	newg := new(g)
	if stacksize >= 0 {
		stacksize = round2(_StackSystem + stacksize)
		systemstack(func() {
			newg.stack = stackalloc(uint32(stacksize))
		})
		newg.stackguard0 = newg.stack.lo + _StackGuard
		newg.stackguard1 = ^uintptr(0)
	}
	return newg
}

// Create a new g running fn with siz bytes of arguments.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
// Cannot split the stack because it assumes that the arguments
// are available sequentially after &fn; they would not be
// copied if a stack split occurred.
//go:nosplit
// 通过汇编我们可以看到Go在编译过程中将go关键字转换成了runtime.newproc方法，该函数原型：
// runtime.newproc的入参是协程函数参数大小和函数的指针，参数在runtime.newproc中又是如何接收的呢，我们看源码
func newproc(siz int32, fn *funcval) {
	// 函数fn需要的参数的位置
	argp := add(unsafe.Pointer(&fn), sys.PtrSize)
	// 获取当前g
	gp := getg()
	// 获取调用者PC
	pc := getcallerpc()
	// 创建新协程
	// 大家可能注意到，runtime.newproc并没有直接调用runtime.newproc1,而是通过函数systemstack进行了间接调用,
	// 函数systemstack的功能是将函数调用转换到g0栈上进行。类似功能的方法还有一个mcall，
	// 它们两个最显著的区别就是systemstack在g0栈上完成函数调用后还会回到原协程栈上继续执行后续的代码，
	// 而mcall不会返回原调用栈，而是继续在g0栈上执行协程调度。
	systemstack(func() {
		newproc1(fn, (*uint8)(argp), siz, gp, pc)
	})
}

// Create a new g running fn with narg bytes of arguments starting
// at argp. callerpc is the address of the go statement that created
// this. The new g is put on the queue of g's waiting to run.
// 创建一个新的g运行fn，参数为narg字节，从argp开始。
// callerpc是创建这个的go语句的地址。新的g被放在等待运行的g的队列中。

// 如果是程序刚启动，经由runtime.rt0_go调用newproc1时，实质干的事情就是创建一个G，
// 把runtime.main(也包含main.main)放进去。在执行mstart时，触发调度。
// 所以main实际是在一个新的G里运行的，而不是g0。

// 1. 复用或者新建一个G
// 2. 如果G运行的函数包含参数，则进行参数拷贝
// 3. g.sched核心字段赋值
// 4. 模拟被goexit调用
// 5. 将新创建的G放入P本地运行队列中
// 6. 检测如果当前包含空闲的P，则唤醒
func newproc1(fn *funcval, argp *uint8, narg int32, callergp *g, callerpc uintptr) {
	// 获取当前G(g0)，goroutine 的指针
	// 因为已经切换到 g0 栈，所以无论什么场景都是 _g_ = g0
	// g0 是指当前工作线程的 g0
	_g_ := getg()

	if fn == nil {
		_g_.m.throwing = -1 // do not dump full stacks
		throw("go of nil func value")
	}
	// 加锁，禁用抢占
	_g_.m.locks++ // disable preemption because it can be holding p in a local var
	// 参数加返回值所需要的空间（经过内存对齐）
	siz := narg
	siz = (siz + 7) &^ 7

	// We could allocate a larger initial stack if necessary.
	// Not worth it: this is almost always an error.
	// 4*sizeof(uintreg): extra space added below
	// sizeof(uintreg): caller's LR (arm) or return address (x86, in gostartcall).
	if siz >= _StackMin-4*sys.RegSize-sys.RegSize {
		throw("newproc: function arguments too large for new goroutine")
	}

	// 空闲列表中获取一个待复用G
	// 当前工作线程所绑定的 p
	// 初始化时 _p_ = g0.m.p，也就是 _p_ = allp[0]
	_p_ := _g_.m.p.ptr()
	// 从 p 的本地缓冲里获取一个没有使用的 g，初始化时为空，返回 nil
	newg := gfget(_p_)
	if newg == nil {
		// 创建新的G
		// new 一个 g 结构体对象，然后从堆上为其分配栈，并设置 g 的 stack 成员和两个 stackgard 成员
		newg = malg(_StackMin)
		// 初始化 g 的状态为 _Gdead
		casgstatus(newg, _Gidle, _Gdead)
		// 放入全局变量 allgs 切片中
		allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
	}
	if newg.stack.hi == 0 {
		throw("newproc1: newg missing stack")
	}

	if readgstatus(newg) != _Gdead {
		throw("newproc1: new g is not Gdead")
	}

	// 计算运行空间大小，对齐
	totalSize := 4*sys.RegSize + uintptr(siz) + sys.MinFrameSize // extra space in case of reads slightly beyond frame
	totalSize += -totalSize & (sys.SpAlign - 1)                  // align to spAlign
	// 计算首个函数栈帧，确定 sp 位置
	sp := newg.stack.hi - totalSize
	// 确定参数入栈位置
	spArg := sp
	if usesLR {
		// caller's LR
		*(*uintptr)(unsafe.Pointer(sp)) = 0
		prepGoExitFrame(sp)
		spArg += sys.MinFrameSize
	}
	if narg > 0 {
		// 拷贝参数，将参数从执行 newproc 函数的栈拷贝到新 g 的栈
		memmove(unsafe.Pointer(spArg), unsafe.Pointer(argp), uintptr(narg))
		// This is a stack-to-stack copy. If write barriers
		// are enabled and the source stack is grey (the
		// destination is always black), then perform a
		// barrier copy. We do this *after* the memmove
		// because the destination stack may have garbage on
		// it.
		if writeBarrier.needed && !_g_.m.curg.gcscandone {
			f := findfunc(fn.fn)
			stkmap := (*stackmap)(funcdata(f, _FUNCDATA_ArgsPointerMaps))
			if stkmap.nbit > 0 {
				// We're in the prologue, so it's always stack map index 0.
				bv := stackmapdata(stkmap, 0)
				bulkBarrierBitmap(spArg, spArg, uintptr(bv.n)*sys.PtrSize, 0, bv.bytedata)
			}
		}
	}

	// 清空g.sched的内存，后各关键参数赋值
	// 把 newg.sched 结构体成员的所有成员设置为 0
	memclrNoHeapPointers(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
	// 设置 newg 的 sched 成员，调度器需要依靠这些字段才能把 goroutine 调度到 CPU 上运行
	newg.sched.sp = sp
	newg.stktopsp = sp
	// newg.sched.pc 表示当 newg 被调度起来运行时从这个地址开始执行指令
	newg.sched.pc = funcPC(goexit) + sys.PCQuantum // +PCQuantum so that previous instruction is in same function
	newg.sched.g = guintptr(unsafe.Pointer(newg))
	// 重置协程的调用栈帧，模拟`goexit`调用本方法
	// 调整sched成员和newg的栈
	gostartcallfn(&newg.sched, fn)
	newg.gopc = callerpc
	newg.ancestors = saveAncestors(callergp)
	// 设置 newg 的 startpc 为 fn.fn，该成员主要用于函数调用栈的 traceback 和栈收缩
	// newg 真正从哪里开始执行并不依赖于这个成员，而是 sched.pc
	newg.startpc = fn.fn
	if _g_.m.curg != nil {
		newg.labels = _g_.m.curg.labels
	}
	if isSystemGoroutine(newg, false) {
		atomic.Xadd(&sched.ngsys, +1)
	}
	newg.gcscanvalid = false
	// 设置 g 的状态为 _Grunnable，可以运行了
	casgstatus(newg, _Gdead, _Grunnable)

	if _p_.goidcache == _p_.goidcacheend {
		// Sched.goidgen is the last allocated id,
		// this batch must be [sched.goidgen+1, sched.goidgen+GoidCacheBatch].
		// At startup sched.goidgen=0, so main goroutine receives goid=1.
		_p_.goidcache = atomic.Xadd64(&sched.goidgen, _GoidCacheBatch)
		_p_.goidcache -= _GoidCacheBatch - 1
		_p_.goidcacheend = _p_.goidcache + _GoidCacheBatch
	}
	// 设置 goid
	newg.goid = int64(_p_.goidcache)
	_p_.goidcache++
	if raceenabled {
		newg.racectx = racegostart(callerpc)
	}
	if trace.enabled {
		traceGoCreate(newg, newg.startpc)
	}
	// 将新创建的G放入P本地的可运行队列中
	runqput(_p_, newg, true)

	// 如果当前存在空闲的P，则唤醒P
	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 && mainStarted {
		wakep()
	}

	// 释放锁
	_g_.m.locks--
	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
		_g_.stackguard0 = stackPreempt
	}
}

// saveAncestors copies previous ancestors of the given caller g and
// includes infor for the current caller into a new set of tracebacks for
// a g being created.
func saveAncestors(callergp *g) *[]ancestorInfo {
	// Copy all prior info, except for the root goroutine (goid 0).
	if debug.tracebackancestors <= 0 || callergp.goid == 0 {
		return nil
	}
	var callerAncestors []ancestorInfo
	if callergp.ancestors != nil {
		callerAncestors = *callergp.ancestors
	}
	n := int32(len(callerAncestors)) + 1
	if n > debug.tracebackancestors {
		n = debug.tracebackancestors
	}
	ancestors := make([]ancestorInfo, n)
	copy(ancestors[1:], callerAncestors)

	var pcs [_TracebackMaxFrames]uintptr
	npcs := gcallers(callergp, 0, pcs[:])
	ipcs := make([]uintptr, npcs)
	copy(ipcs, pcs[:])
	ancestors[0] = ancestorInfo{
		pcs:  ipcs,
		goid: callergp.goid,
		gopc: callergp.gopc,
	}

	ancestorsp := new([]ancestorInfo)
	*ancestorsp = ancestors
	return ancestorsp
}

// Put on gfree list.
// If local list is too long, transfer a batch to the global list.
func gfput(_p_ *p, gp *g) {
	if readgstatus(gp) != _Gdead {
		throw("gfput: bad status (not Gdead)")
	}

	stksize := gp.stack.hi - gp.stack.lo

	if stksize != _FixedStack {
		// non-standard stack size - free it.
		stackfree(gp.stack)
		gp.stack.lo = 0
		gp.stack.hi = 0
		gp.stackguard0 = 0
	}

	_p_.gFree.push(gp)
	_p_.gFree.n++
	if _p_.gFree.n >= 64 {
		lock(&sched.gFree.lock)
		for _p_.gFree.n >= 32 {
			_p_.gFree.n--
			gp = _p_.gFree.pop()
			if gp.stack.lo == 0 {
				sched.gFree.noStack.push(gp)
			} else {
				sched.gFree.stack.push(gp)
			}
			sched.gFree.n++
		}
		unlock(&sched.gFree.lock)
	}
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
func gfget(_p_ *p) *g {
retry:
	if _p_.gFree.empty() && (!sched.gFree.stack.empty() || !sched.gFree.noStack.empty()) {
		lock(&sched.gFree.lock)
		// Move a batch of free Gs to the P.
		for _p_.gFree.n < 32 {
			// Prefer Gs with stacks.
			gp := sched.gFree.stack.pop()
			if gp == nil {
				gp = sched.gFree.noStack.pop()
				if gp == nil {
					break
				}
			}
			sched.gFree.n--
			_p_.gFree.push(gp)
			_p_.gFree.n++
		}
		unlock(&sched.gFree.lock)
		goto retry
	}
	gp := _p_.gFree.pop()
	if gp == nil {
		return nil
	}
	_p_.gFree.n--
	if gp.stack.lo == 0 {
		// Stack was deallocated in gfput. Allocate a new one.
		systemstack(func() {
			gp.stack = stackalloc(_FixedStack)
		})
		gp.stackguard0 = gp.stack.lo + _StackGuard
	} else {
		if raceenabled {
			racemalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
		}
		if msanenabled {
			msanmalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
		}
	}
	return gp
}

// Purge all cached G's from gfree list to the global list.
func gfpurge(_p_ *p) {
	lock(&sched.gFree.lock)
	for !_p_.gFree.empty() {
		gp := _p_.gFree.pop()
		_p_.gFree.n--
		if gp.stack.lo == 0 {
			sched.gFree.noStack.push(gp)
		} else {
			sched.gFree.stack.push(gp)
		}
		sched.gFree.n++
	}
	unlock(&sched.gFree.lock)
}

// Breakpoint executes a breakpoint trap.
func Breakpoint() {
	breakpoint()
}

// dolockOSThread is called by LockOSThread and lockOSThread below
// after they modify m.locked. Do not allow preemption during this call,
// or else the m might be different in this function than in the caller.
//go:nosplit
// LockOSThread最终会分别设置m.lockedg和g.lockedm，这标识着g和m锁定。接下来我们看下它们是如何影响调度的。
func dolockOSThread() {
	if GOARCH == "wasm" {
		return // no threads on wasm yet
	}
	_g_ := getg()
	// 设置m.lockedg
	_g_.m.lockedg.set(_g_)
	// 设置g.lockedm
	_g_.lockedm.set(_g_.m)
}

//go:nosplit

// LockOSThread wires the calling goroutine to its current operating system thread.
// The calling goroutine will always execute in that thread,
// and no other goroutine will execute in it,
// until the calling goroutine has made as many calls to
// UnlockOSThread as to LockOSThread.
// If the calling goroutine exits without unlocking the thread,
// the thread will be terminated.
//
// All init functions are run on the startup thread. Calling LockOSThread
// from an init function will cause the main function to be invoked on
// that thread.
//
// A goroutine should call LockOSThread before calling OS services or
// non-Go library functions that depend on per-thread state.
// 协程调度是Go语言可以高效执行的基石，贯穿于程序的整个生命周期。接下来我们简单的看下可以触发调度的功能点。
// 在main协程的功能介绍中我们提到了两个方法:runtime.LockOSThread和runtime.UnlockOSThread，
// 这一对方法提供了我们将协程和线程绑定执行的能力
func LockOSThread() {
	if atomic.Load(&newmHandoff.haveTemplateThread) == 0 && GOOS != "plan9" {
		// If we need to start a new thread from the locked
		// thread, we need the template thread. Start it now
		// while we're in a known-good state.
		startTemplateThread()
	}
	// 状态检测，不可重复锁定
	_g_ := getg()
	_g_.m.lockedExt++
	if _g_.m.lockedExt == 0 {
		_g_.m.lockedExt--
		panic("LockOSThread nesting overflow")
	}
	dolockOSThread()
}

//go:nosplit
func lockOSThread() {
	getg().m.lockedInt++
	dolockOSThread()
}

// dounlockOSThread is called by UnlockOSThread and unlockOSThread below
// after they update m->locked. Do not allow preemption during this call,
// or else the m might be in different in this function than in the caller.
//go:nosplit
// 如果我们的协程有依赖线程状态的场景，那么可以使用上述两个方法完成协程和线程的绑定/解绑。
// 一般情况下我们很少会遇到这类场景。
func dounlockOSThread() {
	if GOARCH == "wasm" {
		return // no threads on wasm yet
	}
	_g_ := getg()
	if _g_.m.lockedInt != 0 || _g_.m.lockedExt != 0 {
		return
	}
	_g_.m.lockedg = 0
	_g_.lockedm = 0
}

//go:nosplit

// UnlockOSThread undoes an earlier call to LockOSThread.
// If this drops the number of active LockOSThread calls on the
// calling goroutine to zero, it unwires the calling goroutine from
// its fixed operating system thread.
// If there are no active LockOSThread calls, this is a no-op.
//
// Before calling UnlockOSThread, the caller must ensure that the OS
// thread is suitable for running other goroutines. If the caller made
// any permanent changes to the state of the thread that would affect
// other goroutines, it should not call this function and thus leave
// the goroutine locked to the OS thread until the goroutine (and
// hence the thread) exits.
// 解锁的实现比较简单，只需要将g.lockedm和m.lockedg的值置为空即可
func UnlockOSThread() {
	_g_ := getg()
	if _g_.m.lockedExt == 0 {
		return
	}
	_g_.m.lockedExt--
	dounlockOSThread()
}

//go:nosplit
func unlockOSThread() {
	_g_ := getg()
	if _g_.m.lockedInt == 0 {
		systemstack(badunlockosthread)
	}
	_g_.m.lockedInt--
	dounlockOSThread()
}

func badunlockosthread() {
	throw("runtime: internal error: misuse of lockOSThread/unlockOSThread")
}

func gcount() int32 {
	n := int32(allglen) - sched.gFree.n - int32(atomic.Load(&sched.ngsys))
	for _, _p_ := range allp {
		n -= _p_.gFree.n
	}

	// All these variables can be changed concurrently, so the result can be inconsistent.
	// But at least the current goroutine is running.
	if n < 1 {
		n = 1
	}
	return n
}

func mcount() int32 {
	return int32(sched.mnext - sched.nmfreed)
}

var prof struct {
	signalLock uint32
	hz         int32
}

func _System()                    { _System() }
func _ExternalCode()              { _ExternalCode() }
func _LostExternalCode()          { _LostExternalCode() }
func _GC()                        { _GC() }
func _LostSIGPROFDuringAtomic64() { _LostSIGPROFDuringAtomic64() }
func _VDSO()                      { _VDSO() }

// Counts SIGPROFs received while in atomic64 critical section, on mips{,le}
var lostAtomic64Count uint64

// Called if we receive a SIGPROF signal.
// Called by the signal handler, may run during STW.
//go:nowritebarrierrec
func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
	if prof.hz == 0 {
		return
	}

	// On mips{,le}, 64bit atomics are emulated with spinlocks, in
	// runtime/internal/atomic. If SIGPROF arrives while the program is inside
	// the critical section, it creates a deadlock (when writing the sample).
	// As a workaround, create a counter of SIGPROFs while in critical section
	// to store the count, and pass it to sigprof.add() later when SIGPROF is
	// received from somewhere else (with _LostSIGPROFDuringAtomic64 as pc).
	if GOARCH == "mips" || GOARCH == "mipsle" || GOARCH == "arm" {
		if f := findfunc(pc); f.valid() {
			if hasPrefix(funcname(f), "runtime/internal/atomic") {
				lostAtomic64Count++
				return
			}
		}
	}

	// Profiling runs concurrently with GC, so it must not allocate.
	// Set a trap in case the code does allocate.
	// Note that on windows, one thread takes profiles of all the
	// other threads, so mp is usually not getg().m.
	// In fact mp may not even be stopped.
	// See golang.org/issue/17165.
	getg().m.mallocing++

	// Define that a "user g" is a user-created goroutine, and a "system g"
	// is one that is m->g0 or m->gsignal.
	//
	// We might be interrupted for profiling halfway through a
	// goroutine switch. The switch involves updating three (or four) values:
	// g, PC, SP, and (on arm) LR. The PC must be the last to be updated,
	// because once it gets updated the new g is running.
	//
	// When switching from a user g to a system g, LR is not considered live,
	// so the update only affects g, SP, and PC. Since PC must be last, there
	// the possible partial transitions in ordinary execution are (1) g alone is updated,
	// (2) both g and SP are updated, and (3) SP alone is updated.
	// If SP or g alone is updated, we can detect the partial transition by checking
	// whether the SP is within g's stack bounds. (We could also require that SP
	// be changed only after g, but the stack bounds check is needed by other
	// cases, so there is no need to impose an additional requirement.)
	//
	// There is one exceptional transition to a system g, not in ordinary execution.
	// When a signal arrives, the operating system starts the signal handler running
	// with an updated PC and SP. The g is updated last, at the beginning of the
	// handler. There are two reasons this is okay. First, until g is updated the
	// g and SP do not match, so the stack bounds check detects the partial transition.
	// Second, signal handlers currently run with signals disabled, so a profiling
	// signal cannot arrive during the handler.
	//
	// When switching from a system g to a user g, there are three possibilities.
	//
	// First, it may be that the g switch has no PC update, because the SP
	// either corresponds to a user g throughout (as in asmcgocall)
	// or because it has been arranged to look like a user g frame
	// (as in cgocallback_gofunc). In this case, since the entire
	// transition is a g+SP update, a partial transition updating just one of
	// those will be detected by the stack bounds check.
	//
	// Second, when returning from a signal handler, the PC and SP updates
	// are performed by the operating system in an atomic update, so the g
	// update must be done before them. The stack bounds check detects
	// the partial transition here, and (again) signal handlers run with signals
	// disabled, so a profiling signal cannot arrive then anyway.
	//
	// Third, the common case: it may be that the switch updates g, SP, and PC
	// separately. If the PC is within any of the functions that does this,
	// we don't ask for a traceback. C.F. the function setsSP for more about this.
	//
	// There is another apparently viable approach, recorded here in case
	// the "PC within setsSP function" check turns out not to be usable.
	// It would be possible to delay the update of either g or SP until immediately
	// before the PC update instruction. Then, because of the stack bounds check,
	// the only problematic interrupt point is just before that PC update instruction,
	// and the sigprof handler can detect that instruction and simulate stepping past
	// it in order to reach a consistent state. On ARM, the update of g must be made
	// in two places (in R10 and also in a TLS slot), so the delayed update would
	// need to be the SP update. The sigprof handler must read the instruction at
	// the current PC and if it was the known instruction (for example, JMP BX or
	// MOV R2, PC), use that other register in place of the PC value.
	// The biggest drawback to this solution is that it requires that we can tell
	// whether it's safe to read from the memory pointed at by PC.
	// In a correct program, we can test PC == nil and otherwise read,
	// but if a profiling signal happens at the instant that a program executes
	// a bad jump (before the program manages to handle the resulting fault)
	// the profiling handler could fault trying to read nonexistent memory.
	//
	// To recap, there are no constraints on the assembly being used for the
	// transition. We simply require that g and SP match and that the PC is not
	// in gogo.
	traceback := true
	if gp == nil || sp < gp.stack.lo || gp.stack.hi < sp || setsSP(pc) || (mp != nil && mp.vdsoSP != 0) {
		traceback = false
	}
	var stk [maxCPUProfStack]uintptr
	n := 0
	if mp.ncgo > 0 && mp.curg != nil && mp.curg.syscallpc != 0 && mp.curg.syscallsp != 0 {
		cgoOff := 0
		// Check cgoCallersUse to make sure that we are not
		// interrupting other code that is fiddling with
		// cgoCallers.  We are running in a signal handler
		// with all signals blocked, so we don't have to worry
		// about any other code interrupting us.
		if atomic.Load(&mp.cgoCallersUse) == 0 && mp.cgoCallers != nil && mp.cgoCallers[0] != 0 {
			for cgoOff < len(mp.cgoCallers) && mp.cgoCallers[cgoOff] != 0 {
				cgoOff++
			}
			copy(stk[:], mp.cgoCallers[:cgoOff])
			mp.cgoCallers[0] = 0
		}

		// Collect Go stack that leads to the cgo call.
		n = gentraceback(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, 0, &stk[cgoOff], len(stk)-cgoOff, nil, nil, 0)
		if n > 0 {
			n += cgoOff
		}
	} else if traceback {
		n = gentraceback(pc, sp, lr, gp, 0, &stk[0], len(stk), nil, nil, _TraceTrap|_TraceJumpStack)
	}

	if n <= 0 {
		// Normal traceback is impossible or has failed.
		// See if it falls into several common cases.
		n = 0
		if (GOOS == "windows" || GOOS == "solaris" || GOOS == "darwin") && mp.libcallg != 0 && mp.libcallpc != 0 && mp.libcallsp != 0 {
			// Libcall, i.e. runtime syscall on windows.
			// Collect Go stack that leads to the call.
			n = gentraceback(mp.libcallpc, mp.libcallsp, 0, mp.libcallg.ptr(), 0, &stk[0], len(stk), nil, nil, 0)
		}
		if n == 0 && mp != nil && mp.vdsoSP != 0 {
			n = gentraceback(mp.vdsoPC, mp.vdsoSP, 0, gp, 0, &stk[0], len(stk), nil, nil, _TraceTrap|_TraceJumpStack)
		}
		if n == 0 {
			// If all of the above has failed, account it against abstract "System" or "GC".
			n = 2
			if inVDSOPage(pc) {
				pc = funcPC(_VDSO) + sys.PCQuantum
			} else if pc > firstmoduledata.etext {
				// "ExternalCode" is better than "etext".
				pc = funcPC(_ExternalCode) + sys.PCQuantum
			}
			stk[0] = pc
			if mp.preemptoff != "" {
				stk[1] = funcPC(_GC) + sys.PCQuantum
			} else {
				stk[1] = funcPC(_System) + sys.PCQuantum
			}
		}
	}

	if prof.hz != 0 {
		if (GOARCH == "mips" || GOARCH == "mipsle" || GOARCH == "arm") && lostAtomic64Count > 0 {
			cpuprof.addLostAtomic64(lostAtomic64Count)
			lostAtomic64Count = 0
		}
		cpuprof.add(gp, stk[:n])
	}
	getg().m.mallocing--
}

// If the signal handler receives a SIGPROF signal on a non-Go thread,
// it tries to collect a traceback into sigprofCallers.
// sigprofCallersUse is set to non-zero while sigprofCallers holds a traceback.
var sigprofCallers cgoCallers
var sigprofCallersUse uint32

// sigprofNonGo is called if we receive a SIGPROF signal on a non-Go thread,
// and the signal handler collected a stack trace in sigprofCallers.
// When this is called, sigprofCallersUse will be non-zero.
// g is nil, and what we can do is very limited.
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGo() {
	if prof.hz != 0 {
		n := 0
		for n < len(sigprofCallers) && sigprofCallers[n] != 0 {
			n++
		}
		cpuprof.addNonGo(sigprofCallers[:n])
	}

	atomic.Store(&sigprofCallersUse, 0)
}

// sigprofNonGoPC is called when a profiling signal arrived on a
// non-Go thread and we have a single PC value, not a stack trace.
// g is nil, and what we can do is very limited.
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGoPC(pc uintptr) {
	if prof.hz != 0 {
		stk := []uintptr{
			pc,
			funcPC(_ExternalCode) + sys.PCQuantum,
		}
		cpuprof.addNonGo(stk)
	}
}

// Reports whether a function will set the SP
// to an absolute value. Important that
// we don't traceback when these are at the bottom
// of the stack since we can't be sure that we will
// find the caller.
//
// If the function is not on the bottom of the stack
// we assume that it will have set it up so that traceback will be consistent,
// either by being a traceback terminating function
// or putting one on the stack at the right offset.
func setsSP(pc uintptr) bool {
	f := findfunc(pc)
	if !f.valid() {
		// couldn't find the function for this PC,
		// so assume the worst and stop traceback
		return true
	}
	switch f.funcID {
	case funcID_gogo, funcID_systemstack, funcID_mcall, funcID_morestack:
		return true
	}
	return false
}

// setcpuprofilerate sets the CPU profiling rate to hz times per second.
// If hz <= 0, setcpuprofilerate turns off CPU profiling.
func setcpuprofilerate(hz int32) {
	// Force sane arguments.
	if hz < 0 {
		hz = 0
	}

	// Disable preemption, otherwise we can be rescheduled to another thread
	// that has profiling enabled.
	_g_ := getg()
	_g_.m.locks++

	// Stop profiler on this thread so that it is safe to lock prof.
	// if a profiling signal came in while we had prof locked,
	// it would deadlock.
	setThreadCPUProfiler(0)

	for !atomic.Cas(&prof.signalLock, 0, 1) {
		osyield()
	}
	if prof.hz != hz {
		setProcessCPUProfiler(hz)
		prof.hz = hz
	}
	atomic.Store(&prof.signalLock, 0)

	lock(&sched.lock)
	sched.profilehz = hz
	unlock(&sched.lock)

	if hz != 0 {
		setThreadCPUProfiler(hz)
	}

	_g_.m.locks--
}

// Change number of processors. The world is stopped, sched is locked.
// gcworkbufs are not being modified by either the GC or
// the write barrier code.
// Returns list of Ps with local work, they need to be scheduled by the caller.

// 简单总结下procresize主要做了以下事情
// 1. 比对原始p的个数和目标p个数，进行扩容/缩容
// 2. 如扩容，则创建新的p并初始化
// 3. 如缩容，则回收p的所有资源
// 4. 整理p，将空闲的p和持有g的p分别加入空闲列表和待工作列表

// 函数procresize用来初始化系统中的P，此函数运行时会暂停所有用户代码运行，调度器处于锁定状态，GC和写屏障也不会运行。
// 由于可在用户代码运行中更改GOMAXPROCS，所以此方法的实现略显复杂。
func procresize(nprocs int32) *p {
	// 检测参数是否合法
	old := gomaxprocs
	if old < 0 || nprocs <= 0 {
		throw("procresize: invalid arg")
	}
	if trace.enabled {
		traceGomaxprocs(nprocs)
	}

	// update statistics
	// 更新数据
	now := nanotime()
	if sched.procresizetime != 0 {
		sched.totaltime += int64(old) * (now - sched.procresizetime)
	}
	sched.procresizetime = now

	// Grow allp if necessary.
	// 需要增加P的数量。allp : 全局变量，一个切片，存储着所有的P。len(allp) P的个数
	if nprocs > int32(len(allp)) {
		// Synchronize with retake, which could be running
		// concurrently since it doesn't run on a P.
		lock(&allpLock)
		// cap(allp) P切片的容量
		if nprocs <= int32(cap(allp)) {
			allp = allp[:nprocs]
		} else {
			nallp := make([]*p, nprocs)
			// Copy everything up to allp's cap so we
			// never lose old allocated Ps.
			copy(nallp, allp[:cap(allp)])
			allp = nallp
		}
		unlock(&allpLock)
	}

	// initialize new P's
	// 创建并初始化新增加的P
	for i := int32(0); i < nprocs; i++ {
		pp := allp[i]
		if pp == nil {
			// 申请新对象
			pp = new(p)
			pp.id = i
			// pp 的初始状态为 stop
			pp.status = _Pgcstop
			pp.sudogcache = pp.sudogbuf[:0]
			for i := range pp.deferpool {
				pp.deferpool[i] = pp.deferpoolbuf[i][:0]
			}
			pp.wbBuf.reset()
			// 将 pp 存放到 allp 处
			atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(pp))
		}
		if pp.mcache == nil {
			if old == 0 && i == 0 {
				if getg().m.mcache == nil {
					throw("missing mcache?")
				}
				pp.mcache = getg().m.mcache // bootstrap
			} else {
				pp.mcache = allocmcache()
			}
		}
		if raceenabled && pp.racectx == 0 {
			if old == 0 && i == 0 {
				pp.racectx = raceprocctx0
				raceprocctx0 = 0 // bootstrap
			} else {
				pp.racectx = raceproccreate()
			}
		}
	}

	// free unused P's
	// 回收无用的p。释放多余的 P。由于减少了旧的 procs 的数量，因此需要释放
	for i := nprocs; i < old; i++ {
		p := allp[i]
		if trace.enabled && p == getg().m.p.ptr() {
			// moving to p[0], pretend that we were descheduled
			// and then scheduled again to keep the trace sane.
			// 移动到p[0]，假装我们被取消了计划，然后又被计划，以保持跟踪的合理性。
			traceGoSched()
			traceProcStop(p)
		}
		// move all runnable goroutines to the global queue
		// 将所有可运行的goroutines移到全局队列中。
		for p.runqhead != p.runqtail {
			// pop from tail of local queue
			// 从本地队列的尾部弹出
			p.runqtail--
			gp := p.runq[p.runqtail%uint32(len(p.runq))].ptr()
			// push onto head of global queue
			// 推到全局队列的头部
			globrunqputhead(gp)
		}
		if p.runnext != 0 {
			globrunqputhead(p.runnext.ptr())
			p.runnext = 0
		}
		// if there's a background worker, make it runnable and put
		// it on the global queue so it can clean itself up
		// 如果有一个后台工作者，让它可以运行，并把它放在全局队列中，这样它就可以自行清理。
		if gp := p.gcBgMarkWorker.ptr(); gp != nil {
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.enabled {
				traceGoUnpark(gp, 0)
			}
			globrunqput(gp)
			// This assignment doesn't race because the world is stopped.
			// 这项任务并不是因为世界被停止而进行竞争。
			p.gcBgMarkWorker.set(nil)
		}
		// Flush p's write barrier buffer.
		// 冲洗p的写屏障缓冲区。
		if gcphase != _GCoff {
			wbBufFlush1(p)
			p.gcw.dispose()
		}
		for i := range p.sudogbuf {
			p.sudogbuf[i] = nil
		}
		p.sudogcache = p.sudogbuf[:0]
		for i := range p.deferpool {
			for j := range p.deferpoolbuf[i] {
				p.deferpoolbuf[i][j] = nil
			}
			p.deferpool[i] = p.deferpoolbuf[i][:0]
		}
		freemcache(p.mcache)
		p.mcache = nil
		gfpurge(p)
		traceProcFree(p)
		if raceenabled {
			raceprocdestroy(p.racectx)
			p.racectx = 0
		}
		p.gcAssistTime = 0
		p.status = _Pdead
		// can't free P itself because it can be referenced by an M in syscall
	}

	// Trim allp.
	// 修剪切片allp
	if int32(len(allp)) != nprocs {
		lock(&allpLock)
		allp = allp[:nprocs]
		unlock(&allpLock)
	}

	// 获取当前正在运行的 g 指针，初始化时 _g_ = g0
	_g_ := getg()
	if _g_.m.p != 0 && _g_.m.p.ptr().id < nprocs {
		// continue to use the current P
		// 在当前M上继续使用绑定的P
		_g_.m.p.ptr().status = _Prunning
		_g_.m.p.ptr().mcache.prepareForSweep()
	} else {
		// release the current P and acquire allp[0]
		// 释放M上绑定的P并绑定allp[0]
		if _g_.m.p != 0 {
			_g_.m.p.ptr().m = 0
		}
		_g_.m.p = 0
		_g_.m.mcache = nil
		// 取出第 0 号 p
		p := allp[0]
		p.m = 0
		p.status = _Pidle
		// 当前运行的M绑定p
		acquirep(p)
		if trace.enabled {
			traceGoStart()
		}
	}

	// 找到所有需要立即运行的P并返回(p本地队列中包含待运行g)。下面这个 for 循环把所有空闲的 p 放入空闲链表
	var runnablePs *p
	for i := nprocs - 1; i >= 0; i-- {
		p := allp[i]
		// allp[0] 跟 m0 关联了，不会进行之后的“放入空闲链表”
		if _g_.m.p.ptr() == p {
			continue
		}
		// 状态转为 idle
		p.status = _Pidle
		// p 的 LRQ 里没有 G
		if runqempty(p) {
			// 放入全局空闲链表
			pidleput(p)
		} else {
			p.m.set(mget())
			p.link.set(runnablePs)
			runnablePs = p
		}
	}
	// 初始化任务偷取随机因子
	stealOrder.reset(uint32(nprocs))
	var int32p *int32 = &gomaxprocs // make compiler check that gomaxprocs is an int32
	atomic.Store((*uint32)(unsafe.Pointer(int32p)), uint32(nprocs))

	// 返回有本地任务的 P 链表
	return runnablePs
}

// Associate p and the current m.
//
// This function is allowed to have write barriers even if the caller
// isn't because it immediately acquires _p_.
//
//go:yeswritebarrierrec

// 关联p和当前m。
// 即使调用者没有写入障碍，也允许此函数具有写入障碍，因为它会立即获取_p_。
func acquirep(_p_ *p) {
	// Do the part that isn't allowed to have write barriers.
	// 做不允许有写障碍的部分。
	wirep(_p_)

	// Have p; write barriers now allowed.

	// Perform deferred mcache flush before this P can allocate
	// from a potentially stale mcache.
	// 在该P可以从可能过时的mcache分配之前，请执行延迟的mcache刷新。
	_p_.mcache.prepareForSweep()

	if trace.enabled {
		traceProcStart()
	}
}

// wirep is the first step of acquirep, which actually associates the
// current M to _p_. This is broken out so we can disallow write
// barriers for this part, since we don't yet have a P.
//
//go:nowritebarrierrec
//go:nosplit
func wirep(_p_ *p) {
	_g_ := getg()

	if _g_.m.p != 0 || _g_.m.mcache != nil {
		throw("wirep: already in go")
	}
	if _p_.m != 0 || _p_.status != _Pidle {
		id := int64(0)
		if _p_.m != 0 {
			id = _p_.m.ptr().id
		}
		print("wirep: p->m=", _p_.m, "(", id, ") p->status=", _p_.status, "\n")
		throw("wirep: invalid p state")
	}
	_g_.m.mcache = _p_.mcache
	_g_.m.p.set(_p_)
	_p_.m.set(_g_.m)
	_p_.status = _Prunning
}

// Disassociate p and the current m.
// 解除 p 与 m 的关联
func releasep() *p {
	_g_ := getg()

	if _g_.m.p == 0 || _g_.m.mcache == nil {
		throw("releasep: invalid arg")
	}
	_p_ := _g_.m.p.ptr()
	if _p_.m.ptr() != _g_.m || _p_.mcache != _g_.m.mcache || _p_.status != _Prunning {
		print("releasep: m=", _g_.m, " m->p=", _g_.m.p.ptr(), " p->m=", _p_.m, " m->mcache=", _g_.m.mcache, " p->mcache=", _p_.mcache, " p->status=", _p_.status, "\n")
		throw("releasep: invalid p state")
	}
	if trace.enabled {
		traceProcStop(_g_.m.p.ptr())
	}
	// 清空一些字段
	_g_.m.p = 0
	_g_.m.mcache = nil
	_p_.m = 0
	_p_.status = _Pidle
	return _p_
}

func incidlelocked(v int32) {
	lock(&sched.lock)
	sched.nmidlelocked += v
	if v > 0 {
		checkdead()
	}
	unlock(&sched.lock)
}

// Check for deadlock situation.
// The check is based on number of running M's, if 0 -> deadlock.
// sched.lock must be held.
func checkdead() {
	// For -buildmode=c-shared or -buildmode=c-archive it's OK if
	// there are no running goroutines. The calling program is
	// assumed to be running.
	if islibrary || isarchive {
		return
	}

	// If we are dying because of a signal caught on an already idle thread,
	// freezetheworld will cause all running threads to block.
	// And runtime will essentially enter into deadlock state,
	// except that there is a thread that will call exit soon.
	if panicking > 0 {
		return
	}

	// If we are not running under cgo, but we have an extra M then account
	// for it. (It is possible to have an extra M on Windows without cgo to
	// accommodate callbacks created by syscall.NewCallback. See issue #6751
	// for details.)
	var run0 int32
	if !iscgo && cgoHasExtraM {
		run0 = 1
	}

	run := mcount() - sched.nmidle - sched.nmidlelocked - sched.nmsys
	if run > run0 {
		return
	}
	if run < 0 {
		print("runtime: checkdead: nmidle=", sched.nmidle, " nmidlelocked=", sched.nmidlelocked, " mcount=", mcount(), " nmsys=", sched.nmsys, "\n")
		throw("checkdead: inconsistent counts")
	}

	grunning := 0
	lock(&allglock)
	for i := 0; i < len(allgs); i++ {
		gp := allgs[i]
		if isSystemGoroutine(gp, false) {
			continue
		}
		s := readgstatus(gp)
		switch s &^ _Gscan {
		case _Gwaiting:
			grunning++
		case _Grunnable,
			_Grunning,
			_Gsyscall:
			unlock(&allglock)
			print("runtime: checkdead: find g ", gp.goid, " in status ", s, "\n")
			throw("checkdead: runnable g")
		}
	}
	unlock(&allglock)
	if grunning == 0 { // possible if main goroutine calls runtime·Goexit()
		throw("no goroutines (main called runtime.Goexit) - deadlock!")
	}

	// Maybe jump time forward for playground.
	gp := timejump()
	if gp != nil {
		casgstatus(gp, _Gwaiting, _Grunnable)
		globrunqput(gp)
		_p_ := pidleget()
		if _p_ == nil {
			throw("checkdead: no p for timer")
		}
		mp := mget()
		if mp == nil {
			// There should always be a free M since
			// nothing is running.
			throw("checkdead: no m for timer")
		}
		mp.nextp.set(_p_)
		notewakeup(&mp.park)
		return
	}

	getg().m.throwing = -1 // do not dump full stacks
	throw("all goroutines are asleep - deadlock!")
}

// forcegcperiod is the maximum time in nanoseconds between garbage
// collections. If we go this long without a garbage collection, one
// is forced to run.
//
// This is a variable for testing purposes. It normally doesn't change.
var forcegcperiod int64 = 2 * 60 * 1e9

// Always runs without a P, so write barriers are not allowed.
//
//go:nowritebarrierrec
// 系统监控发现协程运行超过10ms时发出抢占请求，而系统监控又是什么呢？
// 在Go运行时中，系统监控是一个常驻系统线程，负责监察系统中协程的资源调度，适当给予干预，相当于“上帝”的存在。
// 我们来看系统监控的实现。在系统启动章节的最后，main协程创建了系统监控线程，我们了解下它的实现
// 系统监控在一个独立的线程上运行，不需要P，永不退出

// 系统监控保持一个死循环一直到应用结束，在每次循环的开始阶段，它都会进行短时间的休眠，20us-10ms不等。
// 休眠结束才会真正的履行自己的工作职责：
// 1. 检查当前是否处于GC阶段，如果正在进行垃圾回收，则继续休眠(设置一个较短的超时).
// 2. 检查网络数据，如果网络轮询器已经创建且已经10ms没有数据了，那么检查网络轮询器。
// 3. 抢占陷入系统调用中的P，运行时间过长的G
// 4. 检查是否需要强制触发垃圾回收
func sysmon() {
	lock(&sched.lock)
	sched.nmsys++
	// 检查死锁
	checkdead()
	unlock(&sched.lock)

	// If a heap span goes unused for 5 minutes after a garbage collection,
	// we hand it back to the operating system.
	// 如果一个堆区在垃圾收集后5分钟内没有被使用。我们就把它交还给操作系统。
	scavengelimit := int64(5 * 60 * 1e9)

	if debug.scavenge > 0 {
		// Scavenge-a-lot for testing.
		forcegcperiod = 10 * 1e6
		scavengelimit = 20 * 1e6
	}

	lastscavenge := nanotime()
	nscavenge := 0

	lasttrace := int64(0)
	// 连续多少个周期我们都没有醒过来的人
	idle := 0 // how many cycles in succession we had not wokeup somebody
	delay := uint32(0)
	// 死循环，永不退出
	for {
		if idle == 0 { // start with 20us sleep...
			delay = 20
		} else if idle > 50 { // start doubling the sleep after 1ms...
			delay *= 2
		}
		if delay > 10*1000 { // up to 10ms
			delay = 10 * 1000
		}
		// 每次循环结束该线程都会进行休眠。最少休眠20us，最长休眠10ms
		usleep(delay)
		// 如果当前处于GC STW阶段 且所有P已经停止，那么挂起sysmon线程
		if debug.schedtrace <= 0 && (sched.gcwaiting != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs)) {
			lock(&sched.lock)
			if atomic.Load(&sched.gcwaiting) != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs) {
				atomic.Store(&sched.sysmonwait, 1)
				unlock(&sched.lock)
				// Make wake-up period small enough for the sampling to be correct.
				// 使唤醒期足够小，以保证采样的正确性。
				maxsleep := forcegcperiod / 2
				if scavengelimit < forcegcperiod {
					maxsleep = scavengelimit / 2
				}
				shouldRelax := true
				if osRelaxMinNS > 0 {
					next := timeSleepUntil()
					now := nanotime()
					if next-now < osRelaxMinNS {
						shouldRelax = false
					}
				}
				if shouldRelax {
					osRelax(true)
				}
				// 休眠，120纳秒超时
				notetsleep(&sched.sysmonnote, maxsleep)
				if shouldRelax {
					osRelax(false)
				}
				lock(&sched.lock)
				atomic.Store(&sched.sysmonwait, 0)
				noteclear(&sched.sysmonnote)
				idle = 0
				delay = 20
			}
			unlock(&sched.lock)
		}
		// trigger libc interceptors if needed
		// 必要时触发libc拦截器
		if *cgo_yield != nil {
			asmcgocall(*cgo_yield, nil)
		}
		// poll network if not polled for more than 10ms
		// 如果10ms没有检查网络数据了，则检查网络轮询器
		lastpoll := int64(atomic.Load64(&sched.lastpoll))
		now := nanotime()
		if netpollinited() && lastpoll != 0 && lastpoll+10*1000*1000 < now {
			atomic.Cas64(&sched.lastpoll, uint64(lastpoll), uint64(now))
			list := netpoll(false) // non-blocking - returns list of goroutines
			if !list.empty() {
				// Need to decrement number of idle locked M's
				// (pretending that one more is running) before injectglist.
				// Otherwise it can lead to the following situation:
				// injectglist grabs all P's but before it starts M's to run the P's,
				// another M returns from syscall, finishes running its G,
				// observes that there is no work to do and no other running M's
				// and reports deadlock.
				incidlelocked(-1)
				// 网络中就绪的G加入全局队列
				injectglist(&list)
				incidlelocked(1)
			}
		}
		// retake P's blocked in syscalls and preempt long running G's
		// 抢占系统调用中的P，或者运行时间过长的G
		if retake(now) != 0 {
			idle = 0
		} else {
			idle++
		}
		// check if we need to force a GC
		// 检查是否需要触发强制GC
		if t := (gcTrigger{kind: gcTriggerTime, now: now}); t.test() && atomic.Load(&forcegc.idle) != 0 {
			lock(&forcegc.lock)
			forcegc.idle = 0
			var list gList
			list.push(forcegc.g)
			injectglist(&list)
			unlock(&forcegc.lock)
		}
		// scavenge heap once in a while
		if lastscavenge+scavengelimit/2 < now {
			mheap_.scavenge(int32(nscavenge), uint64(now), uint64(scavengelimit))
			lastscavenge = now
			nscavenge++
		}
		if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace)*1000000 <= now {
			lasttrace = now
			schedtrace(debug.scheddetail > 0)
		}
	}
}

type sysmontick struct {
	schedtick   uint32
	schedwhen   int64
	syscalltick uint32
	syscallwhen int64
}

// forcePreemptNS is the time slice given to a G before it is
// preempted.
// 触发强制抢占的时间间隔
const forcePreemptNS = 10 * 1000 * 1000 // 10ms

func retake(now int64) uint32 {
	n := 0
	// Prevent allp slice changes. This lock will be completely
	// uncontended unless we're already stopping the world.
	// 这个锁将是完全防止所有分片的变化。除非我们已经停止了这个世界。
	lock(&allpLock)
	// We can't use a range loop over allp because we may
	// temporarily drop the allpLock. Hence, we need to re-fetch
	// allp each time around the loop.
	// 遍历所有P，检查是否陷入系统调用或者是长时间运行的G
	for i := 0; i < len(allp); i++ {
		_p_ := allp[i]
		// 如果用户调用了procresize方法，则可能出现此场景
		if _p_ == nil {
			// This can happen if procresize has grown
			// allp but not yet created new Ps.
			continue
		}
		// 用于 sysmon 线程记录被监控 p 的系统调用时间和运行时间
		pd := &_p_.sysmontick
		// p 的状态
		s := _p_.status
		// 如果P在正常运行阶段或者系统调用中
		if s == _Psyscall {
			// Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
			// 如果g运行了过长时间则触发抢占
			// 如果p陷入系统调用超过一个系统监控循环(20us-10ms)，则将p抢占
			// P 处于系统调用之中，需要检查是否需要抢占
			// _p_.syscalltick 用于记录系统调用的次数，在完成系统调用之后加 1
			t := int64(_p_.syscalltick)
			// 如果上次检查后发生了系统调度，则记录检查状态，不触发抢占
			if int64(pd.syscalltick) != t {
				// pd.syscalltick != _p_.syscalltick，说明已经不是上次观察到的系统调用了，
				// 而是另外一次系统调用，所以需要重新记录 tick 和 when 值
				pd.syscalltick = uint32(t)
				pd.syscallwhen = now
				continue
			}
			// On the one hand we don't want to retake Ps if there is no other work to do,
			// but on the other hand we want to retake them eventually
			// because they can prevent the sysmon thread from deep sleep.
			// 触发抢占
			// p本地队列为空，存在空闲p，存在自旋中的m，未超过10ms 则不发生抢占
			// 只要满足下面三个条件中的任意一个，则抢占该 p，否则不抢占
			// 1. p 的运行队列里面有等待运行的 goroutine
			// 2. 没有无所事事的 p
			// 3. 从上一次监控线程观察到 p 对应的 m 处于系统调用之中到现在已经超过 10 毫秒
			if runqempty(_p_) && atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {
				continue
			}
			// Drop allpLock so we can take sched.lock.
			unlock(&allpLock)
			// Need to decrement number of idle locked M's
			// (pretending that one more is running) before the CAS.
			// Otherwise the M from which we retake can exit the syscall,
			// increment nmidle and report deadlock.
			incidlelocked(-1)
			// 修改p的状态为空闲，并尝试使用p或者添加到P空闲队列中
			if atomic.Cas(&_p_.status, s, _Pidle) {
				if trace.enabled {
					traceGoSysBlock(_p_)
					traceProcStop(_p_)
				}
				n++
				_p_.syscalltick++
				// 寻找一新的 m 接管 p
				handoffp(_p_)
			}
			incidlelocked(1)
			lock(&allpLock)
		} else if s == _Prunning {
			// P 处于运行状态，检查是否运行得太久了
			// Preempt G if it's running for too long.
			// 如果g运行了过长时间则触发抢占
			// 每发生一次调度，调度器 ++ 该值
			t := int64(_p_.schedtick)
			// 如果上次检查后发生了系统调度，则记录检查状态，不触发抢占
			if int64(pd.schedtick) != t {
				pd.schedtick = uint32(t)
				pd.schedwhen = now
				continue
			}
			//pd.schedtick == t 说明(pd.schedwhen ～ now)这段时间未发生过调度
			// 这段时间是同一个goroutine一直在运行，检查是否连续运行超过了 10 毫秒
			if pd.schedwhen+forcePreemptNS > now {
				continue
			}
			// 触发抢占
			// 连续运行超过 10 毫秒了，发起抢占请求
			preemptone(_p_)
		}
	}
	unlock(&allpLock)
	return uint32(n)
}

// Tell all goroutines that they have been preempted and they should stop.
// This function is purely best-effort.
// It can fail to inform a goroutine if a processor just started running it.
// No locks need to be held.
// Returns true if preemption request was issued to at least one goroutine.
/*
告诉所有的Goroutine他们已经被抢先了，他们应该停止。
这个功能纯粹是尽力而为。
如果处理器刚刚开始运行goroutine，它可能无法通知goroutine。
不需要锁。
如果向至少一个goroutine发出了抢占请求，则返回true。
*/
func preemptall() bool {
	res := false
	for _, _p_ := range allp {
		if _p_.status != _Prunning {
			continue
		}
		if preemptone(_p_) {
			res = true
		}
	}
	return res
}

// Tell the goroutine running on processor P to stop.
// This function is purely best-effort. It can incorrectly fail to inform the
// goroutine. It can send inform the wrong goroutine. Even if it informs the
// correct goroutine, that goroutine might ignore the request if it is
// simultaneously executing newstack.
// No lock needs to be held.
// Returns true if preemption request was issued.
// The actual preemption will happen at some point in the future
// and will be indicated by the gp->status no longer being
// Grunning
// 设置当前G可以被抢占
// 函数preemptone最终实现了抢占操作
func preemptone(_p_ *p) bool {
	// 如果当前P未与m进行绑定，则不会触发抢占
	mp := _p_.m.ptr()
	if mp == nil || mp == getg().m {
		return false
	}
	// 如果当前p未运行g或者在g0栈运行，则不会触发抢占
	// 被抢占的 goroutine
	gp := mp.curg
	if gp == nil || gp == mp.g0 {
		return false
	}

	// 允许抢占
	// 设置抢占标志
	gp.preempt = true

	// Every call in a go routine checks for stack overflow by
	// comparing the current stack pointer to gp->stackguard0.
	// Setting gp->stackguard0 to StackPreempt folds
	// preemption into the normal stack overflow check.
	// 具体抢占触发以及抢占过程参见上节。
	// 在 goroutine 内部的每次调用都会比较栈顶指针和 g.stackguard0，
	// 来判断是否发生了栈溢出。stackPreempt 非常大的一个数，比任何栈都大
	// stackPreempt = 0xfffffade
	gp.stackguard0 = stackPreempt
	return true
}

var starttime int64

func schedtrace(detailed bool) {
	now := nanotime()
	if starttime == 0 {
		starttime = now
	}

	lock(&sched.lock)
	print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle, " threads=", mcount(), " spinningthreads=", sched.nmspinning, " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
	if detailed {
		print(" gcwaiting=", sched.gcwaiting, " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait, "\n")
	}
	// We must be careful while reading data from P's, M's and G's.
	// Even if we hold schedlock, most data can be changed concurrently.
	// E.g. (p->m ? p->m->id : -1) can crash if p->m changes from non-nil to nil.
	for i, _p_ := range allp {
		mp := _p_.m.ptr()
		h := atomic.Load(&_p_.runqhead)
		t := atomic.Load(&_p_.runqtail)
		if detailed {
			id := int64(-1)
			if mp != nil {
				id = mp.id
			}
			print("  P", i, ": status=", _p_.status, " schedtick=", _p_.schedtick, " syscalltick=", _p_.syscalltick, " m=", id, " runqsize=", t-h, " gfreecnt=", _p_.gFree.n, "\n")
		} else {
			// In non-detailed mode format lengths of per-P run queues as:
			// [len1 len2 len3 len4]
			print(" ")
			if i == 0 {
				print("[")
			}
			print(t - h)
			if i == len(allp)-1 {
				print("]\n")
			}
		}
	}

	if !detailed {
		unlock(&sched.lock)
		return
	}

	for mp := allm; mp != nil; mp = mp.alllink {
		_p_ := mp.p.ptr()
		gp := mp.curg
		lockedg := mp.lockedg.ptr()
		id1 := int32(-1)
		if _p_ != nil {
			id1 = _p_.id
		}
		id2 := int64(-1)
		if gp != nil {
			id2 = gp.goid
		}
		id3 := int64(-1)
		if lockedg != nil {
			id3 = lockedg.goid
		}
		print("  M", mp.id, ": p=", id1, " curg=", id2, " mallocing=", mp.mallocing, " throwing=", mp.throwing, " preemptoff=", mp.preemptoff, ""+" locks=", mp.locks, " dying=", mp.dying, " spinning=", mp.spinning, " blocked=", mp.blocked, " lockedg=", id3, "\n")
	}

	lock(&allglock)
	for gi := 0; gi < len(allgs); gi++ {
		gp := allgs[gi]
		mp := gp.m
		lockedm := gp.lockedm.ptr()
		id1 := int64(-1)
		if mp != nil {
			id1 = mp.id
		}
		id2 := int64(-1)
		if lockedm != nil {
			id2 = lockedm.id
		}
		print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason.String(), ") m=", id1, " lockedm=", id2, "\n")
	}
	unlock(&allglock)
	unlock(&sched.lock)
}

// schedEnableUser enables or disables the scheduling of user
// goroutines.
//
// This does not stop already running user goroutines, so the caller
// should first stop the world when disabling user goroutines.
func schedEnableUser(enable bool) {
	lock(&sched.lock)
	if sched.disable.user == !enable {
		unlock(&sched.lock)
		return
	}
	sched.disable.user = !enable
	if enable {
		n := sched.disable.n
		sched.disable.n = 0
		globrunqputbatch(&sched.disable.runnable, n)
		unlock(&sched.lock)
		for ; n != 0 && sched.npidle != 0; n-- {
			startm(nil, false)
		}
	} else {
		unlock(&sched.lock)
	}
}

// schedEnabled reports whether gp should be scheduled. It returns
// false is scheduling of gp is disabled.
func schedEnabled(gp *g) bool {
	if sched.disable.user {
		return isSystemGoroutine(gp, true)
	}
	return true
}

// Put mp on midle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func mput(mp *m) {
	mp.schedlink = sched.midle
	sched.midle.set(mp)
	sched.nmidle++
	checkdead()
}

// Try to get an m from midle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
// 获取空闲m的过程比较简单。需要注意的是mget方法操作全局变量，需要加锁。
func mget() *m {
	// 取到链表中首个对象
	mp := sched.midle.ptr()
	if mp != nil {
		// 表头后移
		sched.midle = mp.schedlink
		// 空闲m个数-1
		sched.nmidle--
	}
	return mp
}

// Put gp on the global runnable queue.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func globrunqput(gp *g) {
	sched.runq.pushBack(gp)
	sched.runqsize++
}

// Put gp at the head of the global runnable queue.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func globrunqputhead(gp *g) {
	sched.runq.push(gp)
	sched.runqsize++
}

// Put a batch of runnable goroutines on the global runnable queue.
// This clears *batch.
// Sched must be locked.
func globrunqputbatch(batch *gQueue, n int32) {
	sched.runq.pushBackAll(*batch)
	sched.runqsize += n
	*batch = gQueue{}
}

// Try get a batch of G's from the global runnable queue.
// Sched must be locked.
// globrunqget尝试在全局队列中批量获取可运行g，如果一次尝试获取多个，除了返回的一个外，其他存储在P本地队列中。
// 由于globrunqget操作了全局队列，所以调用过程中需要加锁。
func globrunqget(_p_ *p, max int32) *g {
	// 如果全局队列为空，返回nil
	if sched.runqsize == 0 {
		return nil
	}

	// 确定最终获取的g的数量
	// 根据 p 的数量平分全局运行队列中的 goroutines
	n := sched.runqsize/gomaxprocs + 1
	if n > sched.runqsize {
		// 如果 gomaxprocs 为 1
		n = sched.runqsize
	}
	// 修正"偷"的数量
	if max > 0 && n > max {
		n = max
	}
	// 最多只能"偷"本地工作队列一半的数量
	if n > int32(len(_p_.runq))/2 {
		n = int32(len(_p_.runq)) / 2
	}

	// 更新全局可运行队列长度
	sched.runqsize -= n

	// 取出一个g返回，其他放入p本地队列
	gp := sched.runq.pop()
	n--
	for ; n > 0; n-- {
		// 获取当前队列头->移动队列头
		gp1 := sched.runq.pop()
		// 尝试将 gp1 放入 P 本地，使全局队列得到更多的执行机会
		runqput(_p_, gp1, false)
	}
	// 返回最开始获取到的队列头所指向的 goroutine
	return gp
}

// Put p to on _Pidle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
// 将 p 放到 _Pidle 列表里
//go:nowritebarrierrec
func pidleput(_p_ *p) {
	if !runqempty(_p_) {
		throw("pidleput: P has non-empty run queue")
	}
	_p_.link = sched.pidle
	sched.pidle.set(_p_)
	// 增加全局空闲 P 的数量
	atomic.Xadd(&sched.npidle, 1) // TODO: fast atomic
}

// Try get a p from _Pidle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func pidleget() *p {
	_p_ := sched.pidle.ptr()
	if _p_ != nil {
		sched.pidle = _p_.link
		atomic.Xadd(&sched.npidle, -1) // TODO: fast atomic
	}
	return _p_
}

// runqempty reports whether _p_ has no Gs on its local run queue.
// It never returns true spuriously.
// 如果 _p_ 的本地队列里没有待运行的 G，则返回 true
func runqempty(_p_ *p) bool {
	// Defend against a race where 1) _p_ has G1 in runqnext but runqhead == runqtail,
	// 2) runqput on _p_ kicks G1 to the runq, 3) runqget on _p_ empties runqnext.
	// Simply observing that runqhead == runqtail and then observing that runqnext == nil
	// does not mean the queue is empty.
	// 这里涉及到一些数据竞争，并不是简单地判断 runqhead == runqtail 并且 runqnext == nil 就可以
	for {
		head := atomic.Load(&_p_.runqhead)
		tail := atomic.Load(&_p_.runqtail)
		runnext := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&_p_.runnext)))
		if tail == atomic.Load(&_p_.runqtail) {
			return head == tail && runnext == 0
		}
	}
}

// To shake out latent assumptions about scheduling order,
// we introduce some randomness into scheduling decisions
// when running with the race detector.
// The need for this was made obvious by changing the
// (deterministic) scheduling order in Go 1.5 and breaking
// many poorly-written tests.
// With the randomness here, as long as the tests pass
// consistently with -race, they shouldn't have latent scheduling
// assumptions.
const randomizeScheduler = raceenabled

// runqput tries to put g on the local runnable queue.
// If next is false, runqput adds g to the tail of the runnable queue.
// If next is true, runqput puts g in the _p_.runnext slot.
// If the run queue is full, runnext puts g on the global queue.
// Executed only by the owner P.
// runqput 尝试将 g 放到本地可执行队列里。
// 如果 next 为假，runqput 将 g 添加到可运行队列的尾部
// 如果 next 为真，runqput 将 g 添加到 p.runnext 字段
// 如果 run queue 满了，runnext 将 g 放到全局队列里
// runnext 成员中的 goroutine 会被优先调度起来运行
func runqput(_p_ *p, gp *g, next bool) {
	if randomizeScheduler && next && fastrand()%2 == 0 {
		next = false
	}

	if next {
	retryNext:
		oldnext := _p_.runnext
		if !_p_.runnext.cas(oldnext, guintptr(unsafe.Pointer(gp))) {
			// 有其它线程在操作 runnext 成员，需要重试
			goto retryNext
		}
		// 老的 runnext 为 nil，不用管了
		if oldnext == 0 {
			return
		}
		// Kick the old runnext out to the regular run queue.
		// 把之前的 runnext 放到正常的 runq 中
		// 原本存放在 runnext 的 gp 放入 runq 的尾部
		gp = oldnext.ptr()
	}

retry:
	h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with consumers
	t := _p_.runqtail
	// 如果 P 的本地队列没有满，入队
	if t-h < uint32(len(_p_.runq)) {
		_p_.runq[t%uint32(len(_p_.runq))].set(gp)
		// 原子写入
		atomic.StoreRel(&_p_.runqtail, t+1) // store-release, makes the item available for consumption
		return
	}
	// 可运行队列已经满了，放入全局队列了
	if runqputslow(_p_, gp, h, t) {
		return
	}
	// the queue is not full, now the put above must succeed
	// 没有成功放入全局队列，说明本地队列没满，重试一下
	goto retry
}

// Put g and a batch of work from local runnable queue on global queue.
// Executed only by the owner P.
// 将 g 和 _p_ 本地队列的一半 goroutine 放入全局队列。
// 因为要获取锁，所以会慢
func runqputslow(_p_ *p, gp *g, h, t uint32) bool {
	var batch [len(_p_.runq)/2 + 1]*g

	// First, grab a batch from local queue.
	n := t - h
	n = n / 2
	if n != uint32(len(_p_.runq)/2) {
		throw("runqputslow: queue is not full")
	}
	for i := uint32(0); i < n; i++ {
		batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))].ptr()
	}
	// 如果 cas 操作失败，说明本地队列不满了，直接返回
	if !atomic.CasRel(&_p_.runqhead, h, h+n) { // cas-release, commits consume
		return false
	}
	batch[n] = gp

	if randomizeScheduler {
		for i := uint32(1); i <= n; i++ {
			j := fastrandn(i + 1)
			batch[i], batch[j] = batch[j], batch[i]
		}
	}

	// Link the goroutines.
	// 全局运行队列是一个链表，这里首先把所有需要放入全局运行队列的 g 链接起来，
	// 减小锁粒度，从而降低锁冲突，提升性能
	for i := uint32(0); i < n; i++ {
		batch[i].schedlink.set(batch[i+1])
	}
	var q gQueue
	q.head.set(batch[0])
	q.tail.set(batch[n])

	// Now put the batch on global queue.
	lock(&sched.lock)
	globrunqputbatch(&q, int32(n+1))
	unlock(&sched.lock)
	return true
}

// Get g from local runnable queue.
// If inheritTime is true, gp should inherit the remaining time in the
// current time slice. Otherwise, it should start a new time slice.
// Executed only by the owner P.
// runqget会优先获取p.runnext指向的g，如果不存在runnext，则尝试在本地队列中获取。
// 每个p的本地可运行g队列是一个环形队列，容量为256，通过runqhead,runqtail控制数据存取。
// 环形队列实现细节感兴趣的同学可以自行了解下。
func runqget(_p_ *p) (gp *g, inheritTime bool) {
	// If there's a runnext, it's the next G to run.
	// 如果存在runnext，则获取并返回runnext
	// 如果 runnext 不为空，则 runnext 是下一个待运行的 G
	for {
		next := _p_.runnext
		if next == 0 {
			// 为空，则直接跳出循环
			break
		}
		// 再次比较 next 是否没有变化
		if _p_.runnext.cas(next, 0) {
			// 如果没有变化，则返回 next 所指向的 g。且需要继承时间片
			return next.ptr(), true
		}
	}

	for {
		// 取到队列的头
		h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with other consumers
		// 队尾
		t := _p_.runqtail
		if t == h {
			// 队列为空，返回nil
			// 头和尾相等，说明本地队列为空，找不到 g
			return nil, false
		}
		// 获取队列头位置的g
		gp := _p_.runq[h%uint32(len(_p_.runq))].ptr()
		// 更新队头值。原子操作，防止这中间被其他线程因为偷工作而修改
		if atomic.CasRel(&_p_.runqhead, h, h+1) { // cas-release, commits consume
			return gp, false
		}
	}
}

// Grabs a batch of goroutines from _p_'s runnable queue into batch.
// Batch is a ring buffer starting at batchHead.
// Returns number of grabbed goroutines.
// Can be executed by any P.
// 从 _p_ 批量获取可运行 goroutine，放到 batch 数组里
// batch 是一个环，起始于 batchHead
// 返回偷的数量，返回的 goroutine 可被任何 P 执行
func runqgrab(_p_ *p, batch *[256]guintptr, batchHead uint32, stealRunNextG bool) uint32 {
	for {
		// 队列头
		h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with other consumers
		// 队列尾
		t := atomic.LoadAcq(&_p_.runqtail) // load-acquire, synchronize with the producer
		// g 的数量
		n := t - h
		// 取一半
		n = n - n/2
		if n == 0 {
			if stealRunNextG {
				// Try to steal from _p_.runnext.
				// 连 runnext 都要偷
				if next := _p_.runnext; next != 0 {
					if _p_.status == _Prunning {
						// Sleep to ensure that _p_ isn't about to run the g we are about to steal.
						// The important use case here is when the g running on _p_ ready()s another g and then almost immediately blocks.
						// Instead of stealing runnext in this window, back off to give _p_ a chance to schedule runnext.
						// This will avoid thrashing gs between different Ps.
						// A sync chan send/recv takes ~50ns as of time of writing, so 3us gives ~50x overshoot.
						// 这里是为了防止 _p_ 执行当前 g，并且马上就要阻塞，所以会马上执行 runnext，
						// 这个时候偷就没必要了，因为让 g 在 P 之间"游走"不太划算，就不偷了，给他们一个机会。
						// channel 一次同步的的接收发送需要 50ns 左右，因此 3us 差不多给了他们 50 次机会了，做得还是不错的
						if GOOS != "windows" {
							usleep(3)
						} else {
							// On windows system timer granularity is
							// 1-15ms, which is way too much for this
							// optimization. So just yield.
							osyield()
						}
					}
					if !_p_.runnext.cas(next, 0) {
						continue
					}
					// 真的偷走了 next
					batch[batchHead%uint32(len(batch))] = next
					// 返回偷的数量，只有 1 个
					return 1
				}
			}
			// 没偷到
			return 0
		}
		// 如果 n 这时变得太大了，重新来一遍了，不能偷的太多
		if n > uint32(len(_p_.runq)/2) { // read inconsistent h and t
			continue
		}
		// 将 g 放置到 bacth 中
		for i := uint32(0); i < n; i++ {
			g := _p_.runq[(h+i)%uint32(len(_p_.runq))]
			batch[(batchHead+i)%uint32(len(batch))] = g
		}
		// g被偷走了，更新一下队列头指针
		if atomic.CasRel(&_p_.runqhead, h, h+n) { // cas-release, commits consume
			return n
		}
	}
}

// Steal half of elements from local runnable queue of p2
// and put onto local runnable queue of p.
// Returns one of the stolen elements (or nil if failed).
// 从 p2 偷走一半的工作放到 _p_ 的本地
func runqsteal(_p_, p2 *p, stealRunNextG bool) *g {
	// 队尾
	t := _p_.runqtail
	// 从 p2 偷取工作，放到 _p_.runq 的队尾
	n := runqgrab(p2, &_p_.runq, t, stealRunNextG)
	if n == 0 {
		// 说明只偷了一个 g
		return nil
	}
	n--
	// 找到最后一个 g，准备返回
	gp := _p_.runq[(t+n)%uint32(len(_p_.runq))].ptr()
	if n == 0 {
		return gp
	}
	// 队列头
	h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with consumers
	// 判断是否偷太多了
	if t-h+n >= uint32(len(_p_.runq)) {
		throw("runqsteal: runq overflow")
	}
	// 更新队尾，将偷来的工作加入队列
	atomic.StoreRel(&_p_.runqtail, t+n) // store-release, makes the item available for consumption
	return gp
}

// A gQueue is a dequeue of Gs linked through g.schedlink. A G can only
// be on one gQueue or gList at a time.
// gQueue是一个通过g.schedlink链接的G的dequeue。一个G只能在同一时间出现在一个gQueue或gList上。
type gQueue struct {
	head guintptr
	tail guintptr
}

// empty reports whether q is empty.
func (q *gQueue) empty() bool {
	return q.head == 0
}

// push adds gp to the head of q.
func (q *gQueue) push(gp *g) {
	gp.schedlink = q.head
	q.head.set(gp)
	if q.tail == 0 {
		q.tail.set(gp)
	}
}

// pushBack adds gp to the tail of q.
func (q *gQueue) pushBack(gp *g) {
	gp.schedlink = 0
	if q.tail != 0 {
		q.tail.ptr().schedlink.set(gp)
	} else {
		q.head.set(gp)
	}
	q.tail.set(gp)
}

// pushBackAll adds all Gs in l2 to the tail of q. After this q2 must
// not be used.
func (q *gQueue) pushBackAll(q2 gQueue) {
	if q2.tail == 0 {
		return
	}
	q2.tail.ptr().schedlink = 0
	if q.tail != 0 {
		q.tail.ptr().schedlink = q2.head
	} else {
		q.head = q2.head
	}
	q.tail = q2.tail
}

// pop removes and returns the head of queue q. It returns nil if
// q is empty.
func (q *gQueue) pop() *g {
	// 获取队列头指向的 goroutine
	gp := q.head.ptr()
	if gp != nil {
		// 移动队列头
		q.head = gp.schedlink
		if q.head == 0 {
			q.tail = 0
		}
	}
	return gp
}

// popList takes all Gs in q and returns them as a gList.
func (q *gQueue) popList() gList {
	stack := gList{q.head}
	*q = gQueue{}
	return stack
}

// A gList is a list of Gs linked through g.schedlink. A G can only be
// on one gQueue or gList at a time.
type gList struct {
	head guintptr
}

// empty reports whether l is empty.
func (l *gList) empty() bool {
	return l.head == 0
}

// push adds gp to the head of l.
func (l *gList) push(gp *g) {
	gp.schedlink = l.head
	l.head.set(gp)
}

// pushAll prepends all Gs in q to l.
func (l *gList) pushAll(q gQueue) {
	if !q.empty() {
		q.tail.ptr().schedlink = l.head
		l.head = q.head
	}
}

// pop removes and returns the head of l. If l is empty, it returns nil.
func (l *gList) pop() *g {
	gp := l.head.ptr()
	if gp != nil {
		l.head = gp.schedlink
	}
	return gp
}

//go:linkname setMaxThreads runtime/debug.setMaxThreads
func setMaxThreads(in int) (out int) {
	lock(&sched.lock)
	out = int(sched.maxmcount)
	if in > 0x7fffffff { // MaxInt32
		sched.maxmcount = 0x7fffffff
	} else {
		sched.maxmcount = int32(in)
	}
	checkmcount()
	unlock(&sched.lock)
	return
}

func haveexperiment(name string) bool {
	if name == "framepointer" {
		return framepointer_enabled // set by linker
	}
	x := sys.Goexperiment
	for x != "" {
		xname := ""
		i := index(x, ",")
		if i < 0 {
			xname, x = x, ""
		} else {
			xname, x = x[:i], x[i+1:]
		}
		if xname == name {
			return true
		}
		if len(xname) > 2 && xname[:2] == "no" && xname[2:] == name {
			return false
		}
	}
	return false
}

//go:nosplit
func procPin() int {
	_g_ := getg()
	mp := _g_.m

	mp.locks++
	return int(mp.p.ptr().id)
}

//go:nosplit
func procUnpin() {
	_g_ := getg()
	_g_.m.locks--
}

//go:linkname sync_runtime_procPin sync.runtime_procPin
//go:nosplit
func sync_runtime_procPin() int {
	return procPin()
}

//go:linkname sync_runtime_procUnpin sync.runtime_procUnpin
//go:nosplit
func sync_runtime_procUnpin() {
	procUnpin()
}

//go:linkname sync_atomic_runtime_procPin sync/atomic.runtime_procPin
//go:nosplit
func sync_atomic_runtime_procPin() int {
	return procPin()
}

//go:linkname sync_atomic_runtime_procUnpin sync/atomic.runtime_procUnpin
//go:nosplit
func sync_atomic_runtime_procUnpin() {
	procUnpin()
}

// Active spinning for sync.Mutex.
//go:linkname sync_runtime_canSpin sync.runtime_canSpin
//go:nosplit
func sync_runtime_canSpin(i int) bool {
	// sync.Mutex is cooperative, so we are conservative with spinning.
	// Spin only few times and only if running on a multicore machine and
	// GOMAXPROCS>1 and there is at least one other running P and local runq is empty.
	// As opposed to runtime mutex we don't do passive spinning here,
	// because there can be work on global runq or on other Ps.
	if i >= active_spin || ncpu <= 1 || gomaxprocs <= int32(sched.npidle+sched.nmspinning)+1 {
		return false
	}
	if p := getg().m.p.ptr(); !runqempty(p) {
		return false
	}
	return true
}

//go:linkname sync_runtime_doSpin sync.runtime_doSpin
//go:nosplit
func sync_runtime_doSpin() {
	procyield(active_spin_cnt)
}

var stealOrder randomOrder

// randomOrder/randomEnum are helper types for randomized work stealing.
// They allow to enumerate all Ps in different pseudo-random orders without repetitions.
// The algorithm is based on the fact that if we have X such that X and GOMAXPROCS
// are coprime, then a sequences of (i + X) % GOMAXPROCS gives the required enumeration.
type randomOrder struct {
	count    uint32
	coprimes []uint32
}

type randomEnum struct {
	i     uint32
	count uint32
	pos   uint32
	inc   uint32
}

func (ord *randomOrder) reset(count uint32) {
	ord.count = count
	ord.coprimes = ord.coprimes[:0]
	for i := uint32(1); i <= count; i++ {
		if gcd(i, count) == 1 {
			ord.coprimes = append(ord.coprimes, i)
		}
	}
}

func (ord *randomOrder) start(i uint32) randomEnum {
	return randomEnum{
		count: ord.count,
		pos:   i % ord.count,
		inc:   ord.coprimes[i%uint32(len(ord.coprimes))],
	}
}

func (enum *randomEnum) done() bool {
	return enum.i == enum.count
}

func (enum *randomEnum) next() {
	enum.i++
	enum.pos = (enum.pos + enum.inc) % enum.count
}

func (enum *randomEnum) position() uint32 {
	return enum.pos
}

func gcd(a, b uint32) uint32 {
	for b != 0 {
		a, b = b, a%b
	}
	return a
}