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// Copyright 2009 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.
// Garbage collector: marking and scanning
package runtime
import (
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
const (
fixedRootFinalizers = iota
fixedRootFreeGStacks
fixedRootCount
// rootBlockBytes is the number of bytes to scan per data or
// BSS root.
rootBlockBytes = 256 << 10
// maxObletBytes is the maximum bytes of an object to scan at
// once. Larger objects will be split up into "oblets" of at
// most this size. Since we can scan 1–2 MB/ms, 128 KB bounds
// scan preemption at ~100 µs.
//
// This must be > _MaxSmallSize so that the object base is the
// span base.
maxObletBytes = 128 << 10
// drainCheckThreshold specifies how many units of work to do
// between self-preemption checks in gcDrain. Assuming a scan
// rate of 1 MB/ms, this is ~100 µs. Lower values have higher
// overhead in the scan loop (the scheduler check may perform
// a syscall, so its overhead is nontrivial). Higher values
// make the system less responsive to incoming work.
drainCheckThreshold = 100000
// pagesPerSpanRoot indicates how many pages to scan from a span root
// at a time. Used by special root marking.
//
// Higher values improve throughput by increasing locality, but
// increase the minimum latency of a marking operation.
//
// Must be a multiple of the pageInUse bitmap element size and
// must also evenly divide pagesPerArena.
pagesPerSpanRoot = 512
)
// gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
// some miscellany) and initializes scanning-related state.
//
// The world must be stopped.
func gcMarkRootPrepare() {
assertWorldStopped()
work.nFlushCacheRoots = 0
// Compute how many data and BSS root blocks there are.
nBlocks := func(bytes uintptr) int {
return int(divRoundUp(bytes, rootBlockBytes))
}
work.nDataRoots = 0
work.nBSSRoots = 0
// Scan globals.
for _, datap := range activeModules() {
nDataRoots := nBlocks(datap.edata - datap.data)
if nDataRoots > work.nDataRoots {
work.nDataRoots = nDataRoots
}
}
for _, datap := range activeModules() {
nBSSRoots := nBlocks(datap.ebss - datap.bss)
if nBSSRoots > work.nBSSRoots {
work.nBSSRoots = nBSSRoots
}
}
// Scan span roots for finalizer specials.
//
// We depend on addfinalizer to mark objects that get
// finalizers after root marking.
//
// We're going to scan the whole heap (that was available at the time the
// mark phase started, i.e. markArenas) for in-use spans which have specials.
//
// Break up the work into arenas, and further into chunks.
//
// Snapshot allArenas as markArenas. This snapshot is safe because allArenas
// is append-only.
mheap_.markArenas = mheap_.allArenas[:len(mheap_.allArenas):len(mheap_.allArenas)]
work.nSpanRoots = len(mheap_.markArenas) * (pagesPerArena / pagesPerSpanRoot)
// Scan stacks.
//
// Gs may be created after this point, but it's okay that we
// ignore them because they begin life without any roots, so
// there's nothing to scan, and any roots they create during
// the concurrent phase will be scanned during mark
// termination.
work.nStackRoots = int(atomic.Loaduintptr(&allglen))
work.markrootNext = 0
work.markrootJobs = uint32(fixedRootCount + work.nFlushCacheRoots + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots)
}
// gcMarkRootCheck checks that all roots have been scanned. It is
// purely for debugging.
func gcMarkRootCheck() {
if work.markrootNext < work.markrootJobs {
print(work.markrootNext, " of ", work.markrootJobs, " markroot jobs done\n")
throw("left over markroot jobs")
}
lock(&allglock)
// Check that stacks have been scanned.
var gp *g
for i := 0; i < work.nStackRoots; i++ {
gp = allgs[i]
if !gp.gcscandone {
goto fail
}
}
unlock(&allglock)
return
fail:
println("gp", gp, "goid", gp.goid,
"status", readgstatus(gp),
"gcscandone", gp.gcscandone)
unlock(&allglock) // Avoid self-deadlock with traceback.
throw("scan missed a g")
}
// ptrmask for an allocation containing a single pointer.
var oneptrmask = [...]uint8{1}
// markroot scans the i'th root.
//
// Preemption must be disabled (because this uses a gcWork).
//
// nowritebarrier is only advisory here.
//
//go:nowritebarrier
func markroot(gcw *gcWork, i uint32) {
// TODO(austin): This is a bit ridiculous. Compute and store
// the bases in gcMarkRootPrepare instead of the counts.
baseFlushCache := uint32(fixedRootCount)
baseData := baseFlushCache + uint32(work.nFlushCacheRoots)
baseBSS := baseData + uint32(work.nDataRoots)
baseSpans := baseBSS + uint32(work.nBSSRoots)
baseStacks := baseSpans + uint32(work.nSpanRoots)
end := baseStacks + uint32(work.nStackRoots)
// Note: if you add a case here, please also update heapdump.go:dumproots.
switch {
case baseFlushCache <= i && i < baseData:
flushmcache(int(i - baseFlushCache))
case baseData <= i && i < baseBSS:
for _, datap := range activeModules() {
markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-baseData))
}
case baseBSS <= i && i < baseSpans:
for _, datap := range activeModules() {
markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-baseBSS))
}
case i == fixedRootFinalizers:
for fb := allfin; fb != nil; fb = fb.alllink {
cnt := uintptr(atomic.Load(&fb.cnt))
scanblock(uintptr(unsafe.Pointer(&fb.fin[0])), cnt*unsafe.Sizeof(fb.fin[0]), &finptrmask[0], gcw, nil)
}
case i == fixedRootFreeGStacks:
// Switch to the system stack so we can call
// stackfree.
systemstack(markrootFreeGStacks)
case baseSpans <= i && i < baseStacks:
// mark mspan.specials
markrootSpans(gcw, int(i-baseSpans))
default:
// the rest is scanning goroutine stacks
var gp *g
if baseStacks <= i && i < end {
gp = allgs[i-baseStacks]
} else {
throw("markroot: bad index")
}
// remember when we've first observed the G blocked
// needed only to output in traceback
status := readgstatus(gp) // We are not in a scan state
if (status == _Gwaiting || status == _Gsyscall) && gp.waitsince == 0 {
gp.waitsince = work.tstart
}
// scanstack must be done on the system stack in case
// we're trying to scan our own stack.
systemstack(func() {
// If this is a self-scan, put the user G in
// _Gwaiting to prevent self-deadlock. It may
// already be in _Gwaiting if this is a mark
// worker or we're in mark termination.
userG := getg().m.curg
selfScan := gp == userG && readgstatus(userG) == _Grunning
if selfScan {
casgstatus(userG, _Grunning, _Gwaiting)
userG.waitreason = waitReasonGarbageCollectionScan
}
// TODO: suspendG blocks (and spins) until gp
// stops, which may take a while for
// running goroutines. Consider doing this in
// two phases where the first is non-blocking:
// we scan the stacks we can and ask running
// goroutines to scan themselves; and the
// second blocks.
stopped := suspendG(gp)
if stopped.dead {
gp.gcscandone = true
return
}
if gp.gcscandone {
throw("g already scanned")
}
scanstack(gp, gcw)
gp.gcscandone = true
resumeG(stopped)
if selfScan {
casgstatus(userG, _Gwaiting, _Grunning)
}
})
}
}
// markrootBlock scans the shard'th shard of the block of memory [b0,
// b0+n0), with the given pointer mask.
//
//go:nowritebarrier
func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) {
if rootBlockBytes%(8*sys.PtrSize) != 0 {
// This is necessary to pick byte offsets in ptrmask0.
throw("rootBlockBytes must be a multiple of 8*ptrSize")
}
// Note that if b0 is toward the end of the address space,
// then b0 + rootBlockBytes might wrap around.
// These tests are written to avoid any possible overflow.
off := uintptr(shard) * rootBlockBytes
if off >= n0 {
return
}
b := b0 + off
ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*sys.PtrSize))))
n := uintptr(rootBlockBytes)
if off+n > n0 {
n = n0 - off
}
// Scan this shard.
scanblock(b, n, ptrmask, gcw, nil)
}
// markrootFreeGStacks frees stacks of dead Gs.
//
// This does not free stacks of dead Gs cached on Ps, but having a few
// cached stacks around isn't a problem.
func markrootFreeGStacks() {
// Take list of dead Gs with stacks.
lock(&sched.gFree.lock)
list := sched.gFree.stack
sched.gFree.stack = gList{}
unlock(&sched.gFree.lock)
if list.empty() {
return
}
// Free stacks.
q := gQueue{list.head, list.head}
for gp := list.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
stackfree(gp.stack)
gp.stack.lo = 0
gp.stack.hi = 0
// Manipulate the queue directly since the Gs are
// already all linked the right way.
q.tail.set(gp)
}
// Put Gs back on the free list.
lock(&sched.gFree.lock)
sched.gFree.noStack.pushAll(q)
unlock(&sched.gFree.lock)
}
// markrootSpans marks roots for one shard of markArenas.
//
//go:nowritebarrier
func markrootSpans(gcw *gcWork, shard int) {
// Objects with finalizers have two GC-related invariants:
//
// 1) Everything reachable from the object must be marked.
// This ensures that when we pass the object to its finalizer,
// everything the finalizer can reach will be retained.
//
// 2) Finalizer specials (which are not in the garbage
// collected heap) are roots. In practice, this means the fn
// field must be scanned.
sg := mheap_.sweepgen
// Find the arena and page index into that arena for this shard.
ai := mheap_.markArenas[shard/(pagesPerArena/pagesPerSpanRoot)]
ha := mheap_.arenas[ai.l1()][ai.l2()]
arenaPage := uint(uintptr(shard) * pagesPerSpanRoot % pagesPerArena)
// Construct slice of bitmap which we'll iterate over.
specialsbits := ha.pageSpecials[arenaPage/8:]
specialsbits = specialsbits[:pagesPerSpanRoot/8]
for i := range specialsbits {
// Find set bits, which correspond to spans with specials.
specials := atomic.Load8(&specialsbits[i])
if specials == 0 {
continue
}
for j := uint(0); j < 8; j++ {
if specials&(1<<j) == 0 {
continue
}
// Find the span for this bit.
//
// This value is guaranteed to be non-nil because having
// specials implies that the span is in-use, and since we're
// currently marking we can be sure that we don't have to worry
// about the span being freed and re-used.
s := ha.spans[arenaPage+uint(i)*8+j]
// The state must be mSpanInUse if the specials bit is set, so
// sanity check that.
if state := s.state.get(); state != mSpanInUse {
print("s.state = ", state, "\n")
throw("non in-use span found with specials bit set")
}
// Check that this span was swept (it may be cached or uncached).
if !useCheckmark && !(s.sweepgen == sg || s.sweepgen == sg+3) {
// sweepgen was updated (+2) during non-checkmark GC pass
print("sweep ", s.sweepgen, " ", sg, "\n")
throw("gc: unswept span")
}
// Lock the specials to prevent a special from being
// removed from the list while we're traversing it.
lock(&s.speciallock)
for sp := s.specials; sp != nil; sp = sp.next {
if sp.kind != _KindSpecialFinalizer {
continue
}
// don't mark finalized object, but scan it so we
// retain everything it points to.
spf := (*specialfinalizer)(unsafe.Pointer(sp))
// A finalizer can be set for an inner byte of an object, find object beginning.
p := s.base() + uintptr(spf.special.offset)/s.elemsize*s.elemsize
// Mark everything that can be reached from
// the object (but *not* the object itself or
// we'll never collect it).
scanobject(p, gcw)
// The special itself is a root.
scanblock(uintptr(unsafe.Pointer(&spf.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil)
}
unlock(&s.speciallock)
}
}
}
// gcAssistAlloc performs GC work to make gp's assist debt positive.
// gp must be the calling user gorountine.
//
// This must be called with preemption enabled.
func gcAssistAlloc(gp *g) {
// Don't assist in non-preemptible contexts. These are
// generally fragile and won't allow the assist to block.
if getg() == gp.m.g0 {
return
}
if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
return
}
traced := false
retry:
// Compute the amount of scan work we need to do to make the
// balance positive. When the required amount of work is low,
// we over-assist to build up credit for future allocations
// and amortize the cost of assisting.
assistWorkPerByte := float64frombits(atomic.Load64(&gcController.assistWorkPerByte))
assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
debtBytes := -gp.gcAssistBytes
scanWork := int64(assistWorkPerByte * float64(debtBytes))
if scanWork < gcOverAssistWork {
scanWork = gcOverAssistWork
debtBytes = int64(assistBytesPerWork * float64(scanWork))
}
// Steal as much credit as we can from the background GC's
// scan credit. This is racy and may drop the background
// credit below 0 if two mutators steal at the same time. This
// will just cause steals to fail until credit is accumulated
// again, so in the long run it doesn't really matter, but we
// do have to handle the negative credit case.
bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit)
stolen := int64(0)
if bgScanCredit > 0 {
if bgScanCredit < scanWork {
stolen = bgScanCredit
gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(stolen))
} else {
stolen = scanWork
gp.gcAssistBytes += debtBytes
}
atomic.Xaddint64(&gcController.bgScanCredit, -stolen)
scanWork -= stolen
if scanWork == 0 {
// We were able to steal all of the credit we
// needed.
if traced {
traceGCMarkAssistDone()
}
return
}
}
if trace.enabled && !traced {
traced = true
traceGCMarkAssistStart()
}
// Perform assist work
systemstack(func() {
gcAssistAlloc1(gp, scanWork)
// The user stack may have moved, so this can't touch
// anything on it until it returns from systemstack.
})
completed := gp.param != nil
gp.param = nil
if completed {
gcMarkDone()
}
if gp.gcAssistBytes < 0 {
// We were unable steal enough credit or perform
// enough work to pay off the assist debt. We need to
// do one of these before letting the mutator allocate
// more to prevent over-allocation.
//
// If this is because we were preempted, reschedule
// and try some more.
if gp.preempt {
Gosched()
goto retry
}
// Add this G to an assist queue and park. When the GC
// has more background credit, it will satisfy queued
// assists before flushing to the global credit pool.
//
// Note that this does *not* get woken up when more
// work is added to the work list. The theory is that
// there wasn't enough work to do anyway, so we might
// as well let background marking take care of the
// work that is available.
if !gcParkAssist() {
goto retry
}
// At this point either background GC has satisfied
// this G's assist debt, or the GC cycle is over.
}
if traced {
traceGCMarkAssistDone()
}
}
// gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system
// stack. This is a separate function to make it easier to see that
// we're not capturing anything from the user stack, since the user
// stack may move while we're in this function.
//
// gcAssistAlloc1 indicates whether this assist completed the mark
// phase by setting gp.param to non-nil. This can't be communicated on
// the stack since it may move.
//
//go:systemstack
func gcAssistAlloc1(gp *g, scanWork int64) {
// Clear the flag indicating that this assist completed the
// mark phase.
gp.param = nil
if atomic.Load(&gcBlackenEnabled) == 0 {
// The gcBlackenEnabled check in malloc races with the
// store that clears it but an atomic check in every malloc
// would be a performance hit.
// Instead we recheck it here on the non-preemptable system
// stack to determine if we should perform an assist.
// GC is done, so ignore any remaining debt.
gp.gcAssistBytes = 0
return
}
// Track time spent in this assist. Since we're on the
// system stack, this is non-preemptible, so we can
// just measure start and end time.
startTime := nanotime()
decnwait := atomic.Xadd(&work.nwait, -1)
if decnwait == work.nproc {
println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
throw("nwait > work.nprocs")
}
// gcDrainN requires the caller to be preemptible.
casgstatus(gp, _Grunning, _Gwaiting)
gp.waitreason = waitReasonGCAssistMarking
// drain own cached work first in the hopes that it
// will be more cache friendly.
gcw := &getg().m.p.ptr().gcw
workDone := gcDrainN(gcw, scanWork)
casgstatus(gp, _Gwaiting, _Grunning)
// Record that we did this much scan work.
//
// Back out the number of bytes of assist credit that
// this scan work counts for. The "1+" is a poor man's
// round-up, to ensure this adds credit even if
// assistBytesPerWork is very low.
assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(workDone))
// If this is the last worker and we ran out of work,
// signal a completion point.
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: work.nwait=", incnwait,
"work.nproc=", work.nproc)
throw("work.nwait > work.nproc")
}
if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
// This has reached a background completion point. Set
// gp.param to a non-nil value to indicate this. It
// doesn't matter what we set it to (it just has to be
// a valid pointer).
gp.param = unsafe.Pointer(gp)
}
duration := nanotime() - startTime
_p_ := gp.m.p.ptr()
_p_.gcAssistTime += duration
if _p_.gcAssistTime > gcAssistTimeSlack {
atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime)
_p_.gcAssistTime = 0
}
}
// gcWakeAllAssists wakes all currently blocked assists. This is used
// at the end of a GC cycle. gcBlackenEnabled must be false to prevent
// new assists from going to sleep after this point.
func gcWakeAllAssists() {
lock(&work.assistQueue.lock)
list := work.assistQueue.q.popList()
injectglist(&list)
unlock(&work.assistQueue.lock)
}
// gcParkAssist puts the current goroutine on the assist queue and parks.
//
// gcParkAssist reports whether the assist is now satisfied. If it
// returns false, the caller must retry the assist.
//
//go:nowritebarrier
func gcParkAssist() bool {
lock(&work.assistQueue.lock)
// If the GC cycle finished while we were getting the lock,
// exit the assist. The cycle can't finish while we hold the
// lock.
if atomic.Load(&gcBlackenEnabled) == 0 {
unlock(&work.assistQueue.lock)
return true
}
gp := getg()
oldList := work.assistQueue.q
work.assistQueue.q.pushBack(gp)
// Recheck for background credit now that this G is in
// the queue, but can still back out. This avoids a
// race in case background marking has flushed more
// credit since we checked above.
if atomic.Loadint64(&gcController.bgScanCredit) > 0 {
work.assistQueue.q = oldList
if oldList.tail != 0 {
oldList.tail.ptr().schedlink.set(nil)
}
unlock(&work.assistQueue.lock)
return false
}
// Park.
goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceEvGoBlockGC, 2)
return true
}
// gcFlushBgCredit flushes scanWork units of background scan work
// credit. This first satisfies blocked assists on the
// work.assistQueue and then flushes any remaining credit to
// gcController.bgScanCredit.
//
// Write barriers are disallowed because this is used by gcDrain after
// it has ensured that all work is drained and this must preserve that
// condition.
//
//go:nowritebarrierrec
func gcFlushBgCredit(scanWork int64) {
if work.assistQueue.q.empty() {
// Fast path; there are no blocked assists. There's a
// small window here where an assist may add itself to
// the blocked queue and park. If that happens, we'll
// just get it on the next flush.
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
return
}
assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
scanBytes := int64(float64(scanWork) * assistBytesPerWork)
lock(&work.assistQueue.lock)
for !work.assistQueue.q.empty() && scanBytes > 0 {
gp := work.assistQueue.q.pop()
// Note that gp.gcAssistBytes is negative because gp
// is in debt. Think carefully about the signs below.
if scanBytes+gp.gcAssistBytes >= 0 {
// Satisfy this entire assist debt.
scanBytes += gp.gcAssistBytes
gp.gcAssistBytes = 0
// It's important that we *not* put gp in
// runnext. Otherwise, it's possible for user
// code to exploit the GC worker's high
// scheduler priority to get itself always run
// before other goroutines and always in the
// fresh quantum started by GC.
ready(gp, 0, false)
} else {
// Partially satisfy this assist.
gp.gcAssistBytes += scanBytes
scanBytes = 0
// As a heuristic, we move this assist to the
// back of the queue so that large assists
// can't clog up the assist queue and
// substantially delay small assists.
work.assistQueue.q.pushBack(gp)
break
}
}
if scanBytes > 0 {
// Convert from scan bytes back to work.
assistWorkPerByte := float64frombits(atomic.Load64(&gcController.assistWorkPerByte))
scanWork = int64(float64(scanBytes) * assistWorkPerByte)
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
}
unlock(&work.assistQueue.lock)
}
// scanstack scans gp's stack, greying all pointers found on the stack.
//
// scanstack will also shrink the stack if it is safe to do so. If it
// is not, it schedules a stack shrink for the next synchronous safe
// point.
//
// scanstack is marked go:systemstack because it must not be preempted
// while using a workbuf.
//
//go:nowritebarrier
//go:systemstack
func scanstack(gp *g, gcw *gcWork) {
if readgstatus(gp)&_Gscan == 0 {
print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n")
throw("scanstack - bad status")
}
switch readgstatus(gp) &^ _Gscan {
default:
print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
throw("mark - bad status")
case _Gdead:
return
case _Grunning:
print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
throw("scanstack: goroutine not stopped")
case _Grunnable, _Gsyscall, _Gwaiting:
// ok
}
if gp == getg() {
throw("can't scan our own stack")
}
if isShrinkStackSafe(gp) {
// Shrink the stack if not much of it is being used.
shrinkstack(gp)
} else {
// Otherwise, shrink the stack at the next sync safe point.
gp.preemptShrink = true
}
var state stackScanState
state.stack = gp.stack
if stackTraceDebug {
println("stack trace goroutine", gp.goid)
}
if debugScanConservative && gp.asyncSafePoint {
print("scanning async preempted goroutine ", gp.goid, " stack [", hex(gp.stack.lo), ",", hex(gp.stack.hi), ")\n")
}
// Scan the saved context register. This is effectively a live
// register that gets moved back and forth between the
// register and sched.ctxt without a write barrier.
if gp.sched.ctxt != nil {
scanblock(uintptr(unsafe.Pointer(&gp.sched.ctxt)), sys.PtrSize, &oneptrmask[0], gcw, &state)
}
// Scan the stack. Accumulate a list of stack objects.
scanframe := func(frame *stkframe, unused unsafe.Pointer) bool {
scanframeworker(frame, &state, gcw)
return true
}
gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, scanframe, nil, 0)
// Find additional pointers that point into the stack from the heap.
// Currently this includes defers and panics. See also function copystack.
// Find and trace all defer arguments.
tracebackdefers(gp, scanframe, nil)
// Find and trace other pointers in defer records.
for d := gp._defer; d != nil; d = d.link {
if d.fn != nil {
// tracebackdefers above does not scan the func value, which could
// be a stack allocated closure. See issue 30453.
scanblock(uintptr(unsafe.Pointer(&d.fn)), sys.PtrSize, &oneptrmask[0], gcw, &state)
}
if d.link != nil {
// The link field of a stack-allocated defer record might point
// to a heap-allocated defer record. Keep that heap record live.
scanblock(uintptr(unsafe.Pointer(&d.link)), sys.PtrSize, &oneptrmask[0], gcw, &state)
}
// Retain defers records themselves.
// Defer records might not be reachable from the G through regular heap
// tracing because the defer linked list might weave between the stack and the heap.
if d.heap {
scanblock(uintptr(unsafe.Pointer(&d)), sys.PtrSize, &oneptrmask[0], gcw, &state)
}
}
if gp._panic != nil {
// Panics are always stack allocated.
state.putPtr(uintptr(unsafe.Pointer(gp._panic)), false)
}
// Find and scan all reachable stack objects.
//
// The state's pointer queue prioritizes precise pointers over
// conservative pointers so that we'll prefer scanning stack
// objects precisely.
state.buildIndex()
for {
p, conservative := state.getPtr()
if p == 0 {
break
}
obj := state.findObject(p)
if obj == nil {
continue
}
t := obj.typ
if t == nil {
// We've already scanned this object.
continue
}
obj.setType(nil) // Don't scan it again.
if stackTraceDebug {
printlock()
print(" live stkobj at", hex(state.stack.lo+uintptr(obj.off)), "of type", t.string())
if conservative {
print(" (conservative)")
}
println()
printunlock()
}
gcdata := t.gcdata
var s *mspan
if t.kind&kindGCProg != 0 {
// This path is pretty unlikely, an object large enough
// to have a GC program allocated on the stack.
// We need some space to unpack the program into a straight
// bitmask, which we allocate/free here.
// TODO: it would be nice if there were a way to run a GC
// program without having to store all its bits. We'd have
// to change from a Lempel-Ziv style program to something else.
// Or we can forbid putting objects on stacks if they require
// a gc program (see issue 27447).
s = materializeGCProg(t.ptrdata, gcdata)
gcdata = (*byte)(unsafe.Pointer(s.startAddr))
}
b := state.stack.lo + uintptr(obj.off)
if conservative {
scanConservative(b, t.ptrdata, gcdata, gcw, &state)
} else {
scanblock(b, t.ptrdata, gcdata, gcw, &state)
}
if s != nil {
dematerializeGCProg(s)
}
}
// Deallocate object buffers.
// (Pointer buffers were all deallocated in the loop above.)
for state.head != nil {
x := state.head
state.head = x.next
if stackTraceDebug {
for i := 0; i < x.nobj; i++ {
obj := &x.obj[i]
if obj.typ == nil { // reachable
continue
}
println(" dead stkobj at", hex(gp.stack.lo+uintptr(obj.off)), "of type", obj.typ.string())
// Note: not necessarily really dead - only reachable-from-ptr dead.
}
}
x.nobj = 0
putempty((*workbuf)(unsafe.Pointer(x)))
}
if state.buf != nil || state.cbuf != nil || state.freeBuf != nil {
throw("remaining pointer buffers")
}
}
// Scan a stack frame: local variables and function arguments/results.
//go:nowritebarrier
func scanframeworker(frame *stkframe, state *stackScanState, gcw *gcWork) {
if _DebugGC > 1 && frame.continpc != 0 {
print("scanframe ", funcname(frame.fn), "\n")
}
isAsyncPreempt := frame.fn.valid() && frame.fn.funcID == funcID_asyncPreempt
isDebugCall := frame.fn.valid() && frame.fn.funcID == funcID_debugCallV1
if state.conservative || isAsyncPreempt || isDebugCall {
if debugScanConservative {
println("conservatively scanning function", funcname(frame.fn), "at PC", hex(frame.continpc))
}
// Conservatively scan the frame. Unlike the precise
// case, this includes the outgoing argument space
// since we may have stopped while this function was
// setting up a call.
//
// TODO: We could narrow this down if the compiler
// produced a single map per function of stack slots
// and registers that ever contain a pointer.
if frame.varp != 0 {
size := frame.varp - frame.sp
if size > 0 {
scanConservative(frame.sp, size, nil, gcw, state)
}
}
// Scan arguments to this frame.
if frame.arglen != 0 {
// TODO: We could pass the entry argument map
// to narrow this down further.
scanConservative(frame.argp, frame.arglen, nil, gcw, state)
}
if isAsyncPreempt || isDebugCall {
// This function's frame contained the
// registers for the asynchronously stopped
// parent frame. Scan the parent
// conservatively.
state.conservative = true
} else {
// We only wanted to scan those two frames
// conservatively. Clear the flag for future
// frames.
state.conservative = false
}
return
}
locals, args, objs := getStackMap(frame, &state.cache, false)
// Scan local variables if stack frame has been allocated.
if locals.n > 0 {
size := uintptr(locals.n) * sys.PtrSize
scanblock(frame.varp-size, size, locals.bytedata, gcw, state)
}
// Scan arguments.
if args.n > 0 {
scanblock(frame.argp, uintptr(args.n)*sys.PtrSize, args.bytedata, gcw, state)
}
// Add all stack objects to the stack object list.
if frame.varp != 0 {
// varp is 0 for defers, where there are no locals.
// In that case, there can't be a pointer to its args, either.
// (And all args would be scanned above anyway.)
for _, obj := range objs {
off := obj.off
base := frame.varp // locals base pointer
if off >= 0 {
base = frame.argp // arguments and return values base pointer
}
ptr := base + uintptr(off)
if ptr < frame.sp {
// object hasn't been allocated in the frame yet.
continue
}
if stackTraceDebug {
println("stkobj at", hex(ptr), "of type", obj.typ.string())
}
state.addObject(ptr, obj.typ)
}
}
}
type gcDrainFlags int
const (
gcDrainUntilPreempt gcDrainFlags = 1 << iota
gcDrainFlushBgCredit
gcDrainIdle
gcDrainFractional
)
// gcDrain scans roots and objects in work buffers, blackening grey
// objects until it is unable to get more work. It may return before
// GC is done; it's the caller's responsibility to balance work from
// other Ps.
//
// If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
// is set.
//
// If flags&gcDrainIdle != 0, gcDrain returns when there is other work
// to do.
//
// If flags&gcDrainFractional != 0, gcDrain self-preempts when
// pollFractionalWorkerExit() returns true. This implies
// gcDrainNoBlock.
//
// If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
// credit to gcController.bgScanCredit every gcCreditSlack units of
// scan work.
//
// gcDrain will always return if there is a pending STW.
//
//go:nowritebarrier
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
if !writeBarrier.needed {
throw("gcDrain phase incorrect")
}
gp := getg().m.curg
preemptible := flags&gcDrainUntilPreempt != 0
flushBgCredit := flags&gcDrainFlushBgCredit != 0
idle := flags&gcDrainIdle != 0
initScanWork := gcw.scanWork
// checkWork is the scan work before performing the next
// self-preempt check.
checkWork := int64(1<<63 - 1)
var check func() bool
if flags&(gcDrainIdle|gcDrainFractional) != 0 {
checkWork = initScanWork + drainCheckThreshold
if idle {
check = pollWork
} else if flags&gcDrainFractional != 0 {
check = pollFractionalWorkerExit
}
}
// Drain root marking jobs.
if work.markrootNext < work.markrootJobs {
// Stop if we're preemptible or if someone wants to STW.
for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job >= work.markrootJobs {
break
}
markroot(gcw, job)
if check != nil && check() {
goto done
}
}
}
// Drain heap marking jobs.
// Stop if we're preemptible or if someone wants to STW.
for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
// Try to keep work available on the global queue. We used to
// check if there were waiting workers, but it's better to
// just keep work available than to make workers wait. In the
// worst case, we'll do O(log(_WorkbufSize)) unnecessary
// balances.
if work.full == 0 {
gcw.balance()
}
b := gcw.tryGetFast()
if b == 0 {
b = gcw.tryGet()
if b == 0 {
// Flush the write barrier
// buffer; this may create
// more work.
wbBufFlush(nil, 0)
b = gcw.tryGet()
}
}
if b == 0 {
// Unable to get work.
break
}
scanobject(b, gcw)
// Flush background scan work credit to the global
// account if we've accumulated enough locally so
// mutator assists can draw on it.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
initScanWork = 0
}
checkWork -= gcw.scanWork
gcw.scanWork = 0
if checkWork <= 0 {
checkWork += drainCheckThreshold
if check != nil && check() {
break
}
}
}
}
done:
// Flush remaining scan work credit.
if gcw.scanWork > 0 {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
}
gcw.scanWork = 0
}
}
// gcDrainN blackens grey objects until it has performed roughly
// scanWork units of scan work or the G is preempted. This is
// best-effort, so it may perform less work if it fails to get a work
// buffer. Otherwise, it will perform at least n units of work, but
// may perform more because scanning is always done in whole object
// increments. It returns the amount of scan work performed.
//
// The caller goroutine must be in a preemptible state (e.g.,
// _Gwaiting) to prevent deadlocks during stack scanning. As a
// consequence, this must be called on the system stack.
//
//go:nowritebarrier
//go:systemstack
func gcDrainN(gcw *gcWork, scanWork int64) int64 {
if !writeBarrier.needed {
throw("gcDrainN phase incorrect")
}
// There may already be scan work on the gcw, which we don't
// want to claim was done by this call.
workFlushed := -gcw.scanWork
gp := getg().m.curg
for !gp.preempt && workFlushed+gcw.scanWork < scanWork {
// See gcDrain comment.
if work.full == 0 {
gcw.balance()
}
// This might be a good place to add prefetch code...
// if(wbuf.nobj > 4) {
// PREFETCH(wbuf->obj[wbuf.nobj - 3];
// }
//
b := gcw.tryGetFast()
if b == 0 {
b = gcw.tryGet()
if b == 0 {
// Flush the write barrier buffer;
// this may create more work.
wbBufFlush(nil, 0)
b = gcw.tryGet()
}
}
if b == 0 {
// Try to do a root job.
//
// TODO: Assists should get credit for this
// work.
if work.markrootNext < work.markrootJobs {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job < work.markrootJobs {
markroot(gcw, job)
continue
}
}
// No heap or root jobs.
break
}
scanobject(b, gcw)
// Flush background scan work credit.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
workFlushed += gcw.scanWork
gcw.scanWork = 0
}
}
// Unlike gcDrain, there's no need to flush remaining work
// here because this never flushes to bgScanCredit and
// gcw.dispose will flush any remaining work to scanWork.
return workFlushed + gcw.scanWork
}
// scanblock scans b as scanobject would, but using an explicit
// pointer bitmap instead of the heap bitmap.
//
// This is used to scan non-heap roots, so it does not update
// gcw.bytesMarked or gcw.scanWork.
//
// If stk != nil, possible stack pointers are also reported to stk.putPtr.
//go:nowritebarrier
func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork, stk *stackScanState) {
// Use local copies of original parameters, so that a stack trace
// due to one of the throws below shows the original block
// base and extent.
b := b0
n := n0
for i := uintptr(0); i < n; {
// Find bits for the next word.
bits := uint32(*addb(ptrmask, i/(sys.PtrSize*8)))
if bits == 0 {
i += sys.PtrSize * 8
continue
}
for j := 0; j < 8 && i < n; j++ {
if bits&1 != 0 {
// Same work as in scanobject; see comments there.
p := *(*uintptr)(unsafe.Pointer(b + i))
if p != 0 {
if obj, span, objIndex := findObject(p, b, i); obj != 0 {
greyobject(obj, b, i, span, gcw, objIndex)
} else if stk != nil && p >= stk.stack.lo && p < stk.stack.hi {
stk.putPtr(p, false)
}
}
}
bits >>= 1
i += sys.PtrSize
}
}
}
// scanobject scans the object starting at b, adding pointers to gcw.
// b must point to the beginning of a heap object or an oblet.
// scanobject consults the GC bitmap for the pointer mask and the
// spans for the size of the object.
//
//go:nowritebarrier
func scanobject(b uintptr, gcw *gcWork) {
// Find the bits for b and the size of the object at b.
//
// b is either the beginning of an object, in which case this
// is the size of the object to scan, or it points to an
// oblet, in which case we compute the size to scan below.
hbits := heapBitsForAddr(b)
s := spanOfUnchecked(b)
n := s.elemsize
if n == 0 {
throw("scanobject n == 0")
}
if n > maxObletBytes {
// Large object. Break into oblets for better
// parallelism and lower latency.
if b == s.base() {
// It's possible this is a noscan object (not
// from greyobject, but from other code
// paths), in which case we must *not* enqueue
// oblets since their bitmaps will be
// uninitialized.
if s.spanclass.noscan() {
// Bypass the whole scan.
gcw.bytesMarked += uint64(n)
return
}
// Enqueue the other oblets to scan later.
// Some oblets may be in b's scalar tail, but
// these will be marked as "no more pointers",
// so we'll drop out immediately when we go to
// scan those.
for oblet := b + maxObletBytes; oblet < s.base()+s.elemsize; oblet += maxObletBytes {
if !gcw.putFast(oblet) {
gcw.put(oblet)
}
}
}
// Compute the size of the oblet. Since this object
// must be a large object, s.base() is the beginning
// of the object.
n = s.base() + s.elemsize - b
if n > maxObletBytes {
n = maxObletBytes
}
}
var i uintptr
for i = 0; i < n; i += sys.PtrSize {
// Find bits for this word.
if i != 0 {
// Avoid needless hbits.next() on last iteration.
hbits = hbits.next()
}
// Load bits once. See CL 22712 and issue 16973 for discussion.
bits := hbits.bits()
if bits&bitScan == 0 {
break // no more pointers in this object
}
if bits&bitPointer == 0 {
continue // not a pointer
}
// Work here is duplicated in scanblock and above.
// If you make changes here, make changes there too.
obj := *(*uintptr)(unsafe.Pointer(b + i))
// At this point we have extracted the next potential pointer.
// Quickly filter out nil and pointers back to the current object.
if obj != 0 && obj-b >= n {
// Test if obj points into the Go heap and, if so,
// mark the object.
//
// Note that it's possible for findObject to
// fail if obj points to a just-allocated heap
// object because of a race with growing the
// heap. In this case, we know the object was
// just allocated and hence will be marked by
// allocation itself.
if obj, span, objIndex := findObject(obj, b, i); obj != 0 {
greyobject(obj, b, i, span, gcw, objIndex)
}
}
}
gcw.bytesMarked += uint64(n)
gcw.scanWork += int64(i)
}
// scanConservative scans block [b, b+n) conservatively, treating any
// pointer-like value in the block as a pointer.
//
// If ptrmask != nil, only words that are marked in ptrmask are
// considered as potential pointers.
//
// If state != nil, it's assumed that [b, b+n) is a block in the stack
// and may contain pointers to stack objects.
func scanConservative(b, n uintptr, ptrmask *uint8, gcw *gcWork, state *stackScanState) {
if debugScanConservative {
printlock()
print("conservatively scanning [", hex(b), ",", hex(b+n), ")\n")
hexdumpWords(b, b+n, func(p uintptr) byte {
if ptrmask != nil {
word := (p - b) / sys.PtrSize
bits := *addb(ptrmask, word/8)
if (bits>>(word%8))&1 == 0 {
return '$'
}
}
val := *(*uintptr)(unsafe.Pointer(p))
if state != nil && state.stack.lo <= val && val < state.stack.hi {
return '@'
}
span := spanOfHeap(val)
if span == nil {
return ' '
}
idx := span.objIndex(val)
if span.isFree(idx) {
return ' '
}
return '*'
})
printunlock()
}
for i := uintptr(0); i < n; i += sys.PtrSize {
if ptrmask != nil {
word := i / sys.PtrSize
bits := *addb(ptrmask, word/8)
if bits == 0 {
// Skip 8 words (the loop increment will do the 8th)
//
// This must be the first time we've
// seen this word of ptrmask, so i
// must be 8-word-aligned, but check
// our reasoning just in case.
if i%(sys.PtrSize*8) != 0 {
throw("misaligned mask")
}
i += sys.PtrSize*8 - sys.PtrSize
continue
}
if (bits>>(word%8))&1 == 0 {
continue
}
}
val := *(*uintptr)(unsafe.Pointer(b + i))
// Check if val points into the stack.
if state != nil && state.stack.lo <= val && val < state.stack.hi {
// val may point to a stack object. This
// object may be dead from last cycle and
// hence may contain pointers to unallocated
// objects, but unlike heap objects we can't
// tell if it's already dead. Hence, if all
// pointers to this object are from
// conservative scanning, we have to scan it
// defensively, too.
state.putPtr(val, true)
continue
}
// Check if val points to a heap span.
span := spanOfHeap(val)
if span == nil {
continue
}
// Check if val points to an allocated object.
idx := span.objIndex(val)
if span.isFree(idx) {
continue
}
// val points to an allocated object. Mark it.
obj := span.base() + idx*span.elemsize
greyobject(obj, b, i, span, gcw, idx)
}
}
// Shade the object if it isn't already.
// The object is not nil and known to be in the heap.
// Preemption must be disabled.
//go:nowritebarrier
func shade(b uintptr) {
if obj, span, objIndex := findObject(b, 0, 0); obj != 0 {
gcw := &getg().m.p.ptr().gcw
greyobject(obj, 0, 0, span, gcw, objIndex)
}
}
// obj is the start of an object with mark mbits.
// If it isn't already marked, mark it and enqueue into gcw.
// base and off are for debugging only and could be removed.
//
// See also wbBufFlush1, which partially duplicates this logic.
//
//go:nowritebarrierrec
func greyobject(obj, base, off uintptr, span *mspan, gcw *gcWork, objIndex uintptr) {
// obj should be start of allocation, and so must be at least pointer-aligned.
if obj&(sys.PtrSize-1) != 0 {
throw("greyobject: obj not pointer-aligned")
}
mbits := span.markBitsForIndex(objIndex)
if useCheckmark {
if setCheckmark(obj, base, off, mbits) {
// Already marked.
return
}
} else {
if debug.gccheckmark > 0 && span.isFree(objIndex) {
print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
gcDumpObject("base", base, off)
gcDumpObject("obj", obj, ^uintptr(0))
getg().m.traceback = 2
throw("marking free object")
}
// If marked we have nothing to do.
if mbits.isMarked() {
return
}
mbits.setMarked()
// Mark span.
arena, pageIdx, pageMask := pageIndexOf(span.base())
if arena.pageMarks[pageIdx]&pageMask == 0 {
atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
}
// If this is a noscan object, fast-track it to black
// instead of greying it.
if span.spanclass.noscan() {
gcw.bytesMarked += uint64(span.elemsize)
return
}
}
// Queue the obj for scanning. The PREFETCH(obj) logic has been removed but
// seems like a nice optimization that can be added back in.
// There needs to be time between the PREFETCH and the use.
// Previously we put the obj in an 8 element buffer that is drained at a rate
// to give the PREFETCH time to do its work.
// Use of PREFETCHNTA might be more appropriate than PREFETCH
if !gcw.putFast(obj) {
gcw.put(obj)
}
}
// gcDumpObject dumps the contents of obj for debugging and marks the
// field at byte offset off in obj.
func gcDumpObject(label string, obj, off uintptr) {
s := spanOf(obj)
print(label, "=", hex(obj))
if s == nil {
print(" s=nil\n")
return
}
print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.spanclass=", s.spanclass, " s.elemsize=", s.elemsize, " s.state=")
if state := s.state.get(); 0 <= state && int(state) < len(mSpanStateNames) {
print(mSpanStateNames[state], "\n")
} else {
print("unknown(", state, ")\n")
}
skipped := false
size := s.elemsize
if s.state.get() == mSpanManual && size == 0 {
// We're printing something from a stack frame. We
// don't know how big it is, so just show up to an
// including off.
size = off + sys.PtrSize
}
for i := uintptr(0); i < size; i += sys.PtrSize {
// For big objects, just print the beginning (because
// that usually hints at the object's type) and the
// fields around off.
if !(i < 128*sys.PtrSize || off-16*sys.PtrSize < i && i < off+16*sys.PtrSize) {
skipped = true
continue
}
if skipped {
print(" ...\n")
skipped = false
}
print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + i))))
if i == off {
print(" <==")
}
print("\n")
}
if skipped {
print(" ...\n")
}
}
// gcmarknewobject marks a newly allocated object black. obj must
// not contain any non-nil pointers.
//
// This is nosplit so it can manipulate a gcWork without preemption.
//
//go:nowritebarrier
//go:nosplit
func gcmarknewobject(span *mspan, obj, size, scanSize uintptr) {
if useCheckmark { // The world should be stopped so this should not happen.
throw("gcmarknewobject called while doing checkmark")
}
// Mark object.
objIndex := span.objIndex(obj)
span.markBitsForIndex(objIndex).setMarked()
// Mark span.
arena, pageIdx, pageMask := pageIndexOf(span.base())
if arena.pageMarks[pageIdx]&pageMask == 0 {
atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
}
gcw := &getg().m.p.ptr().gcw
gcw.bytesMarked += uint64(size)
gcw.scanWork += int64(scanSize)
}
// gcMarkTinyAllocs greys all active tiny alloc blocks.
//
// The world must be stopped.
func gcMarkTinyAllocs() {
assertWorldStopped()
for _, p := range allp {
c := p.mcache
if c == nil || c.tiny == 0 {
continue
}
_, span, objIndex := findObject(c.tiny, 0, 0)
gcw := &p.gcw
greyobject(c.tiny, 0, 0, span, gcw, objIndex)
}
}