sync: move lock linearity test and treat it like a performance test

This change moves test/locklinear.go into the sync package tests, and
adds a bit of infrastructure since there are other linearity-checking
tests that could benefit from it too. This infrastructure is also
different than what test/locklinear.go does: instead of trying really
hard to get at least one success, we instead treat this like a
performance test and look for a significant difference via a t-test.

This makes the methodology behind the tests more rigorous, and should
reduce flakiness as transient noise should produce an insignificant
result. A follow-up CL does more to make these tests even more robust.

For #32986.

Change-Id: I408c5f643962b70ea708930edb4ac9df1c6123ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/411396
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/testenv/testenv.go

@@ -16,6 +16,7 @@ import (
 	"flag"
 	"fmt"
 	"internal/cfg"
+	"internal/testmath"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -463,3 +464,67 @@ func RunWithTimeout(t testing.TB, cmd *exec.Cmd) ([]byte, error) {
 
 	return b.Bytes(), err
 }
+
+// CheckLinear checks if the function produced by f scales linearly.
+//
+// f must accept a scale factor which causes the input to the function it
+// produces to scale by that factor.
+func CheckLinear(t *testing.T, f func(scale float64) func(*testing.B)) {
+	MustHaveExec(t)
+
+	if os.Getenv("GO_PERF_UNIT_TEST") == "" {
+		// Invoke the same test as a subprocess with the GO_PERF_UNIT_TEST environment variable set.
+		// We create a subprocess for two reasons:
+		//
+		//   1. There's no other way to set the benchmarking parameters of testing.Benchmark.
+		//   2. Since we're effectively running a performance test, running in a subprocess grants
+		//      us a little bit more isolation than using the same process.
+		//
+		// As an alternative, we could fairly easily reimplement the timing code in testing.Benchmark,
+		// but a subprocess is just as easy to create.
+
+		selfCmd := CleanCmdEnv(exec.Command(os.Args[0], "-test.v", fmt.Sprintf("-test.run=^%s$", t.Name()), "-test.benchtime=1x"))
+		selfCmd.Env = append(selfCmd.Env, "GO_PERF_UNIT_TEST=1")
+		output, err := RunWithTimeout(t, selfCmd)
+		if err != nil {
+			t.Error(err)
+			t.Logf("--- subprocess output ---\n%s", string(output))
+		}
+		if bytes.Contains(output, []byte("insignificant result")) {
+			t.Skip("insignificant result")
+		}
+		return
+	}
+
+	// Pick a reasonable sample count.
+	const count = 10
+
+	// Collect samples for scale factor 1.
+	x1 := make([]testing.BenchmarkResult, 0, count)
+	for i := 0; i < count; i++ {
+		x1 = append(x1, testing.Benchmark(f(1.0)))
+	}
+
+	// Collect samples for scale factor 2.
+	x2 := make([]testing.BenchmarkResult, 0, count)
+	for i := 0; i < count; i++ {
+		x2 = append(x2, testing.Benchmark(f(2.0)))
+	}
+
+	// Run a t-test on the results.
+	r1 := testmath.BenchmarkResults(x1)
+	r2 := testmath.BenchmarkResults(x2)
+	result, err := testmath.TwoSampleWelchTTest(r1, r2, testmath.LocationDiffers)
+	if err != nil {
+		t.Fatalf("failed to run t-test: %v", err)
+	}
+	if result.P > 0.005 {
+		// Insignificant result.
+		t.Skip("insignificant result")
+	}
+
+	// Let ourselves be within 3x; 2x is too strict.
+	if m1, m2 := r1.Mean(), r2.Mean(); 3.0*m1 < m2 {
+		t.Fatalf("failure to scale linearly: µ_1=%s µ_2=%s p=%f", time.Duration(m1), time.Duration(m2), result.P)
+	}
+}

src/internal/testmath/bench.go

@@ -0,0 +1,38 @@
+// Copyright 2022 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 testmath
+
+import (
+	"math"
+	"testing"
+	"time"
+)
+
+type BenchmarkResults []testing.BenchmarkResult
+
+func (b BenchmarkResults) Weight() float64 {
+	var weight int
+	for _, r := range b {
+		weight += r.N
+	}
+	return float64(weight)
+}
+
+func (b BenchmarkResults) Mean() float64 {
+	var dur time.Duration
+	for _, r := range b {
+		dur += r.T * time.Duration(r.N)
+	}
+	return float64(dur) / b.Weight()
+}
+
+func (b BenchmarkResults) Variance() float64 {
+	var num float64
+	mean := b.Mean()
+	for _, r := range b {
+		num += math.Pow(float64(r.T)-mean, 2) * float64(r.N)
+	}
+	return float64(num) / b.Weight()
+}

src/sync/mutex_test.go

@@ -333,3 +333,93 @@ func BenchmarkMutexSpin(b *testing.B) {
 		}
 	})
 }
+
+const runtimeSemaHashTableSize = 251 // known size of runtime hash table
+
+func TestMutexLinearOne(t *testing.T) {
+	testenv.CheckLinear(t, func(scale float64) func(*testing.B) {
+		n := int(1000 * scale)
+		return func(b *testing.B) {
+			ch := make(chan int)
+			locks := make([]RWMutex, runtimeSemaHashTableSize+1)
+			for i := 0; i < n; i++ {
+				go func() {
+					locks[0].Lock()
+					ch <- 1
+				}()
+			}
+			time.Sleep(1 * time.Millisecond)
+
+			go func() {
+				for j := 0; j < n; j++ {
+					locks[1].Lock()
+					locks[runtimeSemaHashTableSize].Lock()
+					locks[1].Unlock()
+					runtime.Gosched()
+					locks[runtimeSemaHashTableSize].Unlock()
+				}
+			}()
+
+			for j := 0; j < n; j++ {
+				locks[1].Lock()
+				locks[runtimeSemaHashTableSize].Lock()
+				locks[1].Unlock()
+				runtime.Gosched()
+				locks[runtimeSemaHashTableSize].Unlock()
+			}
+
+			for i := 0; i < n; i++ {
+				<-ch
+				locks[0].Unlock()
+			}
+		}
+	})
+}
+
+func TestMutexLinearMany(t *testing.T) {
+	if runtime.GOARCH == "arm" && os.Getenv("GOARM") == "5" {
+		// stressLockMany reliably fails on the linux-arm-arm5spacemonkey
+		// builder. See https://golang.org/issue/24221.
+		return
+	}
+	testenv.CheckLinear(t, func(scale float64) func(*testing.B) {
+		n := int(1000 * scale)
+		return func(b *testing.B) {
+			locks := make([]RWMutex, n*runtimeSemaHashTableSize+1)
+
+			var wg WaitGroup
+			for i := 0; i < n; i++ {
+				wg.Add(1)
+				go func(i int) {
+					locks[(i+1)*runtimeSemaHashTableSize].Lock()
+					wg.Done()
+					locks[(i+1)*runtimeSemaHashTableSize].Lock()
+					locks[(i+1)*runtimeSemaHashTableSize].Unlock()
+				}(i)
+			}
+			wg.Wait()
+
+			go func() {
+				for j := 0; j < n; j++ {
+					locks[1].Lock()
+					locks[0].Lock()
+					locks[1].Unlock()
+					runtime.Gosched()
+					locks[0].Unlock()
+				}
+			}()
+
+			for j := 0; j < n; j++ {
+				locks[1].Lock()
+				locks[0].Lock()
+				locks[1].Unlock()
+				runtime.Gosched()
+				locks[0].Unlock()
+			}
+
+			for i := 0; i < n; i++ {
+				locks[(i+1)*runtimeSemaHashTableSize].Unlock()
+			}
+		}
+	})
+}