runtime: Profile goroutines holding contended mutexes.

runtime.SetMutexProfileFraction(n int) will capture 1/n-th of stack
traces of goroutines holding contended mutexes if n > 0. From runtime/pprof,
pprot.Lookup("mutex").WriteTo writes the accumulated
stack traces to w (in essentially the same format that blocking
profiling uses).

Change-Id: Ie0b54fa4226853d99aa42c14cb529ae586a8335a
Reviewed-on: https://go-review.googlesource.com/29650
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/go/test.go

@@ -238,6 +238,15 @@ profile the tests during execution::
 	    To profile all memory allocations, use -test.memprofilerate=1
 	    and pass --alloc_space flag to the pprof tool.
 
+	-mutexprofile mutex.out
+	    Write a mutex contention profile to the specified file
+	    when all tests are complete.
+	    Writes test binary as -c would.
+
+	-mutexprofilefraction n
+ 	    Sample 1 in n stack traces of goroutines holding a
+	    contended mutex.
+
 	-outputdir directory
 	    Place output files from profiling in the specified directory,
 	    by default the directory in which "go test" is running.

src/cmd/go/testflag.go

@@ -50,6 +50,8 @@ var testFlagDefn = []*testFlagSpec{
 	{name: "memprofilerate", passToTest: true},
 	{name: "blockprofile", passToTest: true},
 	{name: "blockprofilerate", passToTest: true},
+	{name: "mutexprofile", passToTest: true},
+	{name: "mutexprofilefraction", passToTest: true},
 	{name: "outputdir", passToTest: true},
 	{name: "parallel", passToTest: true},
 	{name: "run", passToTest: true},
@@ -152,7 +154,7 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 			case "blockprofile", "cpuprofile", "memprofile":
 				testProfile = true
 				testNeedBinary = true
-			case "trace":
+			case "mutexprofile", "trace":
 				testProfile = true
 			case "coverpkg":
 				testCover = true

src/cmd/internal/pprof/profile/legacy_profile.go

@@ -679,20 +679,32 @@ func scaleHeapSample(count, size, rate int64) (int64, int64) {
 	return int64(float64(count) * scale), int64(float64(size) * scale)
 }
 
-// parseContention parses a contentionz profile and returns a newly
-// populated Profile.
-func parseContention(b []byte) (p *Profile, err error) {
+// parseContention parses a mutex or contention profile. There are 2 cases:
+// "--- contentionz " for legacy C++ profiles (and backwards compatibility)
+// "--- mutex:" or "--- contention:" for profiles generated by the Go runtime.
+// This code converts the text output from runtime into a *Profile. (In the future
+// the runtime might write a serialized Profile directly making this unnecessary.)
+func parseContention(b []byte) (*Profile, error) {
 	r := bytes.NewBuffer(b)
 	l, err := r.ReadString('\n')
 	if err != nil {
 		return nil, errUnrecognized
 	}
-
-	if !strings.HasPrefix(l, "--- contention") {
-		return nil, errUnrecognized
+	if strings.HasPrefix(l, "--- contentionz ") {
+		return parseCppContention(r)
+	} else if strings.HasPrefix(l, "--- mutex:") {
+		return parseCppContention(r)
+	} else if strings.HasPrefix(l, "--- contention:") {
+		return parseCppContention(r)
 	}
+	return nil, errUnrecognized
+}
 
-	p = &Profile{
+// parseCppContention parses the output from synchronization_profiling.cc
+// for backward compatibility, and the compatible (non-debug) block profile
+// output from the Go runtime.
+func parseCppContention(r *bytes.Buffer) (*Profile, error) {
+	p := &Profile{
 		PeriodType: &ValueType{Type: "contentions", Unit: "count"},
 		Period:     1,
 		SampleType: []*ValueType{
@@ -702,6 +714,8 @@ func parseContention(b []byte) (p *Profile, err error) {
 	}
 
 	var cpuHz int64
+	var l string
+	var err error
 	// Parse text of the form "attribute = value" before the samples.
 	const delimiter = "="
 	for {

src/runtime/mgc.go

@@ -932,7 +932,7 @@ func gcStart(mode gcMode, forceTrigger bool) {
 	// another thread.
 	useStartSema := mode == gcBackgroundMode
 	if useStartSema {
-		semacquire(&work.startSema, false)
+		semacquire(&work.startSema, 0)
 		// Re-check transition condition under transition lock.
 		if !gcShouldStart(forceTrigger) {
 			semrelease(&work.startSema)
@@ -953,7 +953,7 @@ func gcStart(mode gcMode, forceTrigger bool) {
 	}
 
 	// Ok, we're doing it!  Stop everybody else
-	semacquire(&worldsema, false)
+	semacquire(&worldsema, 0)
 
 	if trace.enabled {
 		traceGCStart()
@@ -1063,7 +1063,7 @@ func gcStart(mode gcMode, forceTrigger bool) {
 // by mark termination.
 func gcMarkDone() {
 top:
-	semacquire(&work.markDoneSema, false)
+	semacquire(&work.markDoneSema, 0)
 
 	// Re-check transition condition under transition lock.
 	if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {

src/runtime/mprof.go

@@ -22,6 +22,7 @@ const (
 	// profile types
 	memProfile bucketType = 1 + iota
 	blockProfile
+	mutexProfile
 
 	// size of bucket hash table
 	buckHashSize = 179999
@@ -47,7 +48,7 @@ type bucketType int
 type bucket struct {
 	next    *bucket
 	allnext *bucket
-	typ     bucketType // memBucket or blockBucket
+	typ     bucketType // memBucket or blockBucket (includes mutexProfile)
 	hash    uintptr
 	size    uintptr
 	nstk    uintptr
@@ -87,7 +88,7 @@ type memRecord struct {
 }
 
 // A blockRecord is the bucket data for a bucket of type blockProfile,
-// part of the blocking profile.
+// which is used in blocking and mutex profiles.
 type blockRecord struct {
 	count  int64
 	cycles int64
@@ -96,6 +97,7 @@ type blockRecord struct {
 var (
 	mbuckets  *bucket // memory profile buckets
 	bbuckets  *bucket // blocking profile buckets
+	xbuckets  *bucket // mutex profile buckets
 	buckhash  *[179999]*bucket
 	bucketmem uintptr
 )
@@ -108,7 +110,7 @@ func newBucket(typ bucketType, nstk int) *bucket {
 		throw("invalid profile bucket type")
 	case memProfile:
 		size += unsafe.Sizeof(memRecord{})
-	case blockProfile:
+	case blockProfile, mutexProfile:
 		size += unsafe.Sizeof(blockRecord{})
 	}
 
@@ -136,7 +138,7 @@ func (b *bucket) mp() *memRecord {
 
 // bp returns the blockRecord associated with the blockProfile bucket b.
 func (b *bucket) bp() *blockRecord {
-	if b.typ != blockProfile {
+	if b.typ != blockProfile && b.typ != mutexProfile {
 		throw("bad use of bucket.bp")
 	}
 	data := add(unsafe.Pointer(b), unsafe.Sizeof(*b)+b.nstk*unsafe.Sizeof(uintptr(0)))
@@ -188,6 +190,9 @@ func stkbucket(typ bucketType, size uintptr, stk []uintptr, alloc bool) *bucket
 	if typ == memProfile {
 		b.allnext = mbuckets
 		mbuckets = b
+	} else if typ == mutexProfile {
+		b.allnext = xbuckets
+		xbuckets = b
 	} else {
 		b.allnext = bbuckets
 		bbuckets = b
@@ -292,10 +297,20 @@ func blockevent(cycles int64, skip int) {
 	if cycles <= 0 {
 		cycles = 1
 	}
+	if blocksampled(cycles) {
+		saveblockevent(cycles, skip+1, blockProfile, &blockprofilerate)
+	}
+}
+
+func blocksampled(cycles int64) bool {
 	rate := int64(atomic.Load64(&blockprofilerate))
 	if rate <= 0 || (rate > cycles && int64(fastrand())%rate > cycles) {
-		return
+		return false
 	}
+	return true
+}
+
+func saveblockevent(cycles int64, skip int, which bucketType, ratep *uint64) {
 	gp := getg()
 	var nstk int
 	var stk [maxStack]uintptr
@@ -305,12 +320,40 @@ func blockevent(cycles int64, skip int) {
 		nstk = gcallers(gp.m.curg, skip, stk[:])
 	}
 	lock(&proflock)
-	b := stkbucket(blockProfile, 0, stk[:nstk], true)
+	b := stkbucket(which, 0, stk[:nstk], true)
 	b.bp().count++
 	b.bp().cycles += cycles
 	unlock(&proflock)
 }
 
+var mutexprofilerate uint64 // fraction sampled
+
+// SetMutexProfileFraction controls the fraction of mutex contention events
+// that are reported in the mutex profile. On average 1/rate events are
+// reported. The previous rate is returned.
+//
+// To turn off profiling entirely, pass rate 0.
+// To just read the current rate, pass rate -1.
+// (For n>1 the details of sampling may change.)
+func SetMutexProfileFraction(rate int) int {
+	if rate < 0 {
+		return int(mutexprofilerate)
+	}
+	old := mutexprofilerate
+	atomic.Store64(&mutexprofilerate, uint64(rate))
+	return int(old)
+}
+
+//go:linkname mutexevent sync.event
+func mutexevent(cycles int64, skip int) {
+	rate := int64(atomic.Load64(&mutexprofilerate))
+	// TODO(pjw): measure impact of always calling fastrand vs using something
+	// like malloc.go:nextSample()
+	if rate > 0 && int64(fastrand())%rate == 0 {
+		saveblockevent(cycles, skip+1, mutexProfile, &mutexprofilerate)
+	}
+}
+
 // Go interface to profile data.
 
 // A StackRecord describes a single execution stack.
@@ -507,6 +550,35 @@ func BlockProfile(p []BlockProfileRecord) (n int, ok bool) {
 	return
 }
 
+// MutexProfile returns n, the number of records in the current mutex profile.
+// If len(p) >= n, MutexProfile copies the profile into p and returns n, true.
+// Otherwise, MutexProfile does not change p, and returns n, false.
+//
+// Most clients should use the runtime/pprof package
+// instead of calling MutexProfile directly.
+func MutexProfile(p []BlockProfileRecord) (n int, ok bool) {
+	lock(&proflock)
+	for b := xbuckets; b != nil; b = b.allnext {
+		n++
+	}
+	if n <= len(p) {
+		ok = true
+		for b := xbuckets; b != nil; b = b.allnext {
+			bp := b.bp()
+			r := &p[0]
+			r.Count = int64(bp.count)
+			r.Cycles = bp.cycles
+			i := copy(r.Stack0[:], b.stk())
+			for ; i < len(r.Stack0); i++ {
+				r.Stack0[i] = 0
+			}
+			p = p[1:]
+		}
+	}
+	unlock(&proflock)
+	return
+}
+
 // ThreadCreateProfile returns n, the number of records in the thread creation profile.
 // If len(p) >= n, ThreadCreateProfile copies the profile into p and returns n, true.
 // If len(p) < n, ThreadCreateProfile does not change p and returns n, false.

src/runtime/pprof/pprof.go

@@ -99,6 +99,7 @@ import (
 //	heap         - a sampling of all heap allocations
 //	threadcreate - stack traces that led to the creation of new OS threads
 //	block        - stack traces that led to blocking on synchronization primitives
+//	mutex        - stack traces of holders of contended mutexes
 //
 // These predefined profiles maintain themselves and panic on an explicit
 // Add or Remove method call.
@@ -152,6 +153,12 @@ var blockProfile = &Profile{
 	write: writeBlock,
 }
 
+var mutexProfile = &Profile{
+	name:  "mutex",
+	count: countMutex,
+	write: writeMutex,
+}
+
 func lockProfiles() {
 	profiles.mu.Lock()
 	if profiles.m == nil {
@@ -161,6 +168,7 @@ func lockProfiles() {
 			"threadcreate": threadcreateProfile,
 			"heap":         heapProfile,
 			"block":        blockProfile,
+			"mutex":        mutexProfile,
 		}
 	}
 }
@@ -729,6 +737,12 @@ func countBlock() int {
 	return n
 }
 
+// countMutex returns the number of records in the mutex profile.
+func countMutex() int {
+	n, _ := runtime.MutexProfile(nil)
+	return n
+}
+
 // writeBlock writes the current blocking profile to w.
 func writeBlock(w io.Writer, debug int) error {
 	var p []runtime.BlockProfileRecord
@@ -772,4 +786,49 @@ func writeBlock(w io.Writer, debug int) error {
 	return b.Flush()
 }
 
+// writeMutex writes the current mutex profile to w.
+func writeMutex(w io.Writer, debug int) error {
+	// TODO(pjw): too much common code with writeBlock. FIX!
+	var p []runtime.BlockProfileRecord
+	n, ok := runtime.MutexProfile(nil)
+	for {
+		p = make([]runtime.BlockProfileRecord, n+50)
+		n, ok = runtime.MutexProfile(p)
+		if ok {
+			p = p[:n]
+			break
+		}
+	}
+
+	sort.Slice(p, func(i, j int) bool { return p[i].Cycles > p[j].Cycles })
+
+	b := bufio.NewWriter(w)
+	var tw *tabwriter.Writer
+	w = b
+	if debug > 0 {
+		tw = tabwriter.NewWriter(w, 1, 8, 1, '\t', 0)
+		w = tw
+	}
+
+	fmt.Fprintf(w, "--- mutex:\n")
+	fmt.Fprintf(w, "cycles/second=%v\n", runtime_cyclesPerSecond())
+	fmt.Fprintf(w, "sampling period=%d\n", runtime.SetMutexProfileFraction(-1))
+	for i := range p {
+		r := &p[i]
+		fmt.Fprintf(w, "%v %v @", r.Cycles, r.Count)
+		for _, pc := range r.Stack() {
+			fmt.Fprintf(w, " %#x", pc)
+		}
+		fmt.Fprint(w, "\n")
+		if debug > 0 {
+			printStackRecord(w, r.Stack(), true)
+		}
+	}
+
+	if tw != nil {
+		tw.Flush()
+	}
+	return b.Flush()
+}
+
 func runtime_cyclesPerSecond() int64

src/runtime/pprof/pprof_test.go

@@ -592,6 +592,42 @@ func blockCond() {
 	mu.Unlock()
 }
 
+func TestMutexProfile(t *testing.T) {
+	old := runtime.SetMutexProfileFraction(1)
+	defer runtime.SetMutexProfileFraction(old)
+	if old != 0 {
+		t.Fatalf("need MutexProfileRate 0, got %d", old)
+	}
+
+	blockMutex()
+
+	var w bytes.Buffer
+	Lookup("mutex").WriteTo(&w, 1)
+	prof := w.String()
+
+	if !strings.HasPrefix(prof, "--- mutex:\ncycles/second=") {
+		t.Errorf("Bad profile header:\n%v", prof)
+	}
+	prof = strings.Trim(prof, "\n")
+	lines := strings.Split(prof, "\n")
+	if len(lines) != 6 {
+		t.Errorf("expected 6 lines, got %d %q\n%s", len(lines), prof, prof)
+	}
+	if len(lines) < 6 {
+		return
+	}
+	// checking that the line is like "35258904 1 @ 0x48288d 0x47cd28 0x458931"
+	r2 := `^\d+ 1 @(?: 0x[[:xdigit:]]+)+`
+	//r2 := "^[0-9]+ 1 @ 0x[0-9a-f x]+$"
+	if ok, err := regexp.MatchString(r2, lines[3]); err != nil || !ok {
+		t.Errorf("%q didn't match %q", lines[3], r2)
+	}
+	r3 := "^#.*runtime/pprof_test.blockMutex.*$"
+	if ok, err := regexp.MatchString(r3, lines[5]); err != nil || !ok {
+		t.Errorf("%q didn't match %q", lines[5], r3)
+	}
+}
+
 func func1(c chan int) { <-c }
 func func2(c chan int) { <-c }
 func func3(c chan int) { <-c }

src/runtime/proc.go

@@ -923,7 +923,7 @@ func restartg(gp *g) {
 // in panic or being exited, this may not reliably stop all
 // goroutines.
 func stopTheWorld(reason string) {
-	semacquire(&worldsema, false)
+	semacquire(&worldsema, 0)
 	getg().m.preemptoff = reason
 	systemstack(stopTheWorldWithSema)
 }
@@ -946,7 +946,7 @@ var worldsema uint32 = 1
 // preemption first and then should stopTheWorldWithSema on the system
 // stack:
 //
-//	semacquire(&worldsema, false)
+//	semacquire(&worldsema, 0)
 //	m.preemptoff = "reason"
 //	systemstack(stopTheWorldWithSema)
 //

src/runtime/runtime2.go

@@ -256,6 +256,7 @@ type sudog struct {
 	// The following fields are never accessed concurrently.
 	// waitlink is only accessed by g.
 
+	acquiretime int64
 	releasetime int64
 	ticket      uint32
 	waitlink    *sudog // g.waiting list

src/runtime/sema.go

@@ -44,12 +44,12 @@ var semtable [semTabSize]struct {
 
 //go:linkname sync_runtime_Semacquire sync.runtime_Semacquire
 func sync_runtime_Semacquire(addr *uint32) {
-	semacquire(addr, true)
+	semacquire(addr, semaBlockProfile)
 }
 
 //go:linkname net_runtime_Semacquire net.runtime_Semacquire
 func net_runtime_Semacquire(addr *uint32) {
-	semacquire(addr, true)
+	semacquire(addr, semaBlockProfile)
 }
 
 //go:linkname sync_runtime_Semrelease sync.runtime_Semrelease
@@ -57,6 +57,11 @@ func sync_runtime_Semrelease(addr *uint32) {
 	semrelease(addr)
 }
 
+//go:linkname sync_runtime_SemacquireMutex sync.runtime_SemacquireMutex
+func sync_runtime_SemacquireMutex(addr *uint32) {
+	semacquire(addr, semaBlockProfile|semaMutexProfile)
+}
+
 //go:linkname net_runtime_Semrelease net.runtime_Semrelease
 func net_runtime_Semrelease(addr *uint32) {
 	semrelease(addr)
@@ -69,8 +74,15 @@ func readyWithTime(s *sudog, traceskip int) {
 	goready(s.g, traceskip)
 }
 
+type semaProfileFlags int
+
+const (
+	semaBlockProfile semaProfileFlags = 1 << iota
+	semaMutexProfile
+)
+
 // Called from runtime.
-func semacquire(addr *uint32, profile bool) {
+func semacquire(addr *uint32, profile semaProfileFlags) {
 	gp := getg()
 	if gp != gp.m.curg {
 		throw("semacquire not on the G stack")
@@ -91,10 +103,17 @@ func semacquire(addr *uint32, profile bool) {
 	root := semroot(addr)
 	t0 := int64(0)
 	s.releasetime = 0
-	if profile && blockprofilerate > 0 {
+	s.acquiretime = 0
+	if profile&semaBlockProfile != 0 && blockprofilerate > 0 {
 		t0 = cputicks()
 		s.releasetime = -1
 	}
+	if profile&semaMutexProfile != 0 && mutexprofilerate > 0 {
+		if t0 == 0 {
+			t0 = cputicks()
+		}
+		s.acquiretime = t0
+	}
 	for {
 		lock(&root.lock)
 		// Add ourselves to nwait to disable "easy case" in semrelease.
@@ -146,8 +165,19 @@ func semrelease(addr *uint32) {
 			break
 		}
 	}
-	unlock(&root.lock)
 	if s != nil {
+		if s.acquiretime != 0 {
+			t0 := cputicks()
+			for x := root.head; x != nil; x = x.next {
+				if x.elem == unsafe.Pointer(addr) {
+					x.acquiretime = t0
+				}
+			}
+			mutexevent(t0-s.acquiretime, 3)
+		}
+	}
+	unlock(&root.lock)
+	if s != nil { // May be slow, so unlock first
 		readyWithTime(s, 5)
 	}
 }

src/runtime/trace.go

@@ -292,7 +292,7 @@ func StopTrace() {
 
 	// The world is started but we've set trace.shutdown, so new tracing can't start.
 	// Wait for the trace reader to flush pending buffers and stop.
-	semacquire(&trace.shutdownSema, false)
+	semacquire(&trace.shutdownSema, 0)
 	if raceenabled {
 		raceacquire(unsafe.Pointer(&trace.shutdownSema))
 	}

src/sync/mutex.go

@@ -84,7 +84,7 @@ func (m *Mutex) Lock() {
 			if old&mutexLocked == 0 {
 				break
 			}
-			runtime_Semacquire(&m.sema)
+			runtime_SemacquireMutex(&m.sema)
 			awoke = true
 			iter = 0
 		}

src/sync/mutex_test.go

@@ -66,6 +66,10 @@ func HammerMutex(m *Mutex, loops int, cdone chan bool) {
 }
 
 func TestMutex(t *testing.T) {
+	if n := runtime.SetMutexProfileFraction(1); n != 0 {
+		t.Logf("got mutexrate %d expected 0", n)
+	}
+	defer runtime.SetMutexProfileFraction(0)
 	m := new(Mutex)
 	c := make(chan bool)
 	for i := 0; i < 10; i++ {

src/sync/runtime.go

@@ -13,6 +13,9 @@ import "unsafe"
 // library and should not be used directly.
 func runtime_Semacquire(s *uint32)
 
+// SemacquireMutex is like Semacquire, but for profiling contended Mutexes.
+func runtime_SemacquireMutex(*uint32)
+
 // Semrelease atomically increments *s and notifies a waiting goroutine
 // if one is blocked in Semacquire.
 // It is intended as a simple wakeup primitive for use by the synchronization

src/testing/testing.go

@@ -233,19 +233,21 @@ var (
 	outputDir = flag.String("test.outputdir", "", "write profiles to `dir`")
 
 	// Report as tests are run; default is silent for success.
-	chatty           = flag.Bool("test.v", false, "verbose: print additional output")
-	count            = flag.Uint("test.count", 1, "run tests and benchmarks `n` times")
-	coverProfile     = flag.String("test.coverprofile", "", "write a coverage profile to `file`")
-	match            = flag.String("test.run", "", "run only tests and examples matching `regexp`")
-	memProfile       = flag.String("test.memprofile", "", "write a memory profile to `file`")
-	memProfileRate   = flag.Int("test.memprofilerate", 0, "set memory profiling `rate` (see runtime.MemProfileRate)")
-	cpuProfile       = flag.String("test.cpuprofile", "", "write a cpu profile to `file`")
-	blockProfile     = flag.String("test.blockprofile", "", "write a goroutine blocking profile to `file`")
-	blockProfileRate = flag.Int("test.blockprofilerate", 1, "set blocking profile `rate` (see runtime.SetBlockProfileRate)")
-	traceFile        = flag.String("test.trace", "", "write an execution trace to `file`")
-	timeout          = flag.Duration("test.timeout", 0, "fail test binary execution after duration `d` (0 means unlimited)")
-	cpuListStr       = flag.String("test.cpu", "", "comma-separated `list` of cpu counts to run each test with")
-	parallel         = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "run at most `n` tests in parallel")
+	chatty               = flag.Bool("test.v", false, "verbose: print additional output")
+	count                = flag.Uint("test.count", 1, "run tests and benchmarks `n` times")
+	coverProfile         = flag.String("test.coverprofile", "", "write a coverage profile to `file`")
+	match                = flag.String("test.run", "", "run only tests and examples matching `regexp`")
+	memProfile           = flag.String("test.memprofile", "", "write a memory profile to `file`")
+	memProfileRate       = flag.Int("test.memprofilerate", 0, "set memory profiling `rate` (see runtime.MemProfileRate)")
+	cpuProfile           = flag.String("test.cpuprofile", "", "write a cpu profile to `file`")
+	blockProfile         = flag.String("test.blockprofile", "", "write a goroutine blocking profile to `file`")
+	blockProfileRate     = flag.Int("test.blockprofilerate", 1, "set blocking profile `rate` (see runtime.SetBlockProfileRate)")
+	mutexProfile         = flag.String("test.mutexprofile", "", "write a mutex contention profile to the named file after execution")
+	mutexProfileFraction = flag.Int("test.mutexprofilefraction", 1, "if >= 0, calls runtime.SetMutexProfileFraction()")
+	traceFile            = flag.String("test.trace", "", "write an execution trace to `file`")
+	timeout              = flag.Duration("test.timeout", 0, "fail test binary execution after duration `d` (0 means unlimited)")
+	cpuListStr           = flag.String("test.cpu", "", "comma-separated `list` of cpu counts to run each test with")
+	parallel             = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "run at most `n` tests in parallel")
 
 	haveExamples bool // are there examples?
 
@@ -874,6 +876,9 @@ func before() {
 	if *blockProfile != "" && *blockProfileRate >= 0 {
 		runtime.SetBlockProfileRate(*blockProfileRate)
 	}
+	if *mutexProfile != "" && *mutexProfileFraction >= 0 {
+		runtime.SetMutexProfileFraction(*mutexProfileFraction)
+	}
 	if *coverProfile != "" && cover.Mode == "" {
 		fmt.Fprintf(os.Stderr, "testing: cannot use -test.coverprofile because test binary was not built with coverage enabled\n")
 		os.Exit(2)
@@ -913,6 +918,18 @@ func after() {
 		}
 		f.Close()
 	}
+	if *mutexProfile != "" && *mutexProfileFraction >= 0 {
+		f, err := os.Create(toOutputDir(*mutexProfile))
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
+			os.Exit(2)
+		}
+		if err = pprof.Lookup("mutex").WriteTo(f, 0); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *blockProfile, err)
+			os.Exit(2)
+		}
+		f.Close()
+	}
 	if cover.Mode != "" {
 		coverReport()
 	}