runtime/metrics: add metric for current Go-owned thread count

Fixes #15490.

Change-Id: I6ce9edc46398030ff639e22d4ca4adebccdfe1b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/690399
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/metrics.go

@@ -532,6 +532,13 @@ func initMetrics() {
 				sched.stwTotalTimeOther.write(out)
 			},
 		},
+		"/sched/threads/total:threads": {
+			deps: makeStatDepSet(schedStatsDep),
+			compute: func(in *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = uint64(in.schedStats.threads)
+			},
+		},
 		"/sync/mutex/wait/total:seconds": {
 			compute: func(_ *statAggregate, out *metricValue) {
 				out.kind = metricKindFloat64
@@ -787,6 +794,7 @@ type schedStatsAggregate struct {
 	gNonGo    uint64
 	gWaiting  uint64
 	gCreated  uint64
+	threads   uint64
 }
 
 // compute populates the schedStatsAggregate with values from the runtime.
@@ -797,6 +805,10 @@ func (a *schedStatsAggregate) compute() {
 	// approximate.
 	lock(&sched.lock)
 
+	// The total count of threads owned by Go is the number of Ms
+	// minus extra Ms on the list or in use.
+	a.threads = uint64(mcount()) - uint64(extraMInUse.Load()) - uint64(extraMLength.Load())
+
 	// Collect running/runnable from per-P run queues.
 	a.gCreated += sched.goroutinesCreated.Load()
 	for _, p := range allp {

src/runtime/metrics/description.go

@@ -498,6 +498,11 @@ var allDesc = []Description{
 		Kind:        KindFloat64Histogram,
 		Cumulative:  true,
 	},
+	{
+		Name:        "/sched/threads/total:threads",
+		Description: "The current count of live threads that are owned by the Go runtime.",
+		Kind:        KindUint64,
+	},
 	{
 		Name:        "/sync/mutex/wait/total:seconds",
 		Description: "Approximate cumulative time goroutines have spent blocked on a sync.Mutex, sync.RWMutex, or runtime-internal lock. This metric is useful for identifying global changes in lock contention. Collect a mutex or block profile using the runtime/pprof package for more detailed contention data.",

src/runtime/metrics/doc.go

@@ -572,6 +572,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 		/sched/pauses/stopping/other:seconds). Bucket counts increase
 		monotonically.
 
+	/sched/threads/total:threads
+		The current count of live threads that are owned by the Go
+		runtime.
+
 	/sync/mutex/wait/total:seconds
 		Approximate cumulative time goroutines have spent blocked on a
 		sync.Mutex, sync.RWMutex, or runtime-internal lock. This metric

src/runtime/metrics_test.go

@@ -1584,13 +1584,16 @@ func TestReadMetricsSched(t *testing.T) {
 		running
 		waiting
 		created
+		threads
+		numSamples
 	)
-	var s [5]metrics.Sample
+	var s [numSamples]metrics.Sample
 	s[notInGo].Name = "/sched/goroutines/not-in-go:goroutines"
 	s[runnable].Name = "/sched/goroutines/runnable:goroutines"
 	s[running].Name = "/sched/goroutines/running:goroutines"
 	s[waiting].Name = "/sched/goroutines/waiting:goroutines"
 	s[created].Name = "/sched/goroutines-created:goroutines"
+	s[threads].Name = "/sched/threads/total:threads"
 
 	logMetrics := func(t *testing.T, s []metrics.Sample) {
 		for i := range s {
@@ -1608,6 +1611,10 @@ func TestReadMetricsSched(t *testing.T) {
 	// goroutines.
 	const waitingSlack = 100
 
+	// threadsSlack is the maximum number of threads left over
+	// from other tests and the runtime (sysmon, the template thread, etc.)
+	const threadsSlack = 20
+
 	// Make sure GC isn't running, since GC workers interfere with
 	// expected counts.
 	defer debug.SetGCPercent(debug.SetGCPercent(-1))
@@ -1694,6 +1701,7 @@ func TestReadMetricsSched(t *testing.T) {
 			}, time.Second)
 			logMetrics(t, s[:])
 			check(t, &s[running], count, count+4)
+			check(t, &s[threads], count, count+4+threadsSlack)
 		})
 
 		// Force runnable count to be high.
@@ -1724,6 +1732,7 @@ func TestReadMetricsSched(t *testing.T) {
 		t.Run("running", func(t *testing.T) {
 			logMetrics(t, s[:])
 			checkEq(t, &s[running], 1)
+			checkEq(t, &s[threads], 1)
 		})
 		t.Run("runnable", func(t *testing.T) {
 			logMetrics(t, s[:])

src/runtime/proc.go

@@ -1007,7 +1007,7 @@ func mcommoninit(mp *m, id int64) {
 	// when it is just in a register or thread-local storage.
 	mp.alllink = allm
 
-	// NumCgoCall() and others iterate over allm w/o schedlock,
+	// NumCgoCall and others iterate over allm w/o schedlock,
 	// so we need to publish it safely.
 	atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
 	unlock(&sched.lock)

src/runtime/runtime2.go

@@ -1225,7 +1225,9 @@ var isIdleInSynctest = [len(waitReasonStrings)]bool{
 }
 
 var (
-	allm          *m
+	// Linked-list of all Ms. Written under sched.lock, read atomically.
+	allm *m
+
 	gomaxprocs    int32
 	numCPUStartup int32
 	forcegc       forcegcstate