runtime,runtime/metrics: track running time per goroutine

Fixes golang#41554.

This commit introduces a /sched/goroutine/running:nanoseconds metric,
defined as the total time spent by a goroutine in the running state.
This measurement is useful for systems that would benefit from
fine-grained CPU attribution.

An alternative to scheduler-backed CPU attribution would be the use of
profiler labels. Given it's common to spawn multiple goroutines for the
same task, goroutine-backed statistics can easily become misleading.
Profiler labels instead let you measure CPU performance across a set of
cooperating goroutines. That said, it has two downsides:
- performance overhead; for high-performance systems that care about
  fine-grained CPU attribution (databases for e.g. that want to measure
  total CPU time spent processing each request), profiler labels are too
  cost-prohibitive, especially given the Go runtime has a much cheaper
  and more granular view of the data needed
- inaccuracy and imprecision, as evaluated in
  golang#36821

It's worth noting that we already export /sched/latencies:seconds to
track scheduling latencies, i.e. time spent by a goroutine in the
runnable state (go-review.googlesource.com/c/go/+/308933). This commit
does effectively the same except for the running state on the requesting
goroutine. Users are free to use this metric to power histograms or
tracking on-CPU time across a set of goroutines.

Change-Id: Ie78336a3ddeca0521ae29cce57bc7a5ea67da297

src/runtime/metrics.go

@@ -286,6 +286,12 @@ func initMetrics() {
 				out.scalar = uint64(gomaxprocs)
 			},
 		},
+		"/sched/goroutine/running:nanoseconds": {
+			compute: func(_ *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = uint64(grunningnanos())
+			},
+		},
 		"/sched/goroutines:goroutines": {
 			compute: func(_ *statAggregate, out *metricValue) {
 				out.kind = metricKindUint64

src/runtime/metrics/description.go

@@ -225,6 +225,11 @@ var allDesc = []Description{
 		Description: "The current runtime.GOMAXPROCS setting, or the number of operating system threads that can execute user-level Go code simultaneously.",
 		Kind:        KindUint64,
 	},
+	{
+		Name:        "/sched/goroutine/running:nanoseconds",
+		Description: "Wall time spent by the current goroutine in the running state.",
+		Kind:        KindUint64,
+	},
 	{
 		Name:        "/sched/goroutines:goroutines",
 		Description: "Count of live goroutines.",

src/runtime/metrics/doc.go

@@ -172,6 +172,9 @@ Below is the full list of supported metrics, ordered lexicographically.
 		operating system threads that can execute user-level Go code
 		simultaneously.
 
+	/sched/goroutine/running:nanoseconds
+		Wall time spent by the current goroutine in the running state.
+
 	/sched/goroutines:goroutines
 		Count of live goroutines.
 

src/runtime/proc.go

@@ -994,8 +994,18 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		}
 	}
 
-	// Handle tracking for scheduling latencies.
+	// Handle tracking for scheduling and running latencies.
+	now := nanotime()
+	if newval == _Grunning {
+		// We're transitioning into the running state, record the timestamp for
+		// subsequent use.
+		gp.lastsched = now
+	}
 	if oldval == _Grunning {
+		// We're transitioning out of running, record how long we were in the
+		// state.
+		gp.runningnanos += now - gp.lastsched
+
 		// Track every 8th time a goroutine transitions out of running.
 		if gp.trackingSeq%gTrackingPeriod == 0 {
 			gp.tracking = true
@@ -1007,14 +1017,12 @@ func casgstatus(gp *g, oldval, newval uint32) {
 			// We transitioned out of runnable, so measure how much
 			// time we spent in this state and add it to
 			// runnableTime.
-			now := nanotime()
 			gp.runnableTime += now - gp.runnableStamp
 			gp.runnableStamp = 0
 		}
 		if newval == _Grunnable {
 			// We just transitioned into runnable, so record what
 			// time that happened.
-			now := nanotime()
 			gp.runnableStamp = now
 		} else if newval == _Grunning {
 			// We're transitioning into running, so turn off
@@ -3258,6 +3266,14 @@ func dropg() {
 	setGNoWB(&_g_.m.curg, nil)
 }
 
+// grunningnanos returns the wall time spent by current g in the running state.
+// A goroutine may be running on an OS thread that's descheduled by the OS
+// scheduler, this time still counts towards the metric.
+func grunningnanos() int64 {
+	gp := getg()
+	return gp.runningnanos + nanotime() - gp.lastsched
+}
+
 // checkTimers runs any timers for the P that are ready.
 // If now is not 0 it is the current time.
 // It returns the passed time or the current time if now was passed as 0.
@@ -3491,6 +3507,8 @@ func goexit0(gp *g) {
 	gp.param = nil
 	gp.labels = nil
 	gp.timer = nil
+	gp.lastsched = 0
+	gp.runningnanos = 0
 
 	if gcBlackenEnabled != 0 && gp.gcAssistBytes > 0 {
 		// Flush assist credit to the global pool. This gives

src/runtime/runtime2.go

@@ -474,7 +474,6 @@ type g struct {
 	traceseq       uint64   // trace event sequencer
 	tracelastp     puintptr // last P emitted an event for this goroutine
 	lockedm        muintptr
-	sig            uint32
 	writebuf       []byte
 	sigcode0       uintptr
 	sigcode1       uintptr
@@ -488,6 +487,9 @@ type g struct {
 	labels         unsafe.Pointer // profiler labels
 	timer          *timer         // cached timer for time.Sleep
 	selectDone     uint32         // are we participating in a select and did someone win the race?
+	sig            uint32
+	lastsched      int64 // timestamp when the G last started running
+	runningnanos   int64 // wall time spent in the running state
 
 	// goroutineProfiled indicates the status of this goroutine's stack for the
 	// current in-progress goroutine profile

src/runtime/sizeof_test.go

@@ -21,7 +21,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr // size on 32bit platforms
 		_64bit uintptr // size on 64bit platforms
 	}{
-		{runtime.G{}, 240, 392},   // g, but exported for testing
+		{runtime.G{}, 256, 408},   // g, but exported for testing
 		{runtime.Sudog{}, 56, 88}, // sudog, but exported for testing
 	}