runtime/metrics: add /sync/mutex/wait/total:seconds metric

This change adds a metric to the runtime/metrics package which tracks
total mutex wait time for sync.Mutex and sync.RWMutex. The purpose of
this metric is to be able to quickly get an idea of the total mutex wait
time.

The implementation of this metric piggybacks off of the existing G
runnable tracking infrastructure, as well as the wait reason set on a G
when it goes into _Gwaiting.

Fixes #49881.

Change-Id: I4691abf64ac3574bec69b4d7d4428b1573130517
Reviewed-on: https://go-review.googlesource.com/c/go/+/427618
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 93cae48211..4aabf90fe7 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -525,6 +525,12 @@ func Getg() *G {
 	return getg()
 }
 
+func GIsWaitingOnMutex(gp *G) bool {
+	return readgstatus(gp) == _Gwaiting && gp.waitreason.isMutexWait()
+}
+
+var CasGStatusAlwaysTrack = &casgstatusAlwaysTrack
+
 //go:noinline
 func PanicForTesting(b []byte, i int) byte {
 	return unexportedPanicForTesting(b, i)
@@ -1617,3 +1623,5 @@ func (s *ScavengeIndex) Mark(base, limit uintptr) {
 func (s *ScavengeIndex) Clear(ci ChunkIdx) {
 	s.i.clear(chunkIdx(ci))
 }
+
+const GTrackingPeriod = gTrackingPeriod