runtime: contended mutex profiling doesn't work for sync.RWMutex #18496
Description
What version of Go are you using (go version)?
go version devel +9def857072 Mon Jan 2 20:21:02 2017 +0000 darwin/amd64
What operating system and processor architecture are you using (go env)?
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/kale/go"
GORACE=""
GOROOT="/Users/kale/gomaster"
GOTOOLDIR="/Users/kale/gomaster/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/jv/fp0q8rhx427f6p892mh4mpx40000gn/T/go-build367788226=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
What did you do?
Ran a benchmark with a RWMutex and -mutexprofile enabled.
Example:
func BenchmarkRWMutex(b *testing.B) {
var mu sync.RWMutex
b.RunParallel(func(pb *testing.PB) {
for pb.Next() {
mu.RLock()
mu.RUnlock()
}
})
}What did you expect to see?
Mutex contention information.
Example from a similar test using Lock/Unlock:
go tool pprof ./mutex.test mutex.out
Entering interactive mode (type "help" for commands)
(pprof) top10
1.60s of 1.60s total ( 100%)
flat flat% sum% cum cum%
1.60s 100% 100% 1.60s 100% sync.(*Mutex).Unlock
0 0% 100% 1.60s 100% github.com/vcabbage/mutexprofile-repro.BenchmarkMutex.func1
0 0% 100% 1.60s 100% runtime.goexit
0 0% 100% 1.60s 100% testing.(*B).RunParallel.func1
What did you see instead?
go tool pprof ./mutex.test mutex.out
Entering interactive mode (type "help" for commands)
(pprof) top10
profile is empty
I see that sync.Mutex was changed to use runtime_SemacquireMutex instead of runtime_Semacquire in https://golang.org/cl/29650/, but sync.RWMutex continues to use runtime_Semacquire.
I'm unsure if sync.RWMutex was excluded from profiling intentionally, but the lack of profiling information was surprising to me.