runtime: remove GODEBUG=runtimecontentionstacks

Go 1.22 promised to remove the setting in a future release once the
semantics of runtime-internal lock contention matched that of
sync.Mutex. That work is done, remove the setting.

Previously reviewed as https://go.dev/cl/585639.

For #66999

Change-Id: I9fe62558ba0ac12824874a0bb1b41efeb7c0853f
Reviewed-on: https://go-review.googlesource.com/c/go/+/668995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

doc/godebug.md

@@ -169,6 +169,9 @@ Go command will follow symlinks to regular files embedding files.
 The default value `embedfollowsymlinks=0` does not allow following
 symlinks. `embedfollowsymlinks=1` will allow following symlinks.
 
+Go 1.25 corrected the semantics of contention reports for runtime-internal locks,
+and so removed the [`runtimecontentionstacks` setting](/pkg/runtime#hdr-Environment_Variable).
+
 ### Go 1.24
 
 Go 1.24 added a new `fips140` setting that controls whether the Go

doc/next/6-stdlib/99-minor/runtime/pprof/66999.md

@@ -0,0 +1,6 @@
+The mutex profile for contention on runtime-internal locks now correctly points
+to the end of the critical section that caused the delay. This matches the
+profile's behavior for contention on `sync.Mutex` values. The
+`runtimecontentionstacks` setting for `GODEBUG`, which allowed opting in to the
+unusual behavior of Go 1.22 through 1.24 for this part of the profile, is now
+gone.

src/runtime/extern.go

@@ -166,15 +166,6 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	panicnil: setting panicnil=1 disables the runtime error when calling panic with nil
 	interface value or an untyped nil.
 
-	runtimecontentionstacks: setting runtimecontentionstacks=1 enables inclusion of call stacks
-	related to contention on runtime-internal locks in the "mutex" profile, subject to the
-	MutexProfileFraction setting. When runtimecontentionstacks=0, contention on
-	runtime-internal locks will report as "runtime._LostContendedRuntimeLock". When
-	runtimecontentionstacks=1, the call stacks will correspond to the unlock call that released
-	the lock. But instead of the value corresponding to the amount of contention that call
-	stack caused, it corresponds to the amount of time the caller of unlock had to wait in its
-	original call to lock. A future release is expected to align those and remove this setting.
-
 	invalidptr: invalidptr=1 (the default) causes the garbage collector and stack
 	copier to crash the program if an invalid pointer value (for example, 1)
 	is found in a pointer-typed location. Setting invalidptr=0 disables this check.

src/runtime/metrics_test.go

@@ -6,7 +6,6 @@ package runtime_test
 
 import (
 	"bytes"
-	"fmt"
 	"internal/abi"
 	"internal/goexperiment"
 	"internal/profile"
@@ -955,17 +954,6 @@ func TestRuntimeLockMetricsAndProfile(t *testing.T) {
 		t.Fatalf("need MutexProfileRate 0, got %d", old)
 	}
 
-	{
-		before := os.Getenv("GODEBUG")
-		for _, s := range strings.Split(before, ",") {
-			if strings.HasPrefix(s, "runtimecontentionstacks=") {
-				t.Logf("GODEBUG includes explicit setting %q", s)
-			}
-		}
-		defer func() { os.Setenv("GODEBUG", before) }()
-		os.Setenv("GODEBUG", fmt.Sprintf("%s,runtimecontentionstacks=1", before))
-	}
-
 	t.Logf("NumCPU %d", runtime.NumCPU())
 	t.Logf("GOMAXPROCS %d", runtime.GOMAXPROCS(0))
 	if minCPU := 2; runtime.NumCPU() < minCPU {
@@ -1229,9 +1217,9 @@ func TestRuntimeLockMetricsAndProfile(t *testing.T) {
 
 			stks[i] = []string{
 				"runtime.unlock",
-				"runtime_test." + name + ".func5.1.1",
+				"runtime_test." + name + ".func4.1.1",
 				marker,
-				"runtime_test." + name + ".func5.1",
+				"runtime_test." + name + ".func4.1",
 				"runtime_test.(*contentionWorker).run",
 			}
 		}
@@ -1411,14 +1399,14 @@ func TestRuntimeLockMetricsAndProfile(t *testing.T) {
 			{
 				"runtime.unlock",
 				"runtime.semrelease1",
-				"runtime_test.TestRuntimeLockMetricsAndProfile.func6.1",
+				"runtime_test.TestRuntimeLockMetricsAndProfile.func5.1",
 				"runtime_test.(*contentionWorker).run",
 			},
 			{
 				"runtime.unlock",
 				"runtime.semacquire1",
 				"runtime.semacquire",
-				"runtime_test.TestRuntimeLockMetricsAndProfile.func6.1",
+				"runtime_test.TestRuntimeLockMetricsAndProfile.func5.1",
 				"runtime_test.(*contentionWorker).run",
 			},
 		}

src/runtime/mprof.go

@@ -732,11 +732,6 @@ func (prof *mLockProfile) captureStack() {
 	prof.haveStack = true
 
 	prof.stack[0] = logicalStackSentinel
-	if debug.runtimeContentionStacks.Load() == 0 {
-		prof.stack[1] = abi.FuncPCABIInternal(_LostContendedRuntimeLock) + sys.PCQuantum
-		prof.stack[2] = 0
-		return
-	}
 
 	var nstk int
 	gp := getg()

src/runtime/pprof/pprof.go

@@ -169,12 +169,6 @@ import (
 // holds a lock for 1s while 5 other goroutines are waiting for the entire
 // second to acquire the lock, its unlock call stack will report 5s of
 // contention.
-//
-// Runtime-internal locks are always reported at the location
-// "runtime._LostContendedRuntimeLock". More detailed stack traces for
-// runtime-internal locks can be obtained by setting
-// `GODEBUG=runtimecontentionstacks=1` (see package [runtime] docs for
-// caveats).
 type Profile struct {
 	name  string
 	mu    sync.Mutex

src/runtime/runtime1.go

@@ -320,7 +320,6 @@ var debug struct {
 	gctrace                  int32
 	invalidptr               int32
 	madvdontneed             int32 // for Linux; issue 28466
-	runtimeContentionStacks  atomic.Int32
 	scavtrace                int32
 	scheddetail              int32
 	schedtrace               int32
@@ -385,7 +384,6 @@ var dbgvars = []*dbgVar{
 	{name: "madvdontneed", value: &debug.madvdontneed},
 	{name: "panicnil", atomic: &debug.panicnil},
 	{name: "profstackdepth", value: &debug.profstackdepth, def: 128},
-	{name: "runtimecontentionstacks", atomic: &debug.runtimeContentionStacks},
 	{name: "sbrk", value: &debug.sbrk},
 	{name: "scavtrace", value: &debug.scavtrace},
 	{name: "scheddetail", value: &debug.scheddetail},