runtime: note custom GOMAXPROCS even if value doesn't change

When an application calls runtime.GOMAXPROCS(runtime.GOMAXPROCS(0)), the
runtime does not need to change the actual GOMAXPROCS value (via STW).
However, this call must still transition from "automatic" to "custom"
GOMAXPROCS state, thus disabling background updates.

Thus this case shouldn't return quite as early as it currently does.

Change-Id: I6a6a636c42f73996532bd9f7beb95e933256c9e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/683815
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

src/runtime/debug.go

@@ -39,7 +39,7 @@ func GOMAXPROCS(n int) int {
 
 	lock(&sched.lock)
 	ret := int(gomaxprocs)
-	if n <= 0 || n == ret {
+	if n <= 0 {
 		unlock(&sched.lock)
 		return ret
 	}
@@ -52,6 +52,12 @@ func GOMAXPROCS(n int) int {
 	lock(&computeMaxProcsLock)
 	unlock(&computeMaxProcsLock)
 
+	if n == ret {
+		// sched.customGOMAXPROCS set, but no need to actually STW
+		// since the gomaxprocs itself isn't changing.
+		return ret
+	}
+
 	stw := stopTheWorldGC(stwGOMAXPROCS)
 
 	// newprocs will be processed by startTheWorld

src/runtime/testdata/testprog/gomaxprocs.go

@@ -133,6 +133,20 @@ func UpdateGOMAXPROCS() {
 	mustSetCPUMax(path, 200000)
 	mustNotChangeMaxProcs(3)
 
+	// Re-enable updates. Change is immediately visible.
+	runtime.SetDefaultGOMAXPROCS()
+	procs = runtime.GOMAXPROCS(0)
+	println("GOMAXPROCS:", procs)
+	if procs != 2 {
+		panic(fmt.Sprintf("GOMAXPROCS got %d want %d", procs, 2))
+	}
+
+	// Setting GOMAXPROCS to itself also disables updates, despite not
+	// changing the value itself.
+	runtime.GOMAXPROCS(runtime.GOMAXPROCS(0))
+	mustSetCPUMax(path, 300000)
+	mustNotChangeMaxProcs(2)
+
 	println("OK")
 }