diff --git a/src/runtime/debug.go b/src/runtime/debug.go
index bdaaa7196d3..c7592d33299 100644
--- a/src/runtime/debug.go
+++ b/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
diff --git a/src/runtime/testdata/testprog/gomaxprocs.go b/src/runtime/testdata/testprog/gomaxprocs.go
index 915e3c4dad4..99bc9f1dbb3 100644
--- a/src/runtime/testdata/testprog/gomaxprocs.go
+++ b/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")
 }