runtime: preempt goroutines for GC

The last patch for preemptive scheduler,
with this change stoptheworld issues preemption
requests every 100us.
Update #543.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/10264044

src/pkg/runtime/proc.c

@@ -99,7 +99,6 @@ static void inclocked(int32);
 static void checkdead(void);
 static void exitsyscall0(G*);
 static void park0(G*);
-static void gosched0(G*);
 static void goexit0(G*);
 static void gfput(P*, G*);
 static G* gfget(P*);
@@ -364,6 +363,7 @@ runtime·stoptheworld(void)
 	runtime·lock(&runtime·sched);
 	runtime·sched.stopwait = runtime·gomaxprocs;
 	runtime·atomicstore((uint32*)&runtime·gcwaiting, 1);
+	preemptall();
 	// stop current P
 	m->p->status = Pgcstop;
 	runtime·sched.stopwait--;
@@ -382,10 +382,16 @@ runtime·stoptheworld(void)
 	wait = runtime·sched.stopwait > 0;
 	runtime·unlock(&runtime·sched);
 
-	// wait for remaining P's to stop voluntary
+	// wait for remaining P's to stop voluntarily
 	if(wait) {
-		runtime·notesleep(&runtime·sched.stopnote);
-		runtime·noteclear(&runtime·sched.stopnote);
+		for(;;) {
+			// wait for 100us, then try to re-preempt in case of any races
+			if(runtime·notetsleep(&runtime·sched.stopnote, 100*1000)) {
+				runtime·noteclear(&runtime·sched.stopnote);
+				break;
+			}
+			preemptall();
+		}
 	}
 	if(runtime·sched.stopwait)
 		runtime·throw("stoptheworld: not stopped");
@@ -1240,12 +1246,12 @@ park0(G *gp)
 void
 runtime·gosched(void)
 {
-	runtime·mcall(gosched0);
+	runtime·mcall(runtime·gosched0);
 }
 
 // runtime·gosched continuation on g0.
-static void
-gosched0(G *gp)
+void
+runtime·gosched0(G *gp)
 {
 	gp->status = Grunnable;
 	gp->m = nil;

src/pkg/runtime/proc_test.go

@@ -157,6 +157,36 @@ func TestTimerFairness2(t *testing.T) {
 	<-done
 }
 
+// The function is used to test preemption at split stack checks.
+// Declaring a var avoids inlining at the call site.
+var preempt = func() int {
+	var a [128]int
+	sum := 0
+	for _, v := range a {
+		sum += v
+	}
+	return sum
+}
+
+func TestPreemptionGC(t *testing.T) {
+	// Test that pending GC preempts running goroutines.
+	const P = 5
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(P + 1))
+	var stop uint32
+	for i := 0; i < P; i++ {
+		go func() {
+			for atomic.LoadUint32(&stop) == 0 {
+				preempt()
+			}
+		}()
+	}
+	for i := 0; i < 10; i++ {
+		runtime.Gosched()
+		runtime.GC()
+	}
+	atomic.StoreUint32(&stop, 1)
+}
+
 func stackGrowthRecursive(i int) {
 	var pad [128]uint64
 	if i != 0 && pad[0] == 0 {

src/pkg/runtime/runtime.h

@@ -809,6 +809,7 @@ void runtime·newextram(void);
 void	runtime·exit(int32);
 void	runtime·breakpoint(void);
 void	runtime·gosched(void);
+void	runtime·gosched0(G*);
 void	runtime·park(void(*)(Lock*), Lock*, int8*);
 void	runtime·tsleep(int64, int8*);
 M*	runtime·newm(void);

src/pkg/runtime/stack.c

@@ -211,7 +211,10 @@ runtime·newstack(void)
 	gp->status = Gwaiting;
 	gp->waitreason = "stack split";
 	reflectcall = framesize==1;
+	if(reflectcall)
+		framesize = 0;
 
+	// For reflectcall the context already points to beginning of reflect·call.
 	if(!reflectcall)
 		runtime·rewindmorestack(&gp->sched);
 
@@ -238,9 +241,24 @@ runtime·newstack(void)
 		runtime·throw("runtime: stack split argsize");
 	}
 
-	reflectcall = framesize==1;
-	if(reflectcall)
-		framesize = 0;
+	if(gp->stackguard0 == StackPreempt) {
+		if(gp == m->g0)
+			runtime·throw("runtime: preempt g0");
+		if(oldstatus == Grunning && (m->p == nil || m->p->status != Prunning))
+			runtime·throw("runtime: g is running but p is not");
+		// Be conservative about where we preempt.
+		// We are interested in preempting user Go code, not runtime code.
+		if(oldstatus != Grunning || m->locks || m->mallocing || m->gcing) {
+			// Let the goroutine keep running for now.
+			// TODO(dvyukov): remember but delay the preemption.
+			gp->stackguard0 = gp->stackguard;
+			gp->status = oldstatus;
+			runtime·gogo(&gp->sched);	// never return
+		}
+		// Act like goroutine called runtime.Gosched.
+		gp->status = oldstatus;
+		runtime·gosched0(gp);	// never return
+	}
 
 	if(reflectcall && m->morebuf.sp - sizeof(Stktop) - argsize - 32 > gp->stackguard) {
 		// special case: called from reflect.call (framesize==1)

src/pkg/runtime/stack.h

@@ -103,9 +103,10 @@ enum {
 	// The actual size can be smaller than this but cannot be larger.
 	// Checked in proc.c's runtime.malg.
 	StackTop = 96,
-
-	// Goroutine preemption request.
-	// Stored into g->stackguard0 to cause split stack check failure.
-	// Must be greater than any real sp.
-	StackPreempt = (uintptr)(intptr)0xfffffade,
 };
+
+// Goroutine preemption request.
+// Stored into g->stackguard0 to cause split stack check failure.
+// Must be greater than any real sp.
+// 0xfffffade in hex.
+#define StackPreempt ((uintptr)-1314)