runtime: check for preemption due to garbage collection

	in various already expensive routines.

helps keep cpu utilization up when GOMAXPROCS > 1,
but not a full solution.

http://groups.google.com/group/golang-nuts/t/7a9535c4136d3e2

R=r
CC=golang-dev
https://golang.org/cl/184043

src/pkg/runtime/chan.c

@@ -174,6 +174,9 @@ chansend(Hchan *c, byte *ep, bool *pres)
 	SudoG *sg;
 	G* gp;
 
+	if(gcwaiting)
+		gosched();
+
 	if(debug) {
 		prints("chansend: chan=");
 		runtime·printpointer(c);
@@ -277,6 +280,9 @@ chanrecv(Hchan* c, byte *ep, bool* pres)
 	SudoG *sg;
 	G *gp;
 
+	if(gcwaiting)
+		gosched();
+
 	if(debug) {
 		prints("chanrecv: chan=");
 		runtime·printpointer(c);
@@ -631,6 +637,9 @@ runtime·selectgo(Select *sel)
 	G *gp;
 	byte *as;
 
+	if(gcwaiting)
+		gosched();
+
 	if(debug) {
 		prints("selectgo: sel=");
 		runtime·printpointer(sel);
@@ -908,6 +917,9 @@ runtime·closechan(Hchan *c)
 	SudoG *sg;
 	G* gp;
 
+	if(gcwaiting)
+		gosched();
+
 	lock(c);
 	incerr(c);
 	c->closed |= Wclosed;

src/pkg/runtime/hashmap.c

@@ -744,6 +744,9 @@ mapaccess(Hmap *h, byte *ak, byte *av, bool *pres)
 {
 	byte *res;
 
+	if(gcwaiting)
+		gosched();
+
 	res = nil;
 	if(hash_lookup(h, ak, (void**)&res)) {
 		*pres = true;
@@ -812,6 +815,9 @@ mapassign(Hmap *h, byte *ak, byte *av)
 	byte *res;
 	int32 hit;
 
+	if(gcwaiting)
+		gosched();
+
 	res = nil;
 	if(av == nil) {
 		hash_remove(h, ak, (void**)&res);
@@ -908,6 +914,9 @@ mapiterinit(Hmap *h)
 void
 runtime·mapiternext(struct hash_iter *it)
 {
+	if(gcwaiting)
+		gosched();
+
 	it->data = hash_next(it);
 	if(debug) {
 		prints("runtime·mapiternext: iter=");

src/pkg/runtime/malloc.cgo

@@ -27,10 +27,11 @@ mallocgc(uintptr size, uint32 refflag, int32 dogc)
 	void *v;
 	uint32 *ref;
 
+	if(gcwaiting && g != m->g0)
+		gosched();
 	if(m->mallocing)
 		throw("malloc/free - deadlock");
 	m->mallocing = 1;
-
 	if(size == 0)
 		size = 1;
 

src/pkg/runtime/mgc0.c

@@ -244,7 +244,7 @@ gc(int32 force)
 		sweep();
 		mstats.next_gc = mstats.inuse_pages+mstats.inuse_pages*gcpercent/100;
 	}
-	starttheworld();
 	m->gcing = 0;
 	semrelease(&gcsema);
+	starttheworld();
 }

src/pkg/runtime/proc.c

@@ -14,6 +14,8 @@ G	g0;	// idle goroutine for m0
 
 static	int32	debug	= 0;
 
+int32	gcwaiting;
+
 // Go scheduler
 //
 // The go scheduler's job is to match ready-to-run goroutines (`g's)
@@ -362,6 +364,7 @@ void
 stoptheworld(void)
 {
 	lock(&sched);
+	gcwaiting = 1;
 	sched.mcpumax = 1;
 	while(sched.mcpu > 1) {
 		noteclear(&sched.stopped);
@@ -379,6 +382,7 @@ void
 starttheworld(void)
 {
 	lock(&sched);
+	gcwaiting = 0;
 	sched.mcpumax = sched.gomaxprocs;
 	matchmg();
 	unlock(&sched);

src/pkg/runtime/runtime.h

@@ -316,6 +316,7 @@ extern	int32	gomaxprocs;
 extern	int32	panicking;
 extern	int32	maxround;
 extern	int32	fd;	// usually 1; set to 2 when panicking
+extern	int32	gcwaiting;		// gc is waiting to run
 int8*	goos;
 
 /*