runtime: clean up GC code

Remove C version of GC.
Convert freeOSMemory to Go.
Restore g0 check in GC.
Remove unknownGCPercent check in GC,
it's initialized explicitly now.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://golang.org/cl/139910043

src/pkg/runtime/malloc.go

@@ -407,33 +407,19 @@ func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
 // force = 1 - do GC regardless of current heap usage
 // force = 2 - go GC and eager sweep
 func gogc(force int32) {
-	if !memstats.enablegc {
-		return
-	}
-
-	// TODO: should never happen?  Only C calls malloc while holding a lock?
+	// The gc is turned off (via enablegc) until the bootstrap has completed.
+	// Also, malloc gets called in the guts of a number of libraries that might be
+	// holding locks. To avoid deadlocks during stoptheworld, don't bother
+	// trying to run gc while holding a lock. The next mallocgc without a lock
+	// will do the gc instead.
 	mp := acquirem()
-	if mp.locks > 1 {
+	if gp := getg(); gp == mp.g0 || mp.locks > 1 || !memstats.enablegc || panicking != 0 || gcpercent < 0 {
 		releasem(mp)
 		return
 	}
 	releasem(mp)
 	mp = nil
 
-	if panicking != 0 {
-		return
-	}
-	if gcpercent == gcpercentUnknown {
-		lock(&mheap_.lock)
-		if gcpercent == gcpercentUnknown {
-			gcpercent = readgogc()
-		}
-		unlock(&mheap_.lock)
-	}
-	if gcpercent < 0 {
-		return
-	}
-
 	semacquire(&worldsema, false)
 
 	if force == 0 && memstats.heap_alloc < memstats.next_gc {

src/pkg/runtime/malloc.h

@@ -519,7 +519,6 @@ void	runtime·MHeap_Scavenge(int32 k, uint64 now, uint64 limit);
 
 void*	runtime·persistentalloc(uintptr size, uintptr align, uint64 *stat);
 int32	runtime·mlookup(void *v, byte **base, uintptr *size, MSpan **s);
-void	runtime·gc(int32 force);
 uintptr	runtime·sweepone(void);
 void	runtime·markspan(void *v, uintptr size, uintptr n, bool leftover);
 void	runtime·unmarkspan(void *v, uintptr size);

src/pkg/runtime/mgc0.c

@@ -1284,82 +1284,6 @@ runtime·gcinit(void)
 	runtime·gcbssmask = unrollglobgcprog(runtime·gcbss, runtime·ebss - runtime·bss);
 }
 
-// force = 1 - do GC regardless of current heap usage
-// force = 2 - go GC and eager sweep
-void
-runtime·gc(int32 force)
-{
-	struct gc_args a;
-	int32 i;
-
-	// The gc is turned off (via enablegc) until
-	// the bootstrap has completed.
-	// Also, malloc gets called in the guts
-	// of a number of libraries that might be
-	// holding locks.  To avoid priority inversion
-	// problems, don't bother trying to run gc
-	// while holding a lock.  The next mallocgc
-	// without a lock will do the gc instead.
-	if(!mstats.enablegc || g == g->m->g0 || g->m->locks > 0 || runtime·panicking)
-		return;
-
-	if(runtime·gcpercent < 0)
-		return;
-
-	runtime·semacquire(&runtime·worldsema, false);
-	if(force==0 && mstats.heap_alloc < mstats.next_gc) {
-		// typically threads which lost the race to grab
-		// worldsema exit here when gc is done.
-		runtime·semrelease(&runtime·worldsema);
-		return;
-	}
-
-	// Ok, we're doing it!  Stop everybody else
-	a.start_time = runtime·nanotime();
-	a.eagersweep = force >= 2;
-	g->m->gcing = 1;
-	runtime·stoptheworld();
-	
-	runtime·clearpools();
-
-	// Run gc on the g0 stack.  We do this so that the g stack
-	// we're currently running on will no longer change.  Cuts
-	// the root set down a bit (g0 stacks are not scanned, and
-	// we don't need to scan gc's internal state).  Also an
-	// enabler for copyable stacks.
-	for(i = 0; i < (runtime·debug.gctrace > 1 ? 2 : 1); i++) {
-		if(i > 0)
-			a.start_time = runtime·nanotime();
-		// switch to g0, call gc(&a), then switch back
-		g->param = &a;
-		runtime·casgstatus(g, Grunning, Gwaiting);
-		g->waitreason = runtime·gostringnocopy((byte*)"garbage collection");
-		runtime·mcall(mgc);
-	}
-
-	// all done
-	g->m->gcing = 0;
-	g->m->locks++;
-	runtime·semrelease(&runtime·worldsema);
-	runtime·starttheworld();
-	g->m->locks--;
-
-	// now that gc is done, kick off finalizer thread if needed
-	if(!ConcurrentSweep) {
-		// give the queued finalizers, if any, a chance to run
-		runtime·gosched();
-	}
-}
-
-static void
-mgc(G *gp)
-{
-	gc(gp->param);
-	gp->param = nil;
-	runtime·casgstatus(gp, Gwaiting, Grunning);
-	runtime·gogo(&gp->sched);
-}
-
 void
 runtime·gc_m(void)
 {
@@ -1502,7 +1426,7 @@ gc(struct gc_args *args)
 	if(ConcurrentSweep && !args->eagersweep) {
 		runtime·lock(&gclock);
 		if(sweep.g == nil)
-			sweep.g = runtime·newproc1(&bgsweepv, nil, 0, 0, runtime·gc);
+			sweep.g = runtime·newproc1(&bgsweepv, nil, 0, 0, gc);
 		else if(sweep.parked) {
 			sweep.parked = false;
 			runtime·ready(sweep.g);

src/pkg/runtime/mgc0.go

@@ -34,3 +34,8 @@ func gc_unixnanotime(now *int64) {
 	sec, nsec := timenow()
 	*now = sec*1e9 + int64(nsec)
 }
+
+func freeOSMemory() {
+	gogc(2) // force GC and do eager sweep
+	onM(&scavenge_m)
+}

src/pkg/runtime/mheap.c

@@ -622,19 +622,10 @@ runtime·MHeap_Scavenge(int32 k, uint64 now, uint64 limit)
 	}
 }
 
-static void
-scavenge_m(G *gp)
-{
-	runtime·MHeap_Scavenge(-1, ~(uintptr)0, 0);
-	runtime·gogo(&gp->sched);
-}
-
 void
-runtime∕debug·freeOSMemory(void)
+runtime·scavenge_m(void)
 {
-	runtime·gc(2);  // force GC and do eager sweep
-
-	runtime·mcall(scavenge_m);
+	runtime·MHeap_Scavenge(-1, ~(uintptr)0, 0);
 }
 
 // Initialize a new span with the given start and npages.

src/pkg/runtime/stubs.go

@@ -78,6 +78,7 @@ var (
 	largeAlloc_m,
 	mprofMalloc_m,
 	gc_m,
+	scavenge_m,
 	setFinalizer_m,
 	removeFinalizer_m,
 	markallocated_m,
@@ -111,8 +112,7 @@ func memmove(to unsafe.Pointer, from unsafe.Pointer, n uintptr)
 func fastrand2() uint32
 
 const (
-	gcpercentUnknown = -2
-	concurrentSweep  = true
+	concurrentSweep = true
 )
 
 func gosched()

src/pkg/runtime/thunk.s

@@ -61,3 +61,6 @@ TEXT reflect·chanlen(SB), NOSPLIT, $0-0
 
 TEXT reflect·chancap(SB), NOSPLIT, $0-0
 	JMP	runtime·reflect_chancap(SB)
+
+TEXT runtime∕debug·freeOSMemory(SB), NOSPLIT, $0-0
+	JMP	runtime·freeOSMemory(SB)