runtime: fix triggering of forced GC

mstats.last_gc is unix time now, it is compared with abstract monotonic time.
On my machine GC is forced every 5 mins regardless of last_gc.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, iant, rsc
https://golang.org/cl/91350045

src/pkg/runtime/mgc0.c

@@ -91,8 +91,6 @@ enum {
 // Initialized from $GOGC.  GOGC=off means no gc.
 static int32 gcpercent = GcpercentUnknown;
 
-void runtime·gc_unixnanotime(int64 *now);
-
 static FuncVal* poolcleanup;
 
 void
@@ -2406,7 +2404,7 @@ gc(struct gc_args *args)
 	mstats.next_gc = mstats.heap_alloc+mstats.heap_alloc*gcpercent/100;
 
 	t4 = runtime·nanotime();
-	runtime·gc_unixnanotime((int64*)&mstats.last_gc);  // must be Unix time to make sense to user
+	mstats.last_gc = runtime·unixnanotime();  // must be Unix time to make sense to user
 	mstats.pause_ns[mstats.numgc%nelem(mstats.pause_ns)] = t4 - t0;
 	mstats.pause_total_ns += t4 - t0;
 	mstats.numgc++;

src/pkg/runtime/mheap.c

@@ -508,6 +508,7 @@ runtime·MHeap_Scavenger(void)
 {
 	MHeap *h;
 	uint64 tick, now, forcegc, limit;
+	int64 unixnow;
 	int32 k;
 	Note note, *notep;
 
@@ -531,8 +532,8 @@ runtime·MHeap_Scavenger(void)
 		runtime·notetsleepg(&note, tick);
 
 		runtime·lock(h);
-		now = runtime·nanotime();
-		if(now - mstats.last_gc > forcegc) {
+		unixnow = runtime·unixnanotime();
+		if(unixnow - mstats.last_gc > forcegc) {
 			runtime·unlock(h);
 			// The scavenger can not block other goroutines,
 			// otherwise deadlock detector can fire spuriously.
@@ -544,8 +545,8 @@ runtime·MHeap_Scavenger(void)
 			if(runtime·debug.gctrace > 0)
 				runtime·printf("scvg%d: GC forced\n", k);
 			runtime·lock(h);
-			now = runtime·nanotime();
 		}
+		now = runtime·nanotime();
 		scavenge(k, now, limit);
 		runtime·unlock(h);
 	}

src/pkg/runtime/runtime.h

@@ -920,7 +920,8 @@ void	runtime·exitsyscall(void);
 G*	runtime·newproc1(FuncVal*, byte*, int32, int32, void*);
 bool	runtime·sigsend(int32 sig);
 int32	runtime·callers(int32, uintptr*, int32);
-int64	runtime·nanotime(void);
+int64	runtime·nanotime(void);	// monotonic time
+int64	runtime·unixnanotime(void); // real time, can skip
 void	runtime·dopanic(int32);
 void	runtime·startpanic(void);
 void	runtime·freezetheworld(void);

src/pkg/runtime/time.goc

@@ -54,6 +54,16 @@ func stopTimer(t *Timer) (stopped bool) {
 
 // C runtime.
 
+void runtime·gc_unixnanotime(int64 *now);
+
+int64 runtime·unixnanotime(void)
+{
+	int64 now;
+
+	runtime·gc_unixnanotime(&now);
+	return now;
+}
+
 static void timerproc(void);
 static void siftup(int32);
 static void siftdown(int32);