runtime: fix freeOSMemory to free memory immediately

Currently freeOSMemory makes only marking phase of GC, but not sweeping phase.
So recently memory is not released after freeOSMemory.
Do both marking and sweeping during freeOSMemory.
Fixes #8019.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://golang.org/cl/97550043

src/pkg/runtime/malloc.goc

@@ -849,16 +849,7 @@ runtime·cnewarray(Type *typ, intgo n)
 }
 
 func GC() {
-	// We assume that the user expects unused memory to have
-	// been freed when GC returns. To ensure this, run gc(1) twice.
-	// The first will do a collection, and the second will force the
-	// first's sweeping to finish before doing a second collection.
-	// The second collection is overkill, but we assume the user
-	// has a good reason for calling runtime.GC and can stand the
-	// expense. At the least, this fixes all the calls to runtime.GC in
-	// tests that expect finalizers to start running when GC returns.
-	runtime·gc(1);
-	runtime·gc(1);
+	runtime·gc(2);  // force GC and do eager sweep
 }
 
 func SetFinalizer(obj Eface, finalizer Eface) {

src/pkg/runtime/mgc0.c

@@ -2239,6 +2239,7 @@ runtime·updatememstats(GCStats *stats)
 struct gc_args
 {
 	int64 start_time; // start time of GC in ns (just before stoptheworld)
+	bool  eagersweep;
 };
 
 static void gc(struct gc_args *args);
@@ -2257,6 +2258,8 @@ readgogc(void)
 	return runtime·atoi(p);
 }
 
+// force = 1 - do GC regardless of current heap usage
+// force = 2 - go GC and eager sweep
 void
 runtime·gc(int32 force)
 {
@@ -2292,7 +2295,7 @@ runtime·gc(int32 force)
 		return;
 
 	runtime·semacquire(&runtime·worldsema, false);
-	if(!force && mstats.heap_alloc < mstats.next_gc) {
+	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);
@@ -2301,6 +2304,7 @@ runtime·gc(int32 force)
 
 	// Ok, we're doing it!  Stop everybody else
 	a.start_time = runtime·nanotime();
+	a.eagersweep = force >= 2;
 	m->gcing = 1;
 	runtime·stoptheworld();
 	
@@ -2490,7 +2494,7 @@ gc(struct gc_args *args)
 	sweep.spanidx = 0;
 
 	// Temporary disable concurrent sweep, because we see failures on builders.
-	if(ConcurrentSweep) {
+	if(ConcurrentSweep && !args->eagersweep) {
 		runtime·lock(&gclock);
 		if(sweep.g == nil)
 			sweep.g = runtime·newproc1(&bgsweepv, nil, 0, 0, runtime·gc);

src/pkg/runtime/mheap.c

@@ -555,7 +555,7 @@ runtime·MHeap_Scavenger(void)
 void
 runtime∕debug·freeOSMemory(void)
 {
-	runtime·gc(1);
+	runtime·gc(2);  // force GC and do eager sweep
 	runtime·lock(&runtime·mheap);
 	scavenge(-1, ~(uintptr)0, 0);
 	runtime·unlock(&runtime·mheap);