runtime: start GC background sweep eagerly

Starting it lazily causes a memory allocation (for the goroutine) during GC.

First use of channels for runtime implementation.

Change-Id: I9cd24dcadbbf0ee5070ee6d0ed7ea415504f316c
Reviewed-on: https://go-review.googlesource.com/6960
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgc.go

@@ -176,6 +176,16 @@ func gcinit() {
 	memstats.next_gc = heapminimum
 }
 
+// gcenable is called after the bulk of the runtime initialization,
+// just before we're about to start letting user code run.
+// It kicks off the background sweeper goroutine and enables GC.
+func gcenable() {
+	c := make(chan int, 1)
+	go bgsweep(c)
+	<-c
+	memstats.enablegc = true // now that runtime is initialized, GC is okay
+}
+
 func setGCPercent(in int32) (out int32) {
 	lock(&mheap_.lock)
 	out = gcpercent
@@ -568,10 +578,7 @@ func gcSweep(mode int) {
 
 	// Background sweep.
 	lock(&sweep.lock)
-	if !sweep.started {
+	if sweep.parked {
-		go bgsweep()
-		sweep.started = true
-	} else if sweep.parked {
 		sweep.parked = false
 		ready(sweep.g)
 	}

src/runtime/mgcsweep.go

@@ -42,8 +42,14 @@ func finishsweep_m() {
 	}
 }
 
-func bgsweep() {
+func bgsweep(c chan int) {
 	sweep.g = getg()
+
+	lock(&sweep.lock)
+	sweep.parked = true
+	c <- 1
+	goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock)
+
 	for {
 		for gosweepone() != ^uintptr(0) {
 			sweep.nbgsweep++

src/runtime/proc.go

@@ -58,7 +58,7 @@ func main() {
 		}
 	}()
 
-	memstats.enablegc = true // now that runtime is initialized, GC is okay
+	gcenable()
 
 	if iscgo {
 		if _cgo_thread_start == nil {