runtime: consolidate "is sweep done" conditions

The runtime currently has two different notions of sweep completion:

1. All spans are either swept or have begun sweeping.

2. The sweeper has *finished* sweeping all spans.

Having both is confusing (it doesn't help that the documentation is
often unclear or wrong). Condition 2 is stronger and the theoretical
slight optimization that condition 1 could impact is never actually
useful. Hence, this CL consolidates both conditions down to condition 2.

Updates #45315.

Change-Id: I55c84d767d74eb31a004a5619eaba2e351162332
Reviewed-on: https://go-review.googlesource.com/c/go/+/307916
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mgc.go

@@ -175,7 +175,7 @@ func gcinit() {
 	}
 
 	// No sweep on the first cycle.
-	mheap_.sweepdone = 1
+	mheap_.sweepDrained = 1
 
 	// Set a reasonable initial GC trigger.
 	memstats.triggerRatio = 7 / 8.0
@@ -1187,7 +1187,7 @@ func GC() {
 	// First, wait for sweeping to finish. (We know there are no
 	// more spans on the sweep queue, but we may be concurrently
 	// sweeping spans, so we have to wait.)
-	for atomic.Load(&work.cycles) == n+1 && atomic.Load(&mheap_.sweepers) != 0 {
+	for atomic.Load(&work.cycles) == n+1 && !isSweepDone() {
 		Gosched()
 	}
 
@@ -2192,7 +2192,7 @@ func gcSweep(mode gcMode) {
 
 	lock(&mheap_.lock)
 	mheap_.sweepgen += 2
-	mheap_.sweepdone = 0
+	mheap_.sweepDrained = 0
 	mheap_.pagesSwept = 0
 	mheap_.sweepArenas = mheap_.allArenas
 	mheap_.reclaimIndex = 0

src/runtime/mgcsweep.go

@@ -238,7 +238,7 @@ func (l *sweepLocker) dispose() {
 	// Decrement the number of active sweepers and if this is the
 	// last one, mark sweep as complete.
 	l.blocking = false
-	if atomic.Xadd(&mheap_.sweepers, -1) == 0 && atomic.Load(&mheap_.sweepdone) != 0 {
+	if atomic.Xadd(&mheap_.sweepers, -1) == 0 && atomic.Load(&mheap_.sweepDrained) != 0 {
 		l.sweepIsDone()
 	}
 }
@@ -257,7 +257,7 @@ func sweepone() uintptr {
 	// increment locks to ensure that the goroutine is not preempted
 	// in the middle of sweep thus leaving the span in an inconsistent state for next GC
 	_g_.m.locks++
-	if atomic.Load(&mheap_.sweepdone) != 0 {
+	if atomic.Load(&mheap_.sweepDrained) != 0 {
 		_g_.m.locks--
 		return ^uintptr(0)
 	}
@@ -271,7 +271,7 @@ func sweepone() uintptr {
 	for {
 		s := mheap_.nextSpanForSweep()
 		if s == nil {
-			noMoreWork = atomic.Cas(&mheap_.sweepdone, 0, 1)
+			noMoreWork = atomic.Cas(&mheap_.sweepDrained, 0, 1)
 			break
 		}
 		if state := s.state.get(); state != mSpanInUse {
@@ -335,14 +335,17 @@ func sweepone() uintptr {
 	return npages
 }
 
-// isSweepDone reports whether all spans are swept or currently being swept.
+// isSweepDone reports whether all spans are swept.
 //
 // Note that this condition may transition from false to true at any
 // time as the sweeper runs. It may transition from true to false if a
 // GC runs; to prevent that the caller must be non-preemptible or must
 // somehow block GC progress.
 func isSweepDone() bool {
-	return mheap_.sweepdone != 0
+	// Check that all spans have at least begun sweeping and there
+	// are no active sweepers. If both are true, then all spans
+	// have finished sweeping.
+	return atomic.Load(&mheap_.sweepDrained) != 0 && atomic.Load(&mheap_.sweepers) == 0
 }
 
 // Returns only when span s has been swept.

src/runtime/mheap.go

@@ -62,11 +62,12 @@ const (
 type mheap struct {
 	// lock must only be acquired on the system stack, otherwise a g
 	// could self-deadlock if its stack grows with the lock held.
-	lock      mutex
-	pages     pageAlloc // page allocation data structure
-	sweepgen  uint32    // sweep generation, see comment in mspan; written during STW
-	sweepdone uint32    // all spans are swept
-	sweepers  uint32    // number of active sweepone calls
+	lock  mutex
+	pages pageAlloc // page allocation data structure
+
+	sweepgen     uint32 // sweep generation, see comment in mspan; written during STW
+	sweepDrained uint32 // all spans are swept or are being swept
+	sweepers     uint32 // number of active sweepone calls
 
 	// allspans is a slice of all mspans ever created. Each mspan
 	// appears exactly once.
@@ -904,7 +905,7 @@ func (h *mheap) alloc(npages uintptr, spanclass spanClass, needzero bool) *mspan
 	systemstack(func() {
 		// To prevent excessive heap growth, before allocating n pages
 		// we need to sweep and reclaim at least n pages.
-		if h.sweepdone == 0 {
+		if !isSweepDone() {
 			h.reclaim(npages)
 		}
 		s = h.allocSpan(npages, spanAllocHeap, spanclass)