runtime: consolidate "out of GC work" checks

We already have gcMarkWorkAvailable, but the check for GC mark work is
open-coded in several places. Generalize gcMarkWorkAvailable slightly
and replace these open-coded checks with calls to gcMarkWorkAvailable.

In addition to cleaning up the code, this puts us in a better position
to make this check slightly more complicated.

Change-Id: I1b29883300ecd82a1bf6be193e9b4ee96582a860
Reviewed-on: https://go-review.googlesource.com/16058Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgc.go

@@ -613,7 +613,7 @@ func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
 		// else for a while, so kick everything out of its run
 		// queue.
 	} else {
-		if _p_.gcw.wbuf == 0 && work.full == 0 {
+		if !gcMarkWorkAvailable(_p_) {
 			// No work to be done right now. This can
 			// happen at the end of the mark phase when
 			// there are still assists tapering off. Don't
@@ -1383,7 +1383,7 @@ func gcBgMarkWorker(p *p) {
 					"work.nwait=", incnwait, "work.nproc=", work.nproc)
 				throw("work.nwait > work.nproc")
 			}
-			done = incnwait == work.nproc && work.full == 0
+			done = incnwait == work.nproc && !gcMarkWorkAvailable(nil)
 		}
 
 		// If this worker reached a background mark completion
@@ -1414,9 +1414,10 @@ func gcBgMarkWorker(p *p) {
 }
 
 // gcMarkWorkAvailable returns true if executing a mark worker
-// on p is potentially useful.
+// on p is potentially useful. p may be nil, in which case it only
+// checks the global sources of work.
 func gcMarkWorkAvailable(p *p) bool {
-	if !p.gcw.empty() {
+	if p != nil && !p.gcw.empty() {
 		return true
 	}
 	if atomicload64(&work.full) != 0 {

src/runtime/mgcmark.go

@@ -386,7 +386,7 @@ retry:
 			throw("work.nwait > work.nproc")
 		}
 
-		if incnwait == work.nproc && work.full == 0 {
+		if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
 			// This has reached a background completion
 			// point.
 			if gcBlackenPromptly {