runtime: account mark worker time before gcMarkDone

Currently gcMarkDone takes basically no time, so it's okay to account
the worker time after calling it. However, gcMarkDone is about to take
potentially *much* longer because it may perform all of mark
termination. Prepare for this by swapping the order so we account the
time before calling gcMarkDone.

Change-Id: I90c7df68192acfc4fd02a7254dae739dda4e2fcb
Reviewed-on: https://go-review.googlesource.com/16390Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgc.go

@@ -1477,6 +1477,19 @@ func gcBgMarkWorker(p *p) {
 			p.gcw.dispose()
 		}
 
+		// Account for time.
+		duration := nanotime() - startTime
+		switch p.gcMarkWorkerMode {
+		case gcMarkWorkerDedicatedMode:
+			xaddint64(&gcController.dedicatedMarkTime, duration)
+			xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1)
+		case gcMarkWorkerFractionalMode:
+			xaddint64(&gcController.fractionalMarkTime, duration)
+			xaddint64(&gcController.fractionalMarkWorkersNeeded, 1)
+		case gcMarkWorkerIdleMode:
+			xaddint64(&gcController.idleMarkTime, duration)
+		}
+
 		// Was this the last worker and did we run out
 		// of work?
 		incnwait := xadd(&work.nwait, +1)
@@ -1491,18 +1504,6 @@ func gcBgMarkWorker(p *p) {
 		if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
 			gcMarkDone()
 		}
-
-		duration := nanotime() - startTime
-		switch p.gcMarkWorkerMode {
-		case gcMarkWorkerDedicatedMode:
-			xaddint64(&gcController.dedicatedMarkTime, duration)
-			xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1)
-		case gcMarkWorkerFractionalMode:
-			xaddint64(&gcController.fractionalMarkTime, duration)
-			xaddint64(&gcController.fractionalMarkWorkersNeeded, 1)
-		case gcMarkWorkerIdleMode:
-			xaddint64(&gcController.idleMarkTime, duration)
-		}
 	}
 }