runtime: update heap profile stats after world is started

Updating the heap profile stats is one of the most expensive parts of
mark termination other than stack rescanning, but there's really no
need to do this with the world stopped. Move it to right after we've
started the world back up. This creates a *very* small window where
allocations from the next cycle can slip into the profile, but the
exact point where mark termination happens is so non-deterministic
already that a slight reordering here is unimportant.

Change-Id: I2f76f22c70329923ad6a594a2c26869f0736d34e
Reviewed-on: https://go-review.googlesource.com/31363
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -1276,6 +1276,18 @@ func gcMarkTermination() {
 
 	systemstack(startTheWorldWithSema)
 
+	// Update heap profile stats if gcSweep didn't do it. This is
+	// relatively expensive, so we don't want to do it while the
+	// world is stopped, but it needs to happen ASAP after
+	// starting the world to prevent too many allocations from the
+	// next cycle leaking in. It must happen before releasing
+	// worldsema since there are applications that do a
+	// runtime.GC() to update the heap profile and then
+	// immediately collect the profile.
+	if _ConcurrentSweep && work.mode != gcForceBlockMode {
+		mProf_GC()
+	}
+
 	// Free stack spans. This must be done between GC cycles.
 	systemstack(freeStackSpans)
 
@@ -1714,7 +1726,6 @@ func gcSweep(mode gcMode) {
 		ready(sweep.g, 0, true)
 	}
 	unlock(&sweep.lock)
-	mProf_GC()
 }
 
 func gcCopySpans() {