runtime: decentralize mark done and mark termination

This moves all of the mark 1 to mark 2 transition and mark termination
to the mark done transition function. This means these transitions are
now handled on the goroutine that detected mark completion. This also
means that the GC coordinator and the background completion barriers
are no longer used and various workarounds to yield to the coordinator
are no longer necessary. These will be removed in follow-up commits.

One consequence of this is that mark workers now need to be
preemptible when performing the mark done transition. This allows them
to stop the world and to perform the final clean-up steps of GC after
restarting the world. They are only made preemptible while performing
this transition, so if the worker findRunnableGCWorker would schedule
isn't available, we didn't want to schedule it anyway.

Fixes #11970.

Change-Id: I9203a2d6287eeff62d589ec02ad9cb1e29ddb837
Reviewed-on: https://go-review.googlesource.com/16391Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/malloc.go

@@ -742,8 +742,7 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 		// down this G's allocation and help the GC stay
 		// scheduled by yielding.
 		//
-		// TODO: This is a workaround. Either help the GC make
-		// the transition or block.
+		// TODO: This is unused. Remove.
 		gp := getg()
 		if gp != gp.m.g0 && gp.m.locks == 0 && gp.m.preemptoff == "" {
 			Gosched()

src/runtime/mgc.go

@@ -166,6 +166,7 @@ func gcinit() {
 	}
 	memstats.next_gc = heapminimum
 	work.startSema = 1
+	work.markDoneSema = 1
 }
 
 func readgogc() int32 {
@@ -615,14 +616,9 @@ func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
 		throw("gcControllerState.findRunnable: blackening not enabled")
 	}
 	if _p_.gcBgMarkWorker == nil {
-		throw("gcControllerState.findRunnable: no background mark worker")
-	}
-	if work.bgMark1.done != 0 && work.bgMark2.done != 0 {
-		// Background mark is done. Don't schedule background
-		// mark worker any more. (This is not just an
-		// optimization. Without this we can spin scheduling
-		// the background worker and having it return
-		// immediately with no work to do.)
+		// The mark worker associated with this P is blocked
+		// performing a mark transition. We can't run it
+		// because it may be on some other run or wait queue.
 		return nil
 	}
 
@@ -821,12 +817,16 @@ var work struct {
 	// startSema protects the transition from "off" to mark or
 	// mark termination.
 	startSema uint32
+	// markDoneSema protects transitions from mark 1 to mark 2 and
+	// from mark 2 to mark termination.
+	markDoneSema uint32
 
 	bgMarkReady note   // signal background mark worker has started
 	bgMarkDone  uint32 // cas to 1 when at a background mark completion point
 	// Background mark completion signaling
 
 	// Coordination for the 2 parts of the mark phase.
+	// TODO(austin): Unused. Remove.
 	bgMark1 bgMarkSignal
 	bgMark2 bgMarkSignal
 
@@ -894,8 +894,7 @@ const (
 
 // startGCCoordinator starts and readies the GC coordinator goroutine.
 //
-// TODO(austin): This function is temporary and will go away when we
-// finish the transition to the decentralized state machine.
+// TODO(austin): This function unused. Remove it and backgroundgc.
 func startGCCoordinator() {
 	// trigger concurrent GC
 	readied := false
@@ -932,7 +931,7 @@ var bggc struct {
 // bggc holds the state of the backgroundgc.
 func backgroundgc() {
 	for {
-		gc(gcBackgroundMode)
+		gcMarkTermination()
 		lock(&bggc.lock)
 		bggc.working--
 		unlock(&bggc.lock)
@@ -1089,26 +1088,23 @@ func gcStart(mode gcMode, forceTrigger bool) {
 		// mutators.
 		atomicstore(&gcBlackenEnabled, 1)
 
+		// Assists and workers can start the moment we start
+		// the world.
+		gcController.assistStartTime = now
+		gcController.bgMarkStartTime = now
+
 		// Concurrent mark.
 		systemstack(startTheWorldWithSema)
 		now = nanotime()
 		work.pauseNS += now - work.pauseStart
-		gcController.assistStartTime = now
 		work.tMark = now
-
-		// Enable background mark workers.
-		gcController.bgMarkStartTime = now
-		work.bgMark1.clear()
-
-		// TODO: Make mark 1 completion handle the transition.
-		startGCCoordinator()
 	} else {
 		t := nanotime()
 		work.tMark, work.tMarkTerm = t, t
 		work.heapGoal = work.heap0
 
 		// Perform mark termination. This will restart the world.
-		gc(mode)
+		gcMarkTermination()
 	}
 
 	if useStartSema {
@@ -1125,60 +1121,74 @@ func gcStart(mode gcMode, forceTrigger bool) {
 // work may still be cached in per-P work buffers. In mark 2, per-P
 // caches are disabled.
 func gcMarkDone() {
-	// TODO(austin): This should perform the transition rather
-	// than handing it off to the coordinator.
-	if gcBlackenPromptly {
-		if work.bgMark1.done == 0 {
-			throw("completing mark 2, but bgMark1.done == 0")
-		}
-		work.bgMark2.complete()
-	} else {
-		work.bgMark1.complete()
-	}
-}
+	semacquire(&work.markDoneSema, false)
 
-func gc(mode gcMode) {
-	// If mode == gcBackgroundMode, world is not stopped.
-	// If mode != gcBackgroundMode, world is stopped.
-	// TODO(austin): This is temporary.
-
-	if mode == gcBackgroundMode {
-		// Wait for background mark completion.
-		work.bgMark1.wait()
+	// Re-check transition condition under transition lock.
+	if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
+		semrelease(&work.markDoneSema)
+		return
+	}
 
-		gcMarkRootCheck()
+	// Disallow starting new workers so that any remaining workers
+	// in the current mark phase will drain out.
+	//
+	// TODO(austin): Should dedicated workers keep an eye on this
+	// and exit gcDrain promptly?
+	xaddint64(&gcController.dedicatedMarkWorkersNeeded, -0xffffffff)
+	xaddint64(&gcController.fractionalMarkWorkersNeeded, -0xffffffff)
 
+	if !gcBlackenPromptly {
+		// Transition from mark 1 to mark 2.
+		//
 		// The global work list is empty, but there can still be work
 		// sitting in the per-P work caches and there can be more
 		// objects reachable from global roots since they don't have write
 		// barriers. Rescan some roots and flush work caches.
-		systemstack(func() {
-			// Disallow caching workbufs.
+
+		gcMarkRootCheck()
+
+		// Disallow caching workbufs and indicate that we're in mark 2.
 		gcBlackenPromptly = true
 
-			// Flush all currently cached workbufs. This
-			// also forces any remaining background
-			// workers out of their loop.
-			forEachP(func(_p_ *p) {
-				_p_.gcw.dispose()
-			})
+		// Prevent completion of mark 2 until we've flushed
+		// cached workbufs.
+		xadd(&work.nwait, -1)
 
-			// Rescan global data and BSS. Bump "jobs"
-			// down before "next" so workers won't try
-			// running root jobs until we set "next".
-			//
-			// This also ensures there will be queued mark
-			// work, which ensures some mark worker will
-			// run and signal mark 2 completion.
+		// Rescan global data and BSS. There may still work
+		// workers running at this point, so bump "jobs" down
+		// before "next" so they won't try running root jobs
+		// until we set next.
 		atomicstore(&work.markrootJobs, uint32(fixedRootCount+work.nDataRoots+work.nBSSRoots))
 		atomicstore(&work.markrootNext, fixedRootCount)
+
+		// GC is set up for mark 2. Let Gs blocked on the
+		// transition lock go while we flush caches.
+		semrelease(&work.markDoneSema)
+
+		systemstack(func() {
+			// Flush all currently cached workbufs and
+			// ensure all Ps see gcBlackenPromptly. This
+			// also blocks until any remaining mark 1
+			// workers have exited their loop so we can
+			// start new mark 2 workers that will observe
+			// the new root marking jobs.
+			forEachP(func(_p_ *p) {
+				_p_.gcw.dispose()
+			})
 		})
 
-		// Wait for this more aggressive background mark to complete.
-		work.bgMark2.clear()
-		work.bgMark2.wait()
+		// Now we can start up mark 2 workers.
+		xaddint64(&gcController.dedicatedMarkWorkersNeeded, 0xffffffff)
+		xaddint64(&gcController.fractionalMarkWorkersNeeded, 0xffffffff)
 
-		// Begin mark termination.
+		incnwait := xadd(&work.nwait, +1)
+		if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
+			// This recursion is safe because the call
+			// can't take this same "if" branch.
+			gcMarkDone()
+		}
+	} else {
+		// Transition to mark termination.
 		now := nanotime()
 		work.tMarkTerm = now
 		work.pauseStart = now
@@ -1200,9 +1210,19 @@ func gc(mode gcMode) {
 		// start the world again.
 		gcWakeAllAssists()
 
+		// Likewise, release the transition lock. Blocked
+		// workers and assists will run when we start the
+		// world again.
+		semrelease(&work.markDoneSema)
+
 		gcController.endCycle()
+
+		// Perform mark termination. This will restart the world.
+		gcMarkTermination()
 	}
+}
 
+func gcMarkTermination() {
 	// World is stopped.
 	// Start marktermination which includes enabling the write barrier.
 	atomicstore(&gcBlackenEnabled, 0)
@@ -1411,13 +1431,13 @@ func gcBgMarkPrepare() {
 
 func gcBgMarkWorker(p *p) {
 	// Register this G as the background mark worker for p.
-	if p.gcBgMarkWorker != nil {
-		throw("P already has a background mark worker")
+	casgp := func(gpp **g, old, new *g) bool {
+		return casp((*unsafe.Pointer)(unsafe.Pointer(gpp)), unsafe.Pointer(old), unsafe.Pointer(new))
 	}
-	gp := getg()
 
+	gp := getg()
 	mp := acquirem()
-	p.gcBgMarkWorker = gp
+	owned := casgp(&p.gcBgMarkWorker, nil, gp)
 	// After this point, the background mark worker is scheduled
 	// cooperatively by gcController.findRunnable. Hence, it must
 	// never be preempted, as this would put it into _Grunnable
@@ -1425,6 +1445,13 @@ func gcBgMarkWorker(p *p) {
 	// is set, this puts itself into _Gwaiting to be woken up by
 	// gcController.findRunnable at the appropriate time.
 	notewakeup(&work.bgMarkReady)
+	if !owned {
+		// A sleeping worker came back and reassociated with
+		// the P. That's fine.
+		releasem(mp)
+		return
+	}
+
 	for {
 		// Go to sleep until woken by gcContoller.findRunnable.
 		// We can't releasem yet since even the call to gopark
@@ -1502,7 +1529,25 @@ func gcBgMarkWorker(p *p) {
 		// If this worker reached a background mark completion
 		// point, signal the main GC goroutine.
 		if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
+			// Make this G preemptible and disassociate it
+			// as the worker for this P so
+			// findRunnableGCWorker doesn't try to
+			// schedule it.
+			p.gcBgMarkWorker = nil
+			releasem(mp)
+
 			gcMarkDone()
+
+			// Disable preemption and reassociate with the P.
+			//
+			// We may be running on a different P at this
+			// point, so this has to be done carefully.
+			mp = acquirem()
+			if !casgp(&p.gcBgMarkWorker, nil, gp) {
+				// The P got a new worker.
+				releasem(mp)
+				break
+			}
 		}
 	}
 }

src/runtime/mgcmark.go

@@ -409,7 +409,6 @@ retry:
 		if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
 			// This has reached a background completion
 			// point.
-			gcMarkDone()
 			completed = true
 		}
 		duration := nanotime() - startTime
@@ -422,9 +421,7 @@ retry:
 	})
 
 	if completed {
-		// We called complete() above, so we should yield to
-		// the now-runnable GC coordinator.
-		Gosched()
+		gcMarkDone()
 	}
 
 	if gp.gcAssistBytes < 0 {
@@ -452,7 +449,7 @@ retry:
 		lock(&work.assistQueue.lock)
 
 		// If the GC cycle is over, just return. This is the
-		// likely path if we called Gosched above. We do this
+		// likely path if we completed above. We do this
 		// under the lock to prevent a GC cycle from ending
 		// between this check and queuing the assist.
 		if atomicload(&gcBlackenEnabled) == 0 {

src/runtime/proc.go

@@ -1057,6 +1057,8 @@ func mstart1() {
 // memory barrier. GC uses this as a "ragged barrier."
 //
 // The caller must hold worldsema.
+//
+//go:systemstack
 func forEachP(fn func(*p)) {
 	mp := acquirem()
 	_p_ := getg().m.p.ptr()
@@ -1115,6 +1117,8 @@ func forEachP(fn func(*p)) {
 		for {
 			// Wait for 100us, then try to re-preempt in
 			// case of any races.
+			//
+			// Requires system stack.
 			if notetsleep(&sched.safePointNote, 100*1000) {
 				noteclear(&sched.safePointNote)
 				break
@@ -1773,10 +1777,9 @@ top:
 stop:
 
 	// We have nothing to do. If we're in the GC mark phase, can
-	// safely scan and blacken objects, can start a worker, and
-	// have work to do, run idle-time marking rather than give up
-	// the P.
-	if _p_ := _g_.m.p.ptr(); gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != nil && (work.bgMark1.done == 0 || work.bgMark2.done == 0) && gcMarkWorkAvailable(_p_) {
+	// safely scan and blacken objects, and have work to do, run
+	// idle-time marking rather than give up the P.
+	if _p_ := _g_.m.p.ptr(); gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != nil && gcMarkWorkAvailable(_p_) {
 		_p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
 		gp := _p_.gcBgMarkWorker
 		casgstatus(gp, _Gwaiting, _Grunnable)