runtime: synchronize P wakeup and dropping Ps

CL 310850 dropped work re-checks on non-spinning Ms to fix #43997.

This introduced a new race condition: a non-spinning M may drop its P
and then park at the same time a spinning M attempts to wake a P to
handle some new work. The spinning M fails to find an idle P (because
the non-spinning M hasn't quite made its P idle yet), and does nothing
assuming that the system is fully loaded. This results in loss of work
conservation. In the worst case we could have a complete deadlock if
injectglist fails to wake anything just as all Ps are going idle.

sched.needspinning adds new synchronization to cover this case. If work
submission fails to find a P, it sets needspinning to indicate that a
spinning M is required. When non-spinning Ms prepare to drop their P,
they check needspinning and abort going idle to become a spinning M
instead. This addresses the race without extra spurious wakeups. In the
normal (non-racing case), an M will become spinning via the normal path
and clear the flag.

injectglist must change in addition to wakep because it is a similar
form of work submission, notably used following netpoll at a point when
we might not have a P that would guarantee the work runs.

Fixes #45867

Change-Id: Ieb623a6d4162fb8c2be7b4ff8acdebcc3a0d69a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/389014
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

src/runtime/proc.go

@@ -73,7 +73,7 @@ var modinfo string
 // If there is at least one spinning thread (sched.nmspinning>1), we don't
 // unpark new threads when submitting work. To compensate for that, if the last
 // spinning thread finds work and stops spinning, it must unpark a new spinning
-// thread.  This approach smooths out unjustified spikes of thread unparking,
+// thread. This approach smooths out unjustified spikes of thread unparking,
 // but at the same time guarantees eventual maximal CPU parallelism
 // utilization.
 //
@@ -827,6 +827,12 @@ func mcommoninit(mp *m, id int64) {
 	}
 }
 
+func (mp *m) becomeSpinning() {
+	mp.spinning = true
+	sched.nmspinning.Add(1)
+	sched.needspinning.Store(0)
+}
+
 var fastrandseed uintptr
 
 func fastrandinit() {
@@ -2242,8 +2248,8 @@ func mspinning() {
 // Schedules some M to run the p (creates an M if necessary).
 // If p==nil, tries to get an idle P, if no idle P's does nothing.
 // May run with m.p==nil, so write barriers are not allowed.
-// If spinning is set, the caller has incremented nmspinning and startm will
-// either decrement nmspinning or set m.spinning in the newly started M.
+// If spinning is set, the caller has incremented nmspinning and must provide a
+// P. startm will set m.spinning in the newly started M.
 //
 // Callers passing a non-nil P must call from a non-preemptible context. See
 // comment on acquirem below.
@@ -2271,16 +2277,15 @@ func startm(pp *p, spinning bool) {
 	mp := acquirem()
 	lock(&sched.lock)
 	if pp == nil {
+		if spinning {
+			// TODO(prattmic): All remaining calls to this function
+			// with _p_ == nil could be cleaned up to find a P
+			// before calling startm.
+			throw("startm: P required for spinning=true")
+		}
 		pp, _ = pidleget(0)
 		if pp == nil {
 			unlock(&sched.lock)
-			if spinning {
-				// The caller incremented nmspinning, but there are no idle Ps,
-				// so it's okay to just undo the increment and give up.
-				if sched.nmspinning.Add(-1) < 0 {
-					throw("startm: negative nmspinning")
-				}
-			}
 			releasem(mp)
 			return
 		}
@@ -2358,6 +2363,7 @@ func handoffp(pp *p) {
 	// no local work, check that there are no spinning/idle M's,
 	// otherwise our help is not required
 	if sched.nmspinning.Load()+sched.npidle.Load() == 0 && sched.nmspinning.CompareAndSwap(0, 1) { // TODO: fast atomic
+		sched.needspinning.Store(0)
 		startm(pp, true)
 		return
 	}
@@ -2404,15 +2410,41 @@ func handoffp(pp *p) {
 
 // Tries to add one more P to execute G's.
 // Called when a G is made runnable (newproc, ready).
+// Must be called with a P.
 func wakep() {
-	if sched.npidle.Load() == 0 {
+	// Be conservative about spinning threads, only start one if none exist
+	// already.
+	if sched.nmspinning.Load() != 0 || !sched.nmspinning.CompareAndSwap(0, 1) {
 		return
 	}
-	// be conservative about spinning threads
-	if sched.nmspinning.Load() != 0 || !sched.nmspinning.CompareAndSwap(0, 1) {
+
+	// Disable preemption until ownership of pp transfers to the next M in
+	// startm. Otherwise preemption here would leave pp stuck waiting to
+	// enter _Pgcstop.
+	//
+	// See preemption comment on acquirem in startm for more details.
+	mp := acquirem()
+
+	var pp *p
+	lock(&sched.lock)
+	pp, _ = pidlegetSpinning(0)
+	if pp == nil {
+		if sched.nmspinning.Add(-1) < 0 {
+			throw("wakep: negative nmspinning")
+		}
+		unlock(&sched.lock)
+		releasem(mp)
 		return
 	}
-	startm(nil, true)
+	// Since we always have a P, the race in the "No M is available"
+	// comment in startm doesn't apply during the small window between the
+	// unlock here and lock in startm. A checkdead in between will always
+	// see at least one running M (ours).
+	unlock(&sched.lock)
+
+	startm(pp, true)
+
+	releasem(mp)
 }
 
 // Stops execution of the current m that is locked to a g until the g is runnable again.
@@ -2646,8 +2678,7 @@ top:
 	// GOMAXPROCS>>1 but the program parallelism is low.
 	if mp.spinning || 2*sched.nmspinning.Load() < gomaxprocs-sched.npidle.Load() {
 		if !mp.spinning {
-			mp.spinning = true
-			sched.nmspinning.Add(1)
+			mp.becomeSpinning()
 		}
 
 		gp, inheritTime, tnow, w, newWork := stealWork(now)
@@ -2723,6 +2754,12 @@ top:
 		unlock(&sched.lock)
 		return gp, false, false
 	}
+	if !mp.spinning && sched.needspinning.Load() == 1 {
+		// See "Delicate dance" comment below.
+		mp.becomeSpinning()
+		unlock(&sched.lock)
+		goto top
+	}
 	if releasep() != pp {
 		throw("findrunnable: wrong p")
 	}
@@ -2743,12 +2780,28 @@ top:
 	// * New/modified-earlier timers on a per-P timer heap.
 	// * Idle-priority GC work (barring golang.org/issue/19112).
 	//
-	// If we discover new work below, we need to restore m.spinning as a signal
-	// for resetspinning to unpark a new worker thread (because there can be more
-	// than one starving goroutine). However, if after discovering new work
-	// we also observe no idle Ps it is OK to skip unparking a new worker
-	// thread: the system is fully loaded so no spinning threads are required.
-	// Also see "Worker thread parking/unparking" comment at the top of the file.
+	// If we discover new work below, we need to restore m.spinning as a
+	// signal for resetspinning to unpark a new worker thread (because
+	// there can be more than one starving goroutine).
+	//
+	// However, if after discovering new work we also observe no idle Ps
+	// (either here or in resetspinning), we have a problem. We may be
+	// racing with a non-spinning M in the block above, having found no
+	// work and preparing to release its P and park. Allowing that P to go
+	// idle will result in loss of work conservation (idle P while there is
+	// runnable work). This could result in complete deadlock in the
+	// unlikely event that we discover new work (from netpoll) right as we
+	// are racing with _all_ other Ps going idle.
+	//
+	// We use sched.needspinning to synchronize with non-spinning Ms going
+	// idle. If needspinning is set when they are about to drop their P,
+	// they abort the drop and instead become a new spinning M on our
+	// behalf. If we are not racing and the system is truly fully loaded
+	// then no spinning threads are required, and the next thread to
+	// naturally become spinning will clear the flag.
+	//
+	// Also see "Worker thread parking/unparking" comment at the top of the
+	// file.
 	wasSpinning := mp.spinning
 	if mp.spinning {
 		mp.spinning = false
@@ -2758,25 +2811,26 @@ top:
 
 		// Note the for correctness, only the last M transitioning from
 		// spinning to non-spinning must perform these rechecks to
-		// ensure no missed work. We are performing it on every M that
-		// transitions as a conservative change to monitor effects on
-		// latency. See golang.org/issue/43997.
+		// ensure no missed work. However, the runtime has some cases
+		// of transient increments of nmspinning that are decremented
+		// without going through this path, so we must be conservative
+		// and perform the check on all spinning Ms.
+		//
+		// See https://go.dev/issue/43997.
 
 		// Check all runqueues once again.
 		pp := checkRunqsNoP(allpSnapshot, idlepMaskSnapshot)
 		if pp != nil {
 			acquirep(pp)
-			mp.spinning = true
-			sched.nmspinning.Add(1)
+			mp.becomeSpinning()
 			goto top
 		}
 
 		// Check for idle-priority GC work again.
 		pp, gp := checkIdleGCNoP()
 		if pp != nil {
 			acquirep(pp)
-			mp.spinning = true
-			sched.nmspinning.Add(1)
+			mp.becomeSpinning()
 
 			// Run the idle worker.
 			pp.gcMarkWorkerMode = gcMarkWorkerIdleMode
@@ -2844,8 +2898,7 @@ top:
 				return gp, false, false
 			}
 			if wasSpinning {
-				mp.spinning = true
-				sched.nmspinning.Add(1)
+				mp.becomeSpinning()
 			}
 			goto top
 		}
@@ -2964,17 +3017,18 @@ func checkRunqsNoP(allpSnapshot []*p, idlepMaskSnapshot pMask) *p {
 	for id, p2 := range allpSnapshot {
 		if !idlepMaskSnapshot.read(uint32(id)) && !runqempty(p2) {
 			lock(&sched.lock)
-			pp, _ := pidleget(0)
-			unlock(&sched.lock)
-			if pp != nil {
-				return pp
+			pp, _ := pidlegetSpinning(0)
+			if pp == nil {
+				// Can't get a P, don't bother checking remaining Ps.
+				unlock(&sched.lock)
+				return nil
 			}
-
-			// Can't get a P, don't bother checking remaining Ps.
-			break
+			unlock(&sched.lock)
+			return pp
 		}
 	}
 
+	// No work available.
 	return nil
 }
 
@@ -3030,7 +3084,7 @@ func checkIdleGCNoP() (*p, *g) {
 	// the assumption in gcControllerState.findRunnableGCWorker that an
 	// empty gcBgMarkWorkerPool is only possible if gcMarkDone is running.
 	lock(&sched.lock)
-	pp, now := pidleget(0)
+	pp, now := pidlegetSpinning(0)
 	if pp == nil {
 		unlock(&sched.lock)
 		return nil, nil
@@ -3130,8 +3184,20 @@ func injectglist(glist *gList) {
 	*glist = gList{}
 
 	startIdle := func(n int) {
-		for ; n != 0 && sched.npidle.Load() != 0; n-- {
-			startm(nil, false)
+		for i := 0; i < n; i++ {
+			mp := acquirem() // See comment in startm.
+			lock(&sched.lock)
+
+			pp, _ := pidlegetSpinning(0)
+			if pp == nil {
+				unlock(&sched.lock)
+				releasem(mp)
+				break
+			}
+
+			unlock(&sched.lock)
+			startm(pp, false)
+			releasem(mp)
 		}
 	}
 
@@ -5406,7 +5472,7 @@ func schedtrace(detailed bool) {
 	}
 
 	lock(&sched.lock)
-	print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle.Load(), " threads=", mcount(), " spinningthreads=", sched.nmspinning.Load(), " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
+	print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle.Load(), " threads=", mcount(), " spinningthreads=", sched.nmspinning.Load(), " needspinning=", sched.needspinning.Load(), " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
 	if detailed {
 		print(" gcwaiting=", sched.gcwaiting.Load(), " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait.Load(), "\n")
 	}
@@ -5742,6 +5808,31 @@ func pidleget(now int64) (*p, int64) {
 	return pp, now
 }
 
+// pidlegetSpinning tries to get a p from the _Pidle list, acquiring ownership.
+// This is called by spinning Ms (or callers than need a spinning M) that have
+// found work. If no P is available, this must synchronized with non-spinning
+// Ms that may be preparing to drop their P without discovering this work.
+//
+// sched.lock must be held.
+//
+// May run during STW, so write barriers are not allowed.
+//
+//go:nowritebarrierrec
+func pidlegetSpinning(now int64) (*p, int64) {
+	assertLockHeld(&sched.lock)
+
+	pp, now := pidleget(now)
+	if pp == nil {
+		// See "Delicate dance" comment in findrunnable. We found work
+		// that we cannot take, we must synchronize with non-spinning
+		// Ms that may be preparing to drop their P.
+		sched.needspinning.Store(1)
+		return nil, now
+	}
+
+	return pp, now
+}
+
 // runqempty reports whether pp has no Gs on its local run queue.
 // It never returns true spuriously.
 func runqempty(pp *p) bool {

src/runtime/runtime2.go

@@ -777,9 +777,10 @@ type schedt struct {
 
 	ngsys atomic.Int32 // number of system goroutines
 
-	pidle      puintptr // idle p's
-	npidle     atomic.Int32
-	nmspinning atomic.Int32 // See "Worker thread parking/unparking" comment in proc.go.
+	pidle        puintptr // idle p's
+	npidle       atomic.Int32
+	nmspinning   atomic.Int32 // See "Worker thread parking/unparking" comment in proc.go.
+	needspinning atomic.Uint32 // See "Delicate dance" comment in proc.go. Boolean. Must hold sched.lock to set to 1.
 
 	// Global runnable queue.
 	runq     gQueue
@@ -840,6 +841,8 @@ type schedt struct {
 	// with the rest of the runtime.
 	sysmonlock mutex
 
+	_ uint32 // ensure timeToRun has 8-byte alignment
+
 	// timeToRun is a distribution of scheduling latencies, defined
 	// as the sum of time a G spends in the _Grunnable state before
 	// it transitions to _Grunning.