runtime: keep P's first timer when in new atomically accessed field

This reduces lock contention when only a few P's are running and
checking for whether they need to run timers on the sleeping P's.
Without this change the running P's would get lock contention
while looking at the sleeping P's timers. With this change a single
atomic load suffices to determine whether there are any ready timers.

Change-Id: Ie843782bd56df49867a01ecf19c47498ec827452
Reviewed-on: https://go-review.googlesource.com/c/go/+/214185
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/runtime/proc.go

@@ -2621,6 +2621,21 @@ func dropg() {
 // We pass now in and out to avoid extra calls of nanotime.
 //go:yeswritebarrierrec
 func checkTimers(pp *p, now int64) (rnow, pollUntil int64, ran bool) {
+	// If there are no timers to adjust, and the first timer on
+	// the heap is not yet ready to run, then there is nothing to do.
+	if atomic.Load(&pp.adjustTimers) == 0 {
+		next := int64(atomic.Load64(&pp.timer0When))
+		if next == 0 {
+			return now, 0, false
+		}
+		if now == 0 {
+			now = nanotime()
+		}
+		if now < next {
+			return now, next, false
+		}
+	}
+
 	lock(&pp.timersLock)
 
 	adjusttimers(pp)
@@ -4095,6 +4110,7 @@ func (pp *p) destroy() {
 		pp.timers = nil
 		pp.adjustTimers = 0
 		pp.deletedTimers = 0
+		atomic.Store64(&pp.timer0When, 0)
 		unlock(&pp.timersLock)
 		unlock(&plocal.timersLock)
 	}
@@ -4421,8 +4437,10 @@ func checkdead() {
 	}
 
 	// Maybe jump time forward for playground.
-	_p_ := timejump()
+	if faketime != 0 {
+		when, _p_ := timeSleepUntil()
 		if _p_ != nil {
+			faketime = when
 			for pp := &sched.pidle; *pp != 0; pp = &(*pp).ptr().link {
 				if (*pp).ptr() == _p_ {
 					*pp = _p_.link
@@ -4439,6 +4457,7 @@ func checkdead() {
 			notewakeup(&mp.park)
 			return
 		}
+	}
 
 	// There are no goroutines running, so we can look at the P's.
 	for _, _p_ := range allp {
@@ -4482,7 +4501,7 @@ func sysmon() {
 		}
 		usleep(delay)
 		now := nanotime()
-		next := timeSleepUntil()
+		next, _ := timeSleepUntil()
 		if debug.schedtrace <= 0 && (sched.gcwaiting != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs)) {
 			lock(&sched.lock)
 			if atomic.Load(&sched.gcwaiting) != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs) {
@@ -4504,7 +4523,7 @@ func sysmon() {
 						osRelax(false)
 					}
 					now = nanotime()
-					next = timeSleepUntil()
+					next, _ = timeSleepUntil()
 					lock(&sched.lock)
 					atomic.Store(&sched.sysmonwait, 0)
 					noteclear(&sched.sysmonnote)

src/runtime/runtime2.go

@@ -613,6 +613,11 @@ type p struct {
 
 	_ uint32 // Alignment for atomic fields below
 
+	// The when field of the first entry on the timer heap.
+	// This is updated using atomic functions.
+	// This is 0 if the timer heap is empty.
+	timer0When uint64
+
 	// Per-P GC state
 	gcAssistTime         int64    // Nanoseconds in assistAlloc
 	gcFractionalMarkTime int64    // Nanoseconds in fractional mark worker (atomic)

src/runtime/time.go

@@ -288,7 +288,11 @@ func doaddtimer(pp *p, t *timer) bool {
 	t.pp.set(pp)
 	i := len(pp.timers)
 	pp.timers = append(pp.timers, t)
-	return siftupTimer(pp.timers, i)
+	ok := siftupTimer(pp.timers, i)
+	if t == pp.timers[0] {
+		atomic.Store64(&pp.timer0When, uint64(t.when))
+	}
+	return ok
 }
 
 // deltimer deletes the timer t. It may be on some other P, so we can't
@@ -363,6 +367,9 @@ func dodeltimer(pp *p, i int) bool {
 			ok = false
 		}
 	}
+	if i == 0 {
+		updateTimer0When(pp)
+	}
 	return ok
 }
 
@@ -386,6 +393,7 @@ func dodeltimer0(pp *p) bool {
 	if last > 0 {
 		ok = siftdownTimer(pp.timers, 0)
 	}
+	updateTimer0When(pp)
 	return ok
 }
 
@@ -729,17 +737,11 @@ func addAdjustedTimers(pp *p, moved []*timer) {
 // The netpoller M will wake up and adjust timers before sleeping again.
 //go:nowritebarrierrec
 func nobarrierWakeTime(pp *p) int64 {
-	lock(&pp.timersLock)
-	ret := int64(0)
-	if len(pp.timers) > 0 {
 	if atomic.Load(&pp.adjustTimers) > 0 {
-			ret = nanotime()
+		return nanotime()
 	} else {
-			ret = pp.timers[0].when
-		}
+		return int64(atomic.Load64(&pp.timer0When))
 	}
-	unlock(&pp.timersLock)
-	return ret
 }
 
 // runtimer examines the first timer in timers. If it is ready based on now,
@@ -847,6 +849,7 @@ func runOneTimer(pp *p, t *timer, now int64) {
 		if !atomic.Cas(&t.status, timerRunning, timerWaiting) {
 			badTimer()
 		}
+		updateTimer0When(pp)
 	} else {
 		// Remove from heap.
 		if !dodeltimer0(pp) {
@@ -958,6 +961,7 @@ nextTimer:
 	pp.timers = timers
 	atomic.Xadd(&pp.deletedTimers, -cdel)
 	atomic.Xadd(&pp.adjustTimers, -cearlier)
+	updateTimer0When(pp)
 }
 
 // verifyTimerHeap verifies that the timer heap is in a valid state.
@@ -979,69 +983,22 @@ func verifyTimerHeap(timers []*timer) {
 	}
 }
 
-func timejump() *p {
-	if faketime == 0 {
-		return nil
-	}
-
-	// Nothing is running, so we can look at all the P's.
-	// Determine a timer bucket with minimum when.
-	var (
-		minT    *timer
-		minWhen int64
-		minP    *p
-	)
-	for _, pp := range allp {
-		if pp.status != _Pidle && pp.status != _Pdead {
-			throw("non-idle P in timejump")
-		}
+// updateTimer0When sets the P's timer0When field.
+// The caller must have locked the timers for pp.
+func updateTimer0When(pp *p) {
 	if len(pp.timers) == 0 {
-			continue
-		}
-		c := pp.adjustTimers
-		for _, t := range pp.timers {
-			switch s := atomic.Load(&t.status); s {
-			case timerWaiting:
-				if minT == nil || t.when < minWhen {
-					minT = t
-					minWhen = t.when
-					minP = pp
-				}
-			case timerModifiedEarlier, timerModifiedLater:
-				if minT == nil || t.nextwhen < minWhen {
-					minT = t
-					minWhen = t.nextwhen
-					minP = pp
-				}
-				if s == timerModifiedEarlier {
-					c--
-				}
-			case timerRunning, timerModifying, timerMoving:
-				badTimer()
-			}
-			// The timers are sorted, so we only have to check
-			// the first timer for each P, unless there are
-			// some timerModifiedEarlier timers. The number
-			// of timerModifiedEarlier timers is in the adjustTimers
-			// field, used to initialize c, above.
-			if c == 0 {
-				break
-			}
-		}
-	}
-
-	if minT == nil || minWhen <= faketime {
-		return nil
+		atomic.Store64(&pp.timer0When, 0)
+	} else {
+		atomic.Store64(&pp.timer0When, uint64(pp.timers[0].when))
 	}
-
-	faketime = minWhen
-	return minP
 }
 
-// timeSleepUntil returns the time when the next timer should fire.
-// This is only called by sysmon.
-func timeSleepUntil() int64 {
+// timeSleepUntil returns the time when the next timer should fire,
+// and the P that holds the timer heap that that timer is on.
+// This is only called by sysmon and checkdead.
+func timeSleepUntil() (int64, *p) {
 	next := int64(maxWhen)
+	var pret *p
 
 	// Prevent allp slice changes. This is like retake.
 	lock(&allpLock)
@@ -1052,8 +1009,17 @@ func timeSleepUntil() int64 {
 			continue
 		}
 
-		lock(&pp.timersLock)
 		c := atomic.Load(&pp.adjustTimers)
+		if c == 0 {
+			w := int64(atomic.Load64(&pp.timer0When))
+			if w != 0 && w < next {
+				next = w
+				pret = pp
+			}
+			continue
+		}
+
+		lock(&pp.timersLock)
 		for _, t := range pp.timers {
 			switch s := atomic.Load(&t.status); s {
 			case timerWaiting:
@@ -1088,7 +1054,7 @@ func timeSleepUntil() int64 {
 	}
 	unlock(&allpLock)
 
-	return next
+	return next, pret
 }
 
 // Heap maintenance algorithms.