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>

1 files changed, 38 insertions, 72 deletions

diff --git a/src/runtime/time.go b/src/runtime/time.go
index 6c34268d88..6c3c1a63c4 100644
--- a/src/runtime/time.go
+++ b/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()
-		} else {
-			ret = pp.timers[0].when
-		}
+	if atomic.Load(&pp.adjustTimers) > 0 {
+		return nanotime()
+	} else {
+		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")
-		}
-		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
+// 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 {
+		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.