1 files changed, 55 insertions, 44 deletions

diff --git a/src/runtime/time.go b/src/runtime/time.go
index f895bf8443..99290f66d0 100644
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -491,6 +491,8 @@ loop:
 			newStatus = timerModifiedEarlier
 		}
 
+		tpp := t.pp.ptr()
+
 		// Update the adjustTimers field.  Subtract one if we
 		// are removing a timerModifiedEarlier, add one if we
 		// are adding a timerModifiedEarlier.
@@ -500,9 +502,10 @@ loop:
 		}
 		if newStatus == timerModifiedEarlier {
 			adjust++
+			updateTimerModifiedEarliest(tpp, when)
 		}
 		if adjust != 0 {
-			atomic.Xadd(&t.pp.ptr().adjustTimers, adjust)
+			atomic.Xadd(&tpp.adjustTimers, adjust)
 		}
 
 		// Set the new status of the timer.
@@ -637,16 +640,36 @@ func moveTimers(pp *p, timers []*timer) {
 // the correct place in the heap. While looking for those timers,
 // it also moves timers that have been modified to run later,
 // and removes deleted timers. The caller must have locked the timers for pp.
-func adjusttimers(pp *p) {
-	if len(pp.timers) == 0 {
-		return
-	}
+func adjusttimers(pp *p, now int64) {
 	if atomic.Load(&pp.adjustTimers) == 0 {
 		if verifyTimers {
 			verifyTimerHeap(pp)
 		}
+		// There are no timers to adjust, so it is safe to clear
+		// timerModifiedEarliest. Do so in case it is stale.
+		// Everything will work if we don't do this,
+		// but clearing here may save future calls to adjusttimers.
+		atomic.Store64(&pp.timerModifiedEarliest, 0)
 		return
 	}
+
+	// If we haven't yet reached the time of the first timerModifiedEarlier
+	// timer, don't do anything. This speeds up programs that adjust
+	// a lot of timers back and forth if the timers rarely expire.
+	// We'll postpone looking through all the adjusted timers until
+	// one would actually expire.
+	if first := atomic.Load64(&pp.timerModifiedEarliest); first != 0 {
+		if int64(first) > now {
+			if verifyTimers {
+				verifyTimerHeap(pp)
+			}
+			return
+		}
+
+		// We are going to clear all timerModifiedEarlier timers.
+		atomic.Store64(&pp.timerModifiedEarliest, 0)
+	}
+
 	var moved []*timer
 loop:
 	for i := 0; i < len(pp.timers); i++ {
@@ -868,6 +891,10 @@ func runOneTimer(pp *p, t *timer, now int64) {
 //
 // The caller must have locked the timers for pp.
 func clearDeletedTimers(pp *p) {
+	// We are going to clear all timerModifiedEarlier timers.
+	// Do this now in case new ones show up while we are looping.
+	atomic.Store64(&pp.timerModifiedEarliest, 0)
+
 	cdel := int32(0)
 	cearlier := int32(0)
 	to := 0
@@ -977,6 +1004,21 @@ func updateTimer0When(pp *p) {
 	}
 }
 
+// updateTimerModifiedEarliest updates the recorded nextwhen field of the
+// earlier timerModifiedEarier value.
+// The timers for pp will not be locked.
+func updateTimerModifiedEarliest(pp *p, nextwhen int64) {
+	for {
+		old := atomic.Load64(&pp.timerModifiedEarliest)
+		if old != 0 && int64(old) < nextwhen {
+			return
+		}
+		if atomic.Cas64(&pp.timerModifiedEarliest, old, uint64(nextwhen)) {
+			return
+		}
+	}
+}
+
 // 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.
@@ -993,48 +1035,17 @@ func timeSleepUntil() (int64, *p) {
 			continue
 		}
 
-		c := atomic.Load(&pp.adjustTimers)
-		if c == 0 {
-			w := int64(atomic.Load64(&pp.timer0When))
-			if w != 0 && w < next {
-				next = w
-				pret = pp
-			}
-			continue
+		w := int64(atomic.Load64(&pp.timer0When))
+		if w != 0 && w < next {
+			next = w
+			pret = pp
 		}
 
-		lock(&pp.timersLock)
-		for _, t := range pp.timers {
-			switch s := atomic.Load(&t.status); s {
-			case timerWaiting:
-				if t.when < next {
-					next = t.when
-				}
-			case timerModifiedEarlier, timerModifiedLater:
-				if t.nextwhen < next {
-					next = t.nextwhen
-				}
-				if s == timerModifiedEarlier {
-					c--
-				}
-			}
-			// 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.
-			//
-			// We don't worry about cases like timerModifying.
-			// New timers can show up at any time,
-			// so this function is necessarily imprecise.
-			// Do a signed check here since we aren't
-			// synchronizing the read of pp.adjustTimers
-			// with the check of a timer status.
-			if int32(c) <= 0 {
-				break
-			}
+		w = int64(atomic.Load64(&pp.timerModifiedEarliest))
+		if w != 0 && w < next {
+			next = w
+			pret = pp
 		}
-		unlock(&pp.timersLock)
 	}
 	unlock(&allpLock)