runtime: simplify, speed up adjusttimers

The current adjusttimers does an O(n) loop and then queues
a bunch of reinsertions, each of which is O(log n), for a worst
case of O(n log n) time plus an allocation of n elements.

Reestablishing the heap invariant from an arbitrarily ordered
slice can be done in O(n) time, so it is both simpler and faster
to avoid the allocated temporary queue and just re-init the
heap if we have damaged it. The cost of doing so is no worse
than the O(n) loop we already did.

This change also avoids holding multiple timers locked (status
set to timerMoving) at any given moment, as well as holding
individual timers locked for unbounded amounts of time,
as opposed to fixed-size critical sections.

[This is one CL in a refactoring stack making very small changes
in each step, so that any subtle bugs that we miss can be more
easily pinpointed to a small change.]

Change-Id: If966c1d1e66db797f4b19e7b1abbc06ab651764d
Reviewed-on: https://go-review.googlesource.com/c/go/+/564115
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/time.go

@@ -372,40 +372,6 @@ func deltimer(t *timer) bool {
 	}
 }
 
-// dodeltimer removes timer i from the current P's heap.
-// We are locked on the P when this is called.
-// It returns the smallest changed index in pp.timers.
-// The caller must have locked the timers for pp.
-func dodeltimer(pp *p, i int) int {
-	if t := pp.timers[i]; t.pp.ptr() != pp {
-		throw("dodeltimer: wrong P")
-	} else {
-		t.pp = 0
-	}
-	last := len(pp.timers) - 1
-	if i != last {
-		pp.timers[i] = pp.timers[last]
-	}
-	pp.timers[last] = nil
-	pp.timers = pp.timers[:last]
-	smallestChanged := i
-	if i != last {
-		// Moving to i may have moved the last timer to a new parent,
-		// so sift up to preserve the heap guarantee.
-		smallestChanged = siftupTimer(pp.timers, i)
-		siftdownTimer(pp.timers, i)
-	}
-	if i == 0 {
-		updateTimer0When(pp)
-	}
-	n := pp.numTimers.Add(-1)
-	if n == 0 {
-		// If there are no timers, then clearly none are modified.
-		pp.timerModifiedEarliest.Store(0)
-	}
-	return smallestChanged
-}
-
 // dodeltimer0 removes timer 0 from the current P's heap.
 // We are locked on the P when this is called.
 // It reports whether it saw no problems due to races.
@@ -683,7 +649,7 @@ func adjusttimers(pp *p, now int64) {
 	// We are going to clear all timerModifiedEarlier timers.
 	pp.timerModifiedEarliest.Store(0)
 
-	var moved []*timer
+	changed := false
 	for i := 0; i < len(pp.timers); i++ {
 		t := pp.timers[i]
 		if t.pp.ptr() != pp {
@@ -692,28 +658,26 @@ func adjusttimers(pp *p, now int64) {
 		switch s := t.status.Load(); s {
 		case timerDeleted:
 			if t.status.CompareAndSwap(s, timerRemoving) {
-				changed := dodeltimer(pp, i)
+				n := len(pp.timers)
+				pp.timers[i] = pp.timers[n-1]
+				pp.timers[n-1] = nil
+				pp.timers = pp.timers[:n-1]
+				t.pp = 0
 				if !t.status.CompareAndSwap(timerRemoving, timerRemoved) {
 					badTimer()
 				}
 				pp.deletedTimers.Add(-1)
-				// Go back to the earliest changed heap entry.
-				// "- 1" because the loop will add 1.
-				i = changed - 1
+				i--
+				changed = true
 			}
 		case timerModifiedEarlier, timerModifiedLater:
 			if t.status.CompareAndSwap(s, timerMoving) {
 				// Now we can change the when field.
 				t.when = t.nextwhen
-				// Take t off the heap, and hold onto it.
-				// We don't add it back yet because the
-				// heap manipulation could cause our
-				// loop to skip some other timer.
-				changed := dodeltimer(pp, i)
-				moved = append(moved, t)
-				// Go back to the earliest changed heap entry.
-				// "- 1" because the loop will add 1.
-				i = changed - 1
+				changed = true
+				if !t.status.CompareAndSwap(timerMoving, timerWaiting) {
+					badTimer()
+				}
 			}
 		case timerNoStatus, timerRunning, timerRemoving, timerRemoved, timerMoving:
 			badTimer()
@@ -728,8 +692,9 @@ func adjusttimers(pp *p, now int64) {
 		}
 	}
 
-	if len(moved) > 0 {
-		addAdjustedTimers(pp, moved)
+	if changed {
+		initTimerHeap(pp.timers)
+		updateTimer0When(pp)
 	}
 
 	if verifyTimers {
@@ -737,17 +702,6 @@ func adjusttimers(pp *p, now int64) {
 	}
 }
 
-// addAdjustedTimers adds any timers we adjusted in adjusttimers
-// back to the timer heap.
-func addAdjustedTimers(pp *p, moved []*timer) {
-	for _, t := range moved {
-		doaddtimer(pp, t)
-		if !t.status.CompareAndSwap(timerMoving, timerWaiting) {
-			badTimer()
-		}
-	}
-}
-
 // nobarrierWakeTime looks at P's timers and returns the time when we
 // should wake up the netpoller. It returns 0 if there are no timers.
 // This function is invoked when dropping a P, and must run without
@@ -1135,6 +1089,19 @@ func siftdownTimer(t []*timer, i int) {
 	}
 }
 
+// initTimerHeap reestablishes the heap order in the slice t.
+// It takes O(n) time for n=len(t), not the O(n log n) of n repeated add operations.
+func initTimerHeap(t []*timer) {
+	// Last possible element that needs sifting down is parent of last element;
+	// last element is len(t)-1; parent of last element is (len(t)-1-1)/4.
+	if len(t) <= 1 {
+		return
+	}
+	for i := (len(t) - 1 - 1) / 4; i >= 0; i-- {
+		siftdownTimer(t, i)
+	}
+}
+
 // badTimer is called if the timer data structures have been corrupted,
 // presumably due to racy use by the program. We panic here rather than
 // panicking due to invalid slice access while holding locks.