runtime: add new adjusttimers function

The adjusttimers function is where we check the adjustTimers field in
the P struct to see if we need to resort the heap. We walk forward in
the heap and find and resort timers that have been modified, until we
find all the timers that were modified to run earlier. Along the way
we remove deleted timers.

Updates #27707

Change-Id: I1cba7fe77b8112b7e9a9dba80b5dfb08fcc7c568
Reviewed-on: https://go-review.googlesource.com/c/go/+/171877
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/time.go

@@ -154,6 +154,9 @@ type timersBucket struct {
 // cleantimers (looks in P's timer heap):
 //   timerDeleted    -> timerRemoving -> timerRemoved
 //   timerModifiedXX -> timerMoving -> timerWaiting
+// adjusttimers (looks in P's timer heap):
+//   timerDeleted    -> timerRemoving -> timerRemoved
+//   timerModifiedXX -> timerMoving -> timerWaiting
 
 // Values for the timer status field.
 const (
@@ -865,13 +868,119 @@ func moveTimers(pp *p, timers []*timer) {
 
 // adjusttimers looks through the timers in the current P's heap for
 // any timers that have been modified to run earlier, and puts them in
-// the correct place in the heap.
-// The caller must have locked the timers for pp.
+// 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
 	}
-	throw("adjusttimers: not yet implemented")
+	if atomic.Load(&pp.adjustTimers) == 0 {
+		return
+	}
+	var moved []*timer
+	for i := 0; i < len(pp.timers); i++ {
+		t := pp.timers[i]
+		if t.pp.ptr() != pp {
+			throw("adjusttimers: bad p")
+		}
+		switch s := atomic.Load(&t.status); s {
+		case timerDeleted:
+			if atomic.Cas(&t.status, s, timerRemoving) {
+				if !dodeltimer(pp, i) {
+					badTimer()
+				}
+				if !atomic.Cas(&t.status, timerRemoving, timerRemoved) {
+					badTimer()
+				}
+				// Look at this heap position again.
+				i--
+			}
+		case timerModifiedEarlier, timerModifiedLater:
+			if atomic.Cas(&t.status, 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.
+				if !dodeltimer(pp, i) {
+					badTimer()
+				}
+				moved = append(moved, t)
+				if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
+					badTimer()
+				}
+				if s == timerModifiedEarlier {
+					if n := atomic.Xadd(&pp.adjustTimers, -1); int32(n) <= 0 {
+						addAdjustedTimers(pp, moved)
+						return
+					}
+				}
+			}
+		case timerNoStatus, timerRunning, timerRemoving, timerRemoved, timerMoving:
+			badTimer()
+		case timerWaiting:
+			// OK, nothing to do.
+		case timerModifying:
+			// Check again after modification is complete.
+			osyield()
+			i--
+		default:
+			badTimer()
+		}
+	}
+
+	if len(moved) > 0 {
+		addAdjustedTimers(pp, moved)
+	}
+}
+
+// addAdjustedTimers adds any timers we adjusted in adjusttimers
+// back to the timer heap.
+func addAdjustedTimers(pp *p, moved []*timer) {
+	for _, t := range moved {
+	loop:
+		for {
+			switch s := atomic.Load(&t.status); s {
+			case timerWaiting:
+				// This is the normal case.
+				if !doaddtimer(pp, t) {
+					badTimer()
+				}
+				break loop
+			case timerDeleted:
+				// Timer has been deleted since we adjusted it.
+				// This timer is already out of the heap.
+				if !atomic.Cas(&t.status, s, timerRemoved) {
+					badTimer()
+				}
+				break loop
+			case timerModifiedEarlier, timerModifiedLater:
+				// Timer has been modified again since
+				// we adjusted it.
+				if atomic.Cas(&t.status, s, timerMoving) {
+					t.when = t.nextwhen
+					if !doaddtimer(pp, t) {
+						badTimer()
+					}
+					if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
+						badTimer()
+					}
+					if s == timerModifiedEarlier {
+						atomic.Xadd(&pp.adjustTimers, -1)
+					}
+				}
+				break loop
+			case timerNoStatus, timerRunning, timerRemoving, timerRemoved, timerMoving:
+				badTimer()
+			case timerModifying:
+				// Wait and try again.
+				osyield()
+				continue
+			}
+		}
+	}
 }
 
 // runtimer examines the first timer in timers. If it is ready based on now,