runtime: switch p.gcFractionalMarkTime to atomic.Int64

atomic.Int64 automatically maintains proper alignment, avoiding the need
to manually adjust alignment back and forth as fields above change.

Change-Id: I6a6a636c4c3c366353f6dc8ecac473c075dd5cd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/716700
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

src/runtime/align_runtime_test.go

@@ -14,7 +14,6 @@ import "unsafe"
 // operations (all the *64 operations in internal/runtime/atomic).
 var AtomicFields = []uintptr{
 	unsafe.Offsetof(m{}.procid),
-	unsafe.Offsetof(p{}.gcFractionalMarkTime),
 	unsafe.Offsetof(profBuf{}.overflow),
 	unsafe.Offsetof(profBuf{}.overflowTime),
 	unsafe.Offsetof(heapStatsDelta{}.tinyAllocCount),

src/runtime/mgc.go

@@ -316,7 +316,7 @@ func pollFractionalWorkerExit() bool {
 		return true
 	}
 	p := getg().m.p.ptr()
-	selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime)
+	selfTime := p.gcFractionalMarkTime.Load() + (now - p.gcMarkWorkerStartTime)
 	// Add some slack to the utilization goal so that the
 	// fractional worker isn't behind again the instant it exits.
 	return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal
@@ -1858,7 +1858,7 @@ func gcBgMarkWorker(ready chan struct{}) {
 			pp.limiterEvent.stop(limiterEventIdleMarkWork, now)
 		}
 		if pp.gcMarkWorkerMode == gcMarkWorkerFractionalMode {
-			atomic.Xaddint64(&pp.gcFractionalMarkTime, duration)
+			pp.gcFractionalMarkTime.Add(duration)
 		}
 
 		// We'll releasem after this point and thus this P may run

src/runtime/mgcpacer.go

@@ -427,7 +427,7 @@ func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger g
 	// Clear per-P state
 	for _, p := range allp {
 		p.gcAssistTime = 0
-		p.gcFractionalMarkTime = 0
+		p.gcFractionalMarkTime.Store(0)
 	}
 
 	if trigger.kind == gcTriggerTime {
@@ -830,7 +830,7 @@ func (c *gcControllerState) findRunnableGCWorker(pp *p, now int64) (*g, int64) {
 		//
 		// This should be kept in sync with pollFractionalWorkerExit.
 		delta := now - c.markStartTime
-		if delta > 0 && float64(pp.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
+		if delta > 0 && float64(pp.gcFractionalMarkTime.Load())/float64(delta) > c.fractionalUtilizationGoal {
 			// Nope. No need to run a fractional worker.
 			gcBgMarkWorkerPool.push(&node.node)
 			return nil, now

src/runtime/runtime2.go

@@ -794,7 +794,7 @@ type p struct {
 
 	// Per-P GC state
 	gcAssistTime         int64 // Nanoseconds in assistAlloc
-	gcFractionalMarkTime int64 // Nanoseconds in fractional mark worker (atomic)
+	gcFractionalMarkTime atomic.Int64 // Nanoseconds in fractional mark worker
 
 	// limiterEvent tracks events for the GC CPU limiter.
 	limiterEvent limiterEvent