runtime: convert gcController.fractionalMarkTime to atomic type

For #53821.

Change-Id: Ic54bda422b87ee9365090fe6b42b82df7b25d2a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/417782
Run-TryBot: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: prattmic

src/runtime/align_runtime_test.go

@@ -22,7 +22,6 @@ var AtomicFields = []uintptr{
 	unsafe.Offsetof(schedt{}.pollUntil),
 	unsafe.Offsetof(schedt{}.timeToRun),
 	unsafe.Offsetof(gcControllerState{}.dedicatedMarkWorkersNeeded),
-	unsafe.Offsetof(gcControllerState{}.fractionalMarkTime),
 	unsafe.Offsetof(gcControllerState{}.idleMarkTime),
 	unsafe.Offsetof(timeHistogram{}.underflow),
 	unsafe.Offsetof(profBuf{}.overflow),

src/runtime/mgc.go

@@ -1009,7 +1009,7 @@ func gcMarkTermination() {
 	sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm)
 	// We report idle marking time below, but omit it from the
 	// overall utilization here since it's "free".
-	markCpu := gcController.assistTime.Load() + gcController.dedicatedMarkTime.Load() + gcController.fractionalMarkTime
+	markCpu := gcController.assistTime.Load() + gcController.dedicatedMarkTime.Load() + gcController.fractionalMarkTime.Load()
 	markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm)
 	cycleCpu := sweepTermCpu + markCpu + markTermCpu
 	work.totaltime += cycleCpu
@@ -1105,7 +1105,7 @@ func gcMarkTermination() {
 		for i, ns := range []int64{
 			sweepTermCpu,
 			gcController.assistTime.Load(),
-			gcController.dedicatedMarkTime.Load() + gcController.fractionalMarkTime,
+			gcController.dedicatedMarkTime.Load() + gcController.fractionalMarkTime.Load(),
 			gcController.idleMarkTime,
 			markTermCpu,
 		} {

src/runtime/mgcpacer.go

@@ -275,11 +275,11 @@ type gcControllerState struct {
 	// phase.
 	dedicatedMarkTime atomic.Int64
 
-	// fractionalMarkTime is the nanoseconds spent in the
-	// fractional mark worker during this cycle. This is updated
-	// atomically throughout the cycle and will be up-to-date if
-	// the fractional mark worker is not currently running.
-	fractionalMarkTime int64
+	// fractionalMarkTime is the nanoseconds spent in the fractional mark
+	// worker during this cycle. This is updated throughout the cycle and
+	// will be up-to-date if the fractional mark worker is not currently
+	// running.
+	fractionalMarkTime atomic.Int64
 
 	// idleMarkTime is the nanoseconds spent in idle marking
 	// during this cycle. This is updated atomically throughout
@@ -419,7 +419,7 @@ func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger g
 	c.bgScanCredit.Store(0)
 	c.assistTime.Store(0)
 	c.dedicatedMarkTime.Store(0)
-	c.fractionalMarkTime = 0
+	c.fractionalMarkTime.Store(0)
 	c.idleMarkTime = 0
 	c.markStartTime = markStartTime
 
@@ -908,7 +908,7 @@ func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64
 		c.dedicatedMarkTime.Add(duration)
 		atomic.Xaddint64(&c.dedicatedMarkWorkersNeeded, 1)
 	case gcMarkWorkerFractionalMode:
-		atomic.Xaddint64(&c.fractionalMarkTime, duration)
+		c.fractionalMarkTime.Add(duration)
 	case gcMarkWorkerIdleMode:
 		atomic.Xaddint64(&c.idleMarkTime, duration)
 		c.removeIdleMarkWorker()