runtime: convert gcController.globalsScan to atomic type

For #53821.

Change-Id: I92bd33e355c868ae229395fd9c98fdb10768d03d
Reviewed-on: https://go-review.googlesource.com/c/go/+/417779
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/align_runtime_test.go

@@ -26,7 +26,6 @@ var AtomicFields = []uintptr{
 	unsafe.Offsetof(gcControllerState{}.dedicatedMarkWorkersNeeded),
 	unsafe.Offsetof(gcControllerState{}.fractionalMarkTime),
 	unsafe.Offsetof(gcControllerState{}.idleMarkTime),
-	unsafe.Offsetof(gcControllerState{}.globalsScan),
 	unsafe.Offsetof(timeHistogram{}.underflow),
 	unsafe.Offsetof(profBuf{}.overflow),
 	unsafe.Offsetof(profBuf{}.overflowTime),

src/runtime/export_test.go

@@ -1323,7 +1323,7 @@ func (c *GCController) StartCycle(stackSize, globalsSize uint64, scannableFrac f
 		trigger = c.heapMarked
 	}
 	c.maxStackScan.Store(stackSize)
-	c.globalsScan = globalsSize
+	c.globalsScan.Store(globalsSize)
 	c.heapLive.Store(trigger)
 	c.heapScan.Add(int64(float64(trigger-c.heapMarked) * scannableFrac))
 	c.startCycle(0, gomaxprocs, gcTrigger{kind: gcTriggerHeap})

src/runtime/mgc.go

@@ -1121,7 +1121,7 @@ func gcMarkTermination() {
 			work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ",
 			gcController.lastHeapGoal>>20, " MB goal, ",
 			gcController.maxStackScan.Load()>>20, " MB stacks, ",
-			gcController.globalsScan>>20, " MB globals, ",
+			gcController.globalsScan.Load()>>20, " MB globals, ",
 			work.maxprocs, " P")
 		if work.userForced {
 			print(" (forced)")

src/runtime/mgcpacer.go

@@ -231,9 +231,7 @@ type gcControllerState struct {
 
 	// globalsScan is the total amount of global variable space
 	// that is scannable.
-	//
-	// Read and updated atomically.
-	globalsScan uint64
+	globalsScan atomic.Uint64
 
 	// heapMarked is the number of bytes marked by the previous
 	// GC. After mark termination, heapLive == heapMarked, but
@@ -555,14 +553,14 @@ func (c *gcControllerState) revise() {
 
 	// The expected scan work is computed as the amount of bytes scanned last
 	// GC cycle (both heap and stack), plus our estimate of globals work for this cycle.
-	scanWorkExpected := int64(c.lastHeapScan + c.lastStackScan.Load() + c.globalsScan)
+	scanWorkExpected := int64(c.lastHeapScan + c.lastStackScan.Load() + c.globalsScan.Load())
 
 	// maxScanWork is a worst-case estimate of the amount of scan work that
 	// needs to be performed in this GC cycle. Specifically, it represents
 	// the case where *all* scannable memory turns out to be live, and
 	// *all* allocated stack space is scannable.
 	maxStackScan := c.maxStackScan.Load()
-	maxScanWork := int64(scan + maxStackScan + c.globalsScan)
+	maxScanWork := int64(scan + maxStackScan + c.globalsScan.Load())
 	if work > scanWorkExpected {
 		// We've already done more scan work than expected. Because our expectation
 		// is based on a steady-state scannable heap size, we assume this means our
@@ -738,7 +736,7 @@ func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) {
 		printlock()
 		goal := gcGoalUtilization * 100
 		print("pacer: ", int(utilization*100), "% CPU (", int(goal), " exp.) for ")
-		print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan, " B exp.) ")
+		print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan.Load(), " B exp.) ")
 		live := c.heapLive.Load()
 		print("in ", c.triggered, " B -> ", live, " B (∆goal ", int64(live)-int64(c.lastHeapGoal), ", cons/mark ", oldConsMark, ")")
 		if !ok {
@@ -953,7 +951,7 @@ func (c *gcControllerState) addScannableStack(pp *p, amount int64) {
 }
 
 func (c *gcControllerState) addGlobals(amount int64) {
-	atomic.Xadd64(&c.globalsScan, amount)
+	c.globalsScan.Add(amount)
 }
 
 // heapGoal returns the current heap goal.
@@ -1255,7 +1253,7 @@ func (c *gcControllerState) commit(isSweepDone bool) {
 	// plus additional runway for non-heap sources of GC work.
 	gcPercentHeapGoal := ^uint64(0)
 	if gcPercent := c.gcPercent.Load(); gcPercent >= 0 {
-		gcPercentHeapGoal = c.heapMarked + (c.heapMarked+c.lastStackScan.Load()+atomic.Load64(&c.globalsScan))*uint64(gcPercent)/100
+		gcPercentHeapGoal = c.heapMarked + (c.heapMarked+c.lastStackScan.Load()+c.globalsScan.Load())*uint64(gcPercent)/100
 	}
 	// Apply the minimum heap size here. It's defined in terms of gcPercent
 	// and is only updated by functions that call commit.
@@ -1287,7 +1285,7 @@ func (c *gcControllerState) commit(isSweepDone bool) {
 	// Furthermore, by setting the runway so that CPU resources are divided
 	// this way, assuming that the cons/mark ratio is correct, we make that
 	// division a reality.
-	c.runway.Store(uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan)))
+	c.runway.Store(uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan.Load())))
 }
 
 // setGCPercent updates gcPercent. commit must be called after.