runtime: use atomic.Float64 for assist ratio

Change-Id: Ie7f09a7c9545ef9dd1860b1e332c4edbcbf8165e
Reviewed-on: https://go-review.googlesource.com/c/go/+/356170
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Knyszek <mknyszek@google.com>

src/runtime/mgcmark.go

@@ -400,8 +400,8 @@ retry:
 	// balance positive. When the required amount of work is low,
 	// we over-assist to build up credit for future allocations
 	// and amortize the cost of assisting.
-	assistWorkPerByte := float64frombits(atomic.Load64(&gcController.assistWorkPerByte))
-	assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
+	assistWorkPerByte := gcController.assistWorkPerByte.Load()
+	assistBytesPerWork := gcController.assistBytesPerWork.Load()
 	debtBytes := -gp.gcAssistBytes
 	scanWork := int64(assistWorkPerByte * float64(debtBytes))
 	if scanWork < gcOverAssistWork {
@@ -545,7 +545,7 @@ func gcAssistAlloc1(gp *g, scanWork int64) {
 	// this scan work counts for. The "1+" is a poor man's
 	// round-up, to ensure this adds credit even if
 	// assistBytesPerWork is very low.
-	assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
+	assistBytesPerWork := gcController.assistBytesPerWork.Load()
 	gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(workDone))
 
 	// If this is the last worker and we ran out of work,
@@ -638,7 +638,7 @@ func gcFlushBgCredit(scanWork int64) {
 		return
 	}
 
-	assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
+	assistBytesPerWork := gcController.assistBytesPerWork.Load()
 	scanBytes := int64(float64(scanWork) * assistBytesPerWork)
 
 	lock(&work.assistQueue.lock)
@@ -672,7 +672,7 @@ func gcFlushBgCredit(scanWork int64) {
 
 	if scanBytes > 0 {
 		// Convert from scan bytes back to work.
-		assistWorkPerByte := float64frombits(atomic.Load64(&gcController.assistWorkPerByte))
+		assistWorkPerByte := gcController.assistWorkPerByte.Load()
 		scanWork = int64(float64(scanBytes) * assistWorkPerByte)
 		atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
 	}

src/runtime/mgcpacer.go

@@ -222,24 +222,14 @@ type gcControllerState struct {
 	// bytes that should be performed by mutator assists. This is
 	// computed at the beginning of each cycle and updated every
 	// time heapScan is updated.
-	//
-	// Stored as a uint64, but it's actually a float64. Use
-	// float64frombits to get the value.
-	//
-	// Read and written atomically.
-	assistWorkPerByte uint64
+	assistWorkPerByte atomic.Float64
 
 	// assistBytesPerWork is 1/assistWorkPerByte.
 	//
-	// Stored as a uint64, but it's actually a float64. Use
-	// float64frombits to get the value.
-	//
-	// Read and written atomically.
-	//
 	// Note that because this is read and written independently
 	// from assistWorkPerByte users may notice a skew between
 	// the two values, and such a state should be safe.
-	assistBytesPerWork uint64
+	assistBytesPerWork atomic.Float64
 
 	// fractionalUtilizationGoal is the fraction of wall clock
 	// time that should be spent in the fractional mark worker on
@@ -333,7 +323,7 @@ func (c *gcControllerState) startCycle() {
 	c.revise()
 
 	if debug.gcpacertrace > 0 {
-		assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte))
+		assistRatio := c.assistWorkPerByte.Load()
 		print("pacer: assist ratio=", assistRatio,
 			" (scan ", gcController.heapScan>>20, " MB in ",
 			work.initialHeapLive>>20, "->",
@@ -439,8 +429,8 @@ func (c *gcControllerState) revise() {
 	// cycle.
 	assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
 	assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
-	atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte))
-	atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork))
+	c.assistWorkPerByte.Store(assistWorkPerByte)
+	c.assistBytesPerWork.Store(assistBytesPerWork)
 }
 
 // endCycle computes the trigger ratio for the next cycle.

src/runtime/proc.go

@@ -3646,7 +3646,7 @@ func goexit0(gp *g) {
 		// Flush assist credit to the global pool. This gives
 		// better information to pacing if the application is
 		// rapidly creating an exiting goroutines.
-		assistWorkPerByte := float64frombits(atomic.Load64(&gcController.assistWorkPerByte))
+		assistWorkPerByte := gcController.assistWorkPerByte.Load()
 		scanCredit := int64(assistWorkPerByte * float64(gp.gcAssistBytes))
 		atomic.Xaddint64(&gcController.bgScanCredit, scanCredit)
 		gp.gcAssistBytes = 0