diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 0727391775..3cf642d9b2 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -1334,7 +1334,7 @@ func gcBgMarkWorker(p *p) {
 		default:
 			throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
 		case gcMarkWorkerDedicatedMode:
-			gcDrain(&p.gcw, gcBgCreditSlack, gcDrainBlock)
+			gcDrain(&p.gcw, gcDrainBlock|gcDrainFlushBgCredit)
 			// gcDrain did the xadd(&work.nwait +1) to
 			// match the decrement above. It only returns
 			// at a mark completion point.
@@ -1343,7 +1343,7 @@ func gcBgMarkWorker(p *p) {
 				throw("gcDrain returned with buffer")
 			}
 		case gcMarkWorkerFractionalMode, gcMarkWorkerIdleMode:
-			gcDrain(&p.gcw, gcBgCreditSlack, gcDrainUntilPreempt)
+			gcDrain(&p.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
 
 			// If we are nearing the end of mark, dispose
 			// of the cache promptly. We must do this
@@ -1454,7 +1454,7 @@ func gcMark(start_time int64) {
 	parfordo(work.markfor)
 
 	var gcw gcWork
-	gcDrain(&gcw, -1, gcDrainBlock)
+	gcDrain(&gcw, gcDrainBlock)
 	gcw.dispose()
 
 	if work.full != 0 {
@@ -1717,7 +1717,7 @@ func gchelper() {
 	parfordo(work.markfor)
 	if gcphase != _GCscan {
 		var gcw gcWork
-		gcDrain(&gcw, -1, gcDrainBlock) // blocks in getfull
+		gcDrain(&gcw, gcDrainBlock) // blocks in getfull
 		gcw.dispose()
 	}
 
diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 4337d71f37..c3134bda29 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -553,6 +553,7 @@ type gcDrainFlags int
 
 const (
 	gcDrainUntilPreempt gcDrainFlags = 1 << iota
+	gcDrainFlushBgCredit
 
 	// gcDrainBlock is the opposite of gcDrainUntilPreempt. This
 	// is the default, but callers should use the constant for
@@ -567,21 +568,22 @@ const (
 // g.preempt is set. Otherwise, this will block until all dedicated
 // workers are blocked in gcDrain.
 //
-// If flushScanCredit != -1, gcDrain flushes accumulated scan work
-// credit to gcController.bgScanCredit whenever gcw's local scan work
-// credit exceeds flushScanCredit.
+// If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
+// credit to gcController.bgScanCredit every gcBgCreditSlack units of
+// scan work.
 //go:nowritebarrier
-func gcDrain(gcw *gcWork, flushScanCredit int64, flags gcDrainFlags) {
+func gcDrain(gcw *gcWork, flags gcDrainFlags) {
 	if !writeBarrierEnabled {
 		throw("gcDrain phase incorrect")
 	}
 
 	blocking := flags&gcDrainUntilPreempt == 0
+	flushBgCredit := flags&gcDrainFlushBgCredit != 0
 
 	var lastScanFlush, nextScanFlush int64
-	if flushScanCredit != -1 {
+	if flushBgCredit {
 		lastScanFlush = gcw.scanWork
-		nextScanFlush = lastScanFlush + flushScanCredit
+		nextScanFlush = lastScanFlush + gcBgCreditSlack
 	} else {
 		nextScanFlush = int64(^uint64(0) >> 1)
 	}
@@ -618,10 +620,10 @@ func gcDrain(gcw *gcWork, flushScanCredit int64, flags gcDrainFlags) {
 			credit := gcw.scanWork - lastScanFlush
 			xaddint64(&gcController.bgScanCredit, credit)
 			lastScanFlush = gcw.scanWork
-			nextScanFlush = lastScanFlush + flushScanCredit
+			nextScanFlush = lastScanFlush + gcBgCreditSlack
 		}
 	}
-	if flushScanCredit != -1 {
+	if flushBgCredit {
 		credit := gcw.scanWork - lastScanFlush
 		xaddint64(&gcController.bgScanCredit, credit)
 	}