diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 185d3201ca..9dc86e13ce 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -302,9 +302,10 @@ const (
 	// gcMarkWorkerFractionalMode indicates that a P is currently
 	// running the "fractional" mark worker. The fractional worker
 	// is necessary when GOMAXPROCS*gcBackgroundUtilization is not
-	// an integer. The fractional worker should run until it is
-	// preempted and will be scheduled to pick up the fractional
-	// part of GOMAXPROCS*gcBackgroundUtilization.
+	// an integer and each P is too far away from the
+	// gcBackgroundUtilization target. The fractional worker should
+	// run until it is preempted and will be scheduled to pick up the
+	// fractional part of GOMAXPROCS*gcBackgroundUtilization.
 	gcMarkWorkerFractionalMode
 
 	// gcMarkWorkerIdleMode indicates that a P is running the mark