diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index b1fbdc91bb5..6d4799a9e29 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -360,7 +360,8 @@ type gcControllerState struct {
 
 	// assistRatio is the ratio of allocated bytes to scan work
 	// that should be performed by mutator assists. This is
-	// computed at the beginning of each cycle.
+	// computed at the beginning of each cycle and updated every
+	// time heap_scan is updated.
 	assistRatio float64
 
 	// fractionalUtilizationGoal is the fraction of wall clock
@@ -379,10 +380,6 @@ type gcControllerState struct {
 	// at the end of of each cycle.
 	triggerRatio float64
 
-	// reviseTimer is a timer that triggers periodic revision of
-	// control variables during the cycle.
-	reviseTimer timer
-
 	_ [_CacheLineSize]byte
 
 	// fractionalMarkWorkersNeeded is the number of fractional
@@ -449,19 +446,11 @@ func (c *gcControllerState) startCycle() {
 			" workers=", c.dedicatedMarkWorkersNeeded,
 			"+", c.fractionalMarkWorkersNeeded, "\n")
 	}
-
-	// Set up a timer to revise periodically
-	c.reviseTimer.f = func(interface{}, uintptr) {
-		gcController.revise()
-	}
-	c.reviseTimer.period = 10 * 1000 * 1000
-	c.reviseTimer.when = nanotime() + c.reviseTimer.period
-	addtimer(&c.reviseTimer)
 }
 
 // revise updates the assist ratio during the GC cycle to account for
-// improved estimates. This should be called periodically during
-// concurrent mark.
+// improved estimates. This should be called either under STW or
+// whenever memstats.heap_scan is updated (with mheap_.lock held).
 func (c *gcControllerState) revise() {
 	// Compute the expected scan work. This is a strict upper
 	// bound on the possible scan work in the current heap.
@@ -502,9 +491,6 @@ func (c *gcControllerState) endCycle() {
 	// transient changes. Values near 1 may be unstable.
 	const triggerGain = 0.5
 
-	// Stop the revise timer
-	deltimer(&c.reviseTimer)
-
 	// Compute next cycle trigger ratio. First, this computes the
 	// "error" for this cycle; that is, how far off the trigger
 	// was from what it should have been, accounting for both heap
diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index d190782580d..be4d6121563 100644
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -420,6 +420,8 @@ func mHeap_Alloc_m(h *mheap, npage uintptr, sizeclass int32, large bool) *mspan
 	memstats.tinyallocs += uint64(_g_.m.mcache.local_tinyallocs)
 	_g_.m.mcache.local_tinyallocs = 0
 
+	gcController.revise()
+
 	s := mHeap_AllocSpanLocked(h, npage)
 	if s != nil {
 		// Record span info, because gc needs to be
@@ -694,6 +696,7 @@ func mHeap_Free(h *mheap, s *mspan, acct int32) {
 		if acct != 0 {
 			memstats.heap_objects--
 		}
+		gcController.revise()
 		mHeap_FreeSpanLocked(h, s, true, true, 0)
 		if trace.enabled {
 			traceHeapAlloc()