diff --git a/src/runtime/proc_test.go b/src/runtime/proc_test.go
index 53cafe8907..9198022ace 100644
--- a/src/runtime/proc_test.go
+++ b/src/runtime/proc_test.go
@@ -119,6 +119,9 @@ func TestGoroutineParallelism(t *testing.T) {
 	// since the goroutines can't be stopped/preempted.
 	// Disable GC for this test (see issue #10958).
 	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+	// Now that GCs are disabled, block until any outstanding GCs
+	// are also done.
+	runtime.GC()
 	for try := 0; try < N; try++ {
 		done := make(chan bool)
 		x := uint32(0)
@@ -163,6 +166,9 @@ func testGoroutineParallelism2(t *testing.T, load, netpoll bool) {
 	// since the goroutines can't be stopped/preempted.
 	// Disable GC for this test (see issue #10958).
 	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+	// Now that GCs are disabled, block until any outstanding GCs
+	// are also done.
+	runtime.GC()
 	for try := 0; try < N; try++ {
 		if load {
 			// Create P goroutines and wait until they all run.
@@ -623,6 +629,9 @@ func TestSchedLocalQueueEmpty(t *testing.T) {
 	// If runtime triggers a forced GC during this test then it will deadlock,
 	// since the goroutines can't be stopped/preempted during spin wait.
 	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+	// Now that GCs are disabled, block until any outstanding GCs
+	// are also done.
+	runtime.GC()
 
 	iters := int(1e5)
 	if testing.Short() {
diff --git a/src/runtime/testdata/testprog/badtraceback.go b/src/runtime/testdata/testprog/badtraceback.go
index d558adceec..09aa2b877e 100644
--- a/src/runtime/testdata/testprog/badtraceback.go
+++ b/src/runtime/testdata/testprog/badtraceback.go
@@ -17,6 +17,9 @@ func init() {
 func BadTraceback() {
 	// Disable GC to prevent traceback at unexpected time.
 	debug.SetGCPercent(-1)
+	// Out of an abundance of caution, also make sure that there are
+	// no GCs actively in progress.
+	runtime.GC()
 
 	// Run badLR1 on its own stack to minimize the stack size and
 	// exercise the stack bounds logic in the hex dump.
diff --git a/src/runtime/testdata/testprog/preempt.go b/src/runtime/testdata/testprog/preempt.go
index 1c74d0e435..eb9f59053c 100644
--- a/src/runtime/testdata/testprog/preempt.go
+++ b/src/runtime/testdata/testprog/preempt.go
@@ -20,6 +20,9 @@ func AsyncPreempt() {
 	runtime.GOMAXPROCS(1)
 	// Disable GC so we have complete control of what we're testing.
 	debug.SetGCPercent(-1)
+	// Out of an abundance of caution, also make sure that there are
+	// no GCs actively in progress.
+	runtime.GC()
 
 	// Start a goroutine with no sync safe-points.
 	var ready, ready2 uint32