runtime: fix comments on the behavior of SetGCPercent

Fixes for #49680, #49695, #45867, and #49370 all assumed that
SetGCPercent(-1) doesn't block until the GC's mark phase is done, but
it actually does. The cause of 3 of those 4 failures comes from the fact
that at the beginning of the sweep phase, the GC does try to preempt
every P once, and this may run concurrently with test code. In the
fourth case, the issue was likely that only *one* of the debug_test.go
tests was missing a call to SetGCPercent(-1). Just to be safe, leave a
TODO there for now to remove the extraneous runtime.GC calls, but leave
the calls in.

Updates #49680, #49695, #45867, and #49370.

Change-Id: Ibf4e64addfba18312526968bcf40f1f5d54eb3f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/369815
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

misc/cgo/test/testdata/issue9400_linux.go

@@ -50,7 +50,9 @@ func test9400(t *testing.T) {
 	// Disable GC for the duration of the test.
 	// This avoids a potential GC deadlock when spinning in uninterruptable ASM below #49695.
 	defer debug.SetGCPercent(debug.SetGCPercent(-1))
-	// And finish any pending GC after we pause, if any.
+	// SetGCPercent waits until the mark phase is over, but the runtime
+	// also preempts at the start of the sweep phase, so make sure that's
+	// done too. See #49695.
 	runtime.GC()
 
 	// Temporarily rewind the stack and trigger SIGSETXID

src/runtime/debug_test.go

@@ -34,10 +34,17 @@ func startDebugCallWorker(t *testing.T) (g *runtime.G, after func()) {
 	skipUnderDebugger(t)
 
 	// This can deadlock if there aren't enough threads or if a GC
-	// tries to interrupt an atomic loop (see issue #10958). A GC
-	// could also actively be in progress (see issue #49370), so we
-	// need to call runtime.GC to block until it has complete. We
-	// use 8 Ps so there's room for the debug call worker,
+	// tries to interrupt an atomic loop (see issue #10958). Execute
+	// an extra GC to ensure even the sweep phase is done (out of
+	// caution to prevent #49370 from happening).
+	// TODO(mknyszek): This extra GC cycle is likely unnecessary
+	// because preemption (which may happen during the sweep phase)
+	// isn't much of an issue anymore thanks to asynchronous preemption.
+	// The biggest risk is having a write barrier in the debug call
+	// injection test code fire, because it runs in a signal handler
+	// and may not have a P.
+	//
+	// We use 8 Ps so there's room for the debug call worker,
 	// something that's trying to preempt the call worker, and the
 	// goroutine that's trying to stop the call worker.
 	ogomaxprocs := runtime.GOMAXPROCS(8)
@@ -270,8 +277,14 @@ func TestDebugCallPanic(t *testing.T) {
 	// progress. Wait until the current GC is done, and turn it off.
 	//
 	// See #10958 and #49370.
-	runtime.GC()
 	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+	// TODO(mknyszek): This extra GC cycle is likely unnecessary
+	// because preemption (which may happen during the sweep phase)
+	// isn't much of an issue anymore thanks to asynchronous preemption.
+	// The biggest risk is having a write barrier in the debug call
+	// injection test code fire, because it runs in a signal handler
+	// and may not have a P.
+	runtime.GC()
 
 	ready := make(chan *runtime.G)
 	var stop uint32

src/runtime/proc_test.go

@@ -119,8 +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.
+	// SetGCPercent waits until the mark phase is over, but the runtime
+	// also preempts at the start of the sweep phase, so make sure that's
+	// done too. See #45867.
 	runtime.GC()
 	for try := 0; try < N; try++ {
 		done := make(chan bool)
@@ -166,8 +167,9 @@ 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.
+	// SetGCPercent waits until the mark phase is over, but the runtime
+	// also preempts at the start of the sweep phase, so make sure that's
+	// done too. See #45867.
 	runtime.GC()
 	for try := 0; try < N; try++ {
 		if load {
@@ -629,8 +631,9 @@ 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.
+	// SetGCPercent waits until the mark phase is over, but the runtime
+	// also preempts at the start of the sweep phase, so make sure that's
+	// done too. See #45867.
 	runtime.GC()
 
 	iters := int(1e5)

src/runtime/rwmutex_test.go

@@ -55,7 +55,9 @@ func TestParallelRWMutexReaders(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))
-	// Finish any in-progress GCs and get ourselves to a clean slate.
+	// SetGCPercent waits until the mark phase is over, but the runtime
+	// also preempts at the start of the sweep phase, so make sure that's
+	// done too.
 	GC()
 
 	doTestParallelReaders(1)

src/runtime/testdata/testprog/preempt.go

@@ -21,7 +21,8 @@ func AsyncPreempt() {
 	// 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.
+	// no GCs actively in progress. The sweep phase of a GC cycle
+	// for instance tries to preempt Ps at the very beginning.
 	runtime.GC()
 
 	// Start a goroutine with no sync safe-points.