runtime: use hard heap goal if we've done more scan work than expected

This change makes it so that if we're already finding ourselves in a
situation where we've done more scan work than expected in the
steady-state (that is, 50% of heap_scan for GOGC=100), then we fall back
on the hard heap goal instead of continuing to assume the expected case.

In some cases its possible that we're already doing more scan work than
expected, and if GC assists come in just at that window where we notice
it, they might accumulate way too much assist credit, causing undue heap
growths if GOMAXPROCS=1 (since the fractional background worker isn't
guaranteed to fire). This case seems awfully specific, and that's
because it's exactly the case for TestGcSys, which has been flaky for
some time as a result.

Fixes #28574, #27636, and #27156.

Change-Id: I771f42bed34739dbb1b84ad82cfe247f70836031
Reviewed-on: https://go-review.googlesource.com/c/go/+/184097
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/gc_test.go

@@ -21,12 +21,6 @@ func TestGcSys(t *testing.T) {
 	if os.Getenv("GOGC") == "off" {
 		t.Skip("skipping test; GOGC=off in environment")
 	}
-	if runtime.GOOS == "windows" {
-		t.Skip("skipping test; GOOS=windows http://golang.org/issue/27156")
-	}
-	if runtime.GOOS == "linux" && runtime.GOARCH == "arm64" {
-		t.Skip("skipping test; GOOS=linux GOARCH=arm64 https://github.com/golang/go/issues/27636")
-	}
 	got := runTestProg(t, "testprog", "GCSys")
 	want := "OK\n"
 	if got != want {

src/runtime/mgc.go

@@ -488,25 +488,25 @@ func (c *gcControllerState) revise() {
 	}
 	live := atomic.Load64(&memstats.heap_live)
 
-	var heapGoal, scanWorkExpected int64
-	if live <= memstats.next_gc {
-		// We're under the soft goal. Pace GC to complete at
-		// next_gc assuming the heap is in steady-state.
-		heapGoal = int64(memstats.next_gc)
+	// Assume we're under the soft goal. Pace GC to complete at
+	// next_gc assuming the heap is in steady-state.
+	heapGoal := int64(memstats.next_gc)
 
-		// Compute the expected scan work remaining.
-		//
-		// This is estimated based on the expected
-		// steady-state scannable heap. For example, with
-		// GOGC=100, only half of the scannable heap is
-		// expected to be live, so that's what we target.
-		//
-		// (This is a float calculation to avoid overflowing on
-		// 100*heap_scan.)
-		scanWorkExpected = int64(float64(memstats.heap_scan) * 100 / float64(100+gcpercent))
-	} else {
-		// We're past the soft goal. Pace GC so that in the
-		// worst case it will complete by the hard goal.
+	// Compute the expected scan work remaining.
+	//
+	// This is estimated based on the expected
+	// steady-state scannable heap. For example, with
+	// GOGC=100, only half of the scannable heap is
+	// expected to be live, so that's what we target.
+	//
+	// (This is a float calculation to avoid overflowing on
+	// 100*heap_scan.)
+	scanWorkExpected := int64(float64(memstats.heap_scan) * 100 / float64(100+gcpercent))
+
+	if live > memstats.next_gc || c.scanWork > scanWorkExpected {
+		// We're past the soft goal, or we've already done more scan
+		// work than we expected. Pace GC so that in the worst case it
+		// will complete by the hard goal.
 		const maxOvershoot = 1.1
 		heapGoal = int64(float64(memstats.next_gc) * maxOvershoot)
 
@@ -518,7 +518,7 @@ func (c *gcControllerState) revise() {
 	//
 	// Note that we currently count allocations during GC as both
 	// scannable heap (heap_scan) and scan work completed
-	// (scanWork), so allocation will change this difference will
+	// (scanWork), so allocation will change this difference
 	// slowly in the soft regime and not at all in the hard
 	// regime.
 	scanWorkRemaining := scanWorkExpected - c.scanWork