Revert "runtime: fix missing pprof labels"

This reverts CL 351751.

Reason for revert: new test is failing on many builders.

Change-Id: I066211c9f25607ca9eb5299aedea2ecc5069e34f
Reviewed-on: https://go-review.googlesource.com/c/go/+/360757
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/cpuprof.go

@@ -89,7 +89,7 @@ func SetCPUProfileRate(hz int) {
 // held at the time of the signal, nor can it use substantial amounts
 // of stack.
 //go:nowritebarrierrec
-func (p *cpuProfile) add(tagPtr *unsafe.Pointer, stk []uintptr) {
+func (p *cpuProfile) add(gp *g, stk []uintptr) {
 	// Simple cas-lock to coordinate with setcpuprofilerate.
 	for !atomic.Cas(&prof.signalLock, 0, 1) {
 		osyield()
@@ -104,6 +104,15 @@ func (p *cpuProfile) add(tagPtr *unsafe.Pointer, stk []uintptr) {
 		// because otherwise its write barrier behavior may not
 		// be correct. See the long comment there before
 		// changing the argument here.
+		//
+		// Note: it can happen on Windows, where we are calling
+		// p.add with a gp that is not the current g, that gp is nil,
+		// meaning we interrupted a system thread with no g.
+		// Avoid faulting in that case.
+		var tagPtr *unsafe.Pointer
+		if gp != nil {
+			tagPtr = &gp.labels
+		}
 		cpuprof.log.write(tagPtr, nanotime(), hdr[:], stk)
 	}
 

src/runtime/pprof/pprof_test.go

@@ -1361,73 +1361,6 @@ func TestLabelRace(t *testing.T) {
 	})
 }
 
-func TestLabelSystemstack(t *testing.T) {
-	// See http://golang.org/cl/351751.
-	prof := testCPUProfile(t, stackContainsLabeled, []string{"runtime.systemstack;key=value"}, avoidFunctions(), func(dur time.Duration) {
-		Do(context.Background(), Labels("key", "value"), func(context.Context) {
-			var wg sync.WaitGroup
-			stop := make(chan struct{})
-			for i := 0; i < runtime.GOMAXPROCS(0); i++ {
-				wg.Add(1)
-				go func() {
-					defer wg.Done()
-					labelHog(stop)
-				}()
-			}
-
-			time.Sleep(dur)
-			close(stop)
-			wg.Wait()
-		})
-	})
-
-	var withLabel, withoutLabel int64
-	for _, s := range prof.Sample {
-		var systemstack, labelHog bool
-		for _, loc := range s.Location {
-			for _, l := range loc.Line {
-				switch l.Function.Name {
-				case "runtime.systemstack":
-					systemstack = true
-				case "runtime/pprof.labelHog":
-					labelHog = true
-				}
-			}
-		}
-
-		if systemstack && labelHog {
-			if s.Label != nil && contains(s.Label["key"], "value") {
-				withLabel += s.Value[0]
-			} else {
-				withoutLabel += s.Value[0]
-			}
-		}
-	}
-
-	// ratio on 2019 Intel MBP before/after CL 351751 for n=30 runs:
-	// before: mean=0.013 stddev=0.013 min=0.000 max=0.039
-	// after : mean=0.996 stddev=0.007 min=0.967 max=1.000
-	//
-	// TODO: Figure out why some samples still contain labelHog without labels.
-	// Once fixed this test case can be simplified to just check that all samples
-	// containing labelHog() have the label, and no other samples do.
-	ratio := float64(withLabel) / float64((withLabel + withoutLabel))
-	if ratio < 0.9 {
-		t.Fatalf("only %.1f%% of labelHog(systemstack()) samples have label", ratio*100)
-	}
-}
-
-func labelHog(stop chan struct{}) {
-	for i := 0; ; i++ {
-		select {
-		case <-stop:
-			return
-		default:
-			fmt.Fprintf(io.Discard, "%d", i)
-		}
-	}
-}
-
 // Check that there is no deadlock when the program receives SIGPROF while in
 // 64bit atomics' critical section. Used to happen on mips{,le}. See #20146.
 func TestAtomicLoadStore64(t *testing.T) {

src/runtime/proc.go

@@ -4711,14 +4711,7 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
 	}
 
 	if prof.hz != 0 {
-		// Note: it can happen on Windows that we interrupted a system thread
-		// with no g, so gp could nil. The other nil checks are done out of
-		// caution, but not expected to be nil in practice.
-		var tagPtr *unsafe.Pointer
-		if gp != nil && gp.m != nil && gp.m.curg != nil {
-			tagPtr = &gp.m.curg.labels
-		}
-		cpuprof.add(tagPtr, stk[:n])
+		cpuprof.add(gp, stk[:n])
 	}
 	getg().m.mallocing--
 }