runtime/pprof: scale mutex profile samples when they are recorded

Samples in the mutex profile have their count and duration scaled
according to the probability they were sampled. This is done when the
profile is actually requested. The adjustment is done using to the
current configured sampling rate. However, if the sample rate is changed
after a specific sample is recorded, then the sample will be scaled
incorrectly. In particular, if the sampling rate is changed to 0, all of
the samples in the encoded profile will have 0 count and duration. This
means the profile will be "empty", even if it should have had samples.

This CL scales the samples in the profile when they are recorded, rather
than when the profile is requested. This matches what is currently done
for the block profile.

With this change, neither the block profile nor mutex profile are scaled
when they are encoded, so the logic for scaling the samples can be
removed.

Change-Id: If228cf39284385aa8fb9a2d62492d839e02f027f
Reviewed-on: https://go-review.googlesource.com/c/go/+/443056
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 7 insertions, 22 deletions

diff --git a/src/runtime/pprof/pprof.go b/src/runtime/pprof/pprof.go
index 78445b6264..725fd26c13 100644
--- a/src/runtime/pprof/pprof.go
+++ b/src/runtime/pprof/pprof.go
@@ -371,8 +371,7 @@ type countProfile interface {
 // as the pprof-proto format output. Translations from cycle count to time duration
 // are done because The proto expects count and time (nanoseconds) instead of count
 // and the number of cycles for block, contention profiles.
-// Possible 'scaler' functions are scaleBlockProfile and scaleMutexProfile.
-func printCountCycleProfile(w io.Writer, countName, cycleName string, scaler func(int64, float64) (int64, float64), records []runtime.BlockProfileRecord) error {
+func printCountCycleProfile(w io.Writer, countName, cycleName string, records []runtime.BlockProfileRecord) error {
 	// Output profile in protobuf form.
 	b := newProfileBuilder(w)
 	b.pbValueType(tagProfile_PeriodType, countName, "count")
@@ -385,9 +384,8 @@ func printCountCycleProfile(w io.Writer, countName, cycleName string, scaler fun
 	values := []int64{0, 0}
 	var locs []uint64
 	for _, r := range records {
-		count, nanosec := scaler(r.Count, float64(r.Cycles)/cpuGHz)
-		values[0] = count
-		values[1] = int64(nanosec)
+		values[0] = r.Count
+		values[1] = int64(float64(r.Cycles) / cpuGHz)
 		// For count profiles, all stack addresses are
 		// return PCs, which is what appendLocsForStack expects.
 		locs = b.appendLocsForStack(locs[:0], r.Stack())
@@ -855,24 +853,16 @@ func countMutex() int {
 
 // writeBlock writes the current blocking profile to w.
 func writeBlock(w io.Writer, debug int) error {
-	return writeProfileInternal(w, debug, "contention", runtime.BlockProfile, scaleBlockProfile)
-}
-
-func scaleBlockProfile(cnt int64, ns float64) (int64, float64) {
-	// Do nothing.
-	// The current way of block profile sampling makes it
-	// hard to compute the unsampled number. The legacy block
-	// profile parse doesn't attempt to scale or unsample.
-	return cnt, ns
+	return writeProfileInternal(w, debug, "contention", runtime.BlockProfile)
 }
 
 // writeMutex writes the current mutex profile to w.
 func writeMutex(w io.Writer, debug int) error {
-	return writeProfileInternal(w, debug, "mutex", runtime.MutexProfile, scaleMutexProfile)
+	return writeProfileInternal(w, debug, "mutex", runtime.MutexProfile)
 }
 
 // writeProfileInternal writes the current blocking or mutex profile depending on the passed parameters
-func writeProfileInternal(w io.Writer, debug int, name string, runtimeProfile func([]runtime.BlockProfileRecord) (int, bool), scaleProfile func(int64, float64) (int64, float64)) error {
+func writeProfileInternal(w io.Writer, debug int, name string, runtimeProfile func([]runtime.BlockProfileRecord) (int, bool)) error {
 	var p []runtime.BlockProfileRecord
 	n, ok := runtimeProfile(nil)
 	for {
@@ -887,7 +877,7 @@ func writeProfileInternal(w io.Writer, debug int, name string, runtimeProfile fu
 	sort.Slice(p, func(i, j int) bool { return p[i].Cycles > p[j].Cycles })
 
 	if debug <= 0 {
-		return printCountCycleProfile(w, "contentions", "delay", scaleProfile, p)
+		return printCountCycleProfile(w, "contentions", "delay", p)
 	}
 
 	b := bufio.NewWriter(w)
@@ -917,9 +907,4 @@ func writeProfileInternal(w io.Writer, debug int, name string, runtimeProfile fu
 	return b.Flush()
 }
 
-func scaleMutexProfile(cnt int64, ns float64) (int64, float64) {
-	period := runtime.SetMutexProfileFraction(-1)
-	return cnt * int64(period), ns * float64(period)
-}
-
 func runtime_cyclesPerSecond() int64