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runtime: MutexProfile missing root frames in go1.23 #69335

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felixge opened this issue Sep 7, 2024 · 8 comments
Closed

runtime: MutexProfile missing root frames in go1.23 #69335

felixge opened this issue Sep 7, 2024 · 8 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. FixPending Issues that have a fix which has not yet been reviewed or submitted. NeedsFix The path to resolution is known, but the work has not been done.
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@felixge
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felixge commented Sep 7, 2024

Go version

go1.23.1

What did you do?

Print the stack traces of a runtime.MutexProfile() using the code below.

package main

import (
	"fmt"
	"runtime"
	"sync"
	"time"
)

func main() {
	produceMutexContention()
	printMutexProfile()
}

func produceMutexContention() {
	runtime.SetMutexProfileFraction(1)
	var mu sync.Mutex
	done := make(chan struct{})
	defer close(done)
	for range runtime.GOMAXPROCS(-1) * 2 {
		go func() {
			for finish := false; !finish; {
				mu.Lock()
				select {
				case <-done:
					finish = true
				default:
					time.Sleep(time.Millisecond)
				}
				mu.Unlock()
			}
		}()
	}
	time.Sleep(time.Second)
}

func printMutexProfile() {
	var records []runtime.BlockProfileRecord
	for {
		n, ok := runtime.MutexProfile(records)
		if ok {
			records = records[:n]
			break
		}
		records = make([]runtime.BlockProfileRecord, n*2)
	}

	for _, r := range records {
		var funcs []string
		frames := runtime.CallersFrames(r.Stack())
		for {
			frame, more := frames.Next()
			funcs = append(funcs, frame.Function)
			if !more {
				break
			}
		}
		fmt.Printf("%#v\n", funcs)
	}
}

What did you see happen?

go1.23.1 produces the following output:

[]string{"sync.(*Mutex).Unlock", "main.produceMutexContention.func1"}
[]string{""}

What did you expect to see?

The same output as go1.22.7:

[]string{"sync.(*Mutex).Unlock", "main.produceMutexContention.func1", "runtime.goexit"}
[]string{"runtime._LostContendedRuntimeLock"}

Additional Notes

This problem only happens when using the runtime.MutexProfile API. runtime/pprof output is not impacted.

I have submitted https://go-review.googlesource.com/c/go/+/611615 to fix this problem.

@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Sep 7, 2024
@felixge felixge changed the title runtime: MutexProfile misses _LostContendedRuntimeLock in go1.23 runtime: MutexProfile missing root frames in go1.23 Sep 7, 2024
@gopherbot
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Change https://go.dev/cl/611615 mentions this issue: runtime: fix MutexProfile missing root frames

@timothy-king timothy-king added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Sep 9, 2024
@timothy-king timothy-king added this to the Go1.23.2 milestone Sep 9, 2024
@timothy-king
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CC @golang/runtime, @mknyszek, @prattmic.

@timothy-king timothy-king modified the milestones: Go1.23.2, Go1.24 Sep 13, 2024
@dmitshur dmitshur added NeedsFix The path to resolution is known, but the work has not been done. FixPending Issues that have a fix which has not yet been reviewed or submitted. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Sep 13, 2024
@mknyszek
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mknyszek commented Oct 2, 2024

Ping @felixge @nsrip-dd re: @cagedmantis's question from #69243. Does this need a backport as well? Thanks.

@nsrip-dd
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nsrip-dd commented Oct 2, 2024

@mknyszek I'd say yes, since this bug causes mutex profiles obtained via runtime.MutexProfile to miss information, specifically runtime._LostContendedRuntimeLock records will have no stack.

@felixge
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felixge commented Oct 13, 2024

@gopherbot Please backport to 1.23. This is a regression in mutex profiles with no workaround.

(not sure if this works, i might not have permissions)

@gopherbot
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Backport issue(s) opened: #69865 (for 1.23).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

@gopherbot
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Change https://go.dev/cl/621276 mentions this issue: runtime: fix MutexProfile missing root frames

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Labels
compiler/runtime Issues related to the Go compiler and/or runtime. FixPending Issues that have a fix which has not yet been reviewed or submitted. NeedsFix The path to resolution is known, but the work has not been done.
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