From 433c1d3b4ab41fa4272bd61f8ad2918ccd1e390d Mon Sep 17 00:00:00 2001
From: Rhys Hiltner <rhys.hiltner@gmail.com>
Date: Thu, 15 Aug 2024 18:47:38 -0700
Subject: [PATCH] runtime: store zero-delay mutex contention events
Mutex contention events with delay of 0 need more than CL 604355 added:
When deciding which event to store in the M's single available slot,
always choose to drop the zero-delay event. Store an explicit flag for
whether we have an event to store, rather than relying on a non-zero
delay.
And, fix a test of sync.Mutex contention that expects those events to
have non-zero delay. The reporting of non-runtime contention like this
has long allowed zero-delay events, which we see when cputicks has low
resolution.
Fixes #68892
Fixes #68906
Change-Id: Id412141e4eb09724f3ce195899a20d59c92d7b78
Reviewed-on: https://go-review.googlesource.com/c/go/+/606115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
---
src/runtime/mprof.go | 8 +++++++-
src/runtime/pprof/pprof_test.go | 2 +-
2 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/src/runtime/mprof.go b/src/runtime/mprof.go
index f82f6a6d37d..2629c600fd3 100644
--- a/src/runtime/mprof.go
+++ b/src/runtime/mprof.go
@@ -718,6 +718,7 @@ type mLockProfile struct {
pending uintptr // *mutex that experienced contention (to be traceback-ed)
cycles int64 // cycles attributable to "pending" (if set), otherwise to "stack"
cyclesLost int64 // contention for which we weren't able to record a call stack
+ haveStack bool // stack and cycles are to be added to the mutex profile
disabled bool // attribute all time to "lost"
}
@@ -745,6 +746,9 @@ func (prof *mLockProfile) recordLock(cycles int64, l *mutex) {
// We can only store one call stack for runtime-internal lock contention
// on this M, and we've already got one. Decide which should stay, and
// add the other to the report for runtime._LostContendedRuntimeLock.
+ if cycles == 0 {
+ return
+ }
prevScore := uint64(cheaprand64()) % uint64(prev)
thisScore := uint64(cheaprand64()) % uint64(cycles)
if prevScore > thisScore {
@@ -769,7 +773,7 @@ func (prof *mLockProfile) recordUnlock(l *mutex) {
if uintptr(unsafe.Pointer(l)) == prof.pending {
prof.captureStack()
}
- if gp := getg(); gp.m.locks == 1 && gp.m.mLockProfile.cycles != 0 {
+ if gp := getg(); gp.m.locks == 1 && gp.m.mLockProfile.haveStack {
prof.store()
}
}
@@ -795,6 +799,7 @@ func (prof *mLockProfile) captureStack() {
skip += 1 // runtime.unlockWithRank.func1
}
prof.pending = 0
+ prof.haveStack = true
prof.stack[0] = logicalStackSentinel
if debug.runtimeContentionStacks.Load() == 0 {
@@ -835,6 +840,7 @@ func (prof *mLockProfile) store() {
cycles, lost := prof.cycles, prof.cyclesLost
prof.cycles, prof.cyclesLost = 0, 0
+ prof.haveStack = false
rate := int64(atomic.Load64(&mutexprofilerate))
saveBlockEventStack(cycles, rate, prof.stack[:nstk], mutexProfile)
diff --git a/src/runtime/pprof/pprof_test.go b/src/runtime/pprof/pprof_test.go
index 30ef50b1c0f..0b4e353bb12 100644
--- a/src/runtime/pprof/pprof_test.go
+++ b/src/runtime/pprof/pprof_test.go
@@ -1371,7 +1371,7 @@ func TestMutexProfileRateAdjust(t *testing.T) {
blockMutex(t)
contentions, delay := readProfile()
- if contentions == 0 || delay == 0 {
+ if contentions == 0 { // low-resolution timers can have delay of 0 in mutex profile
t.Fatal("did not see expected function in profile")
}
runtime.SetMutexProfileFraction(0)
--
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