return
}
+ timer := &lockTimer{lock: l}
+ timer.begin()
+
// If a goroutine's stack needed to grow during a lock2 call, the M could
// end up with two active lock2 calls (one each on curg and g0). If both are
// contended, the call on g0 will corrupt mWaitList. Disable stack growth.
gp.stackguard0, gp.throwsplit = stackPreempt, true
}
- gp.m.mWaitList.acquireTimes = timePair{nanotime: nanotime(), cputicks: cputicks()}
// On uniprocessors, no point spinning.
// On multiprocessors, spin for ACTIVE_SPIN attempts.
spin := 0
if v == old || atomic.Casuintptr(&l.key, old, v) {
gp.m.mWaitList.clearLinks()
- gp.m.mWaitList.acquireTimes = timePair{}
break
}
v = atomic.Loaduintptr(&l.key)
if gp == gp.m.curg {
gp.stackguard0, gp.throwsplit = stackguard0, throwsplit
}
+ timer.end()
return
}
i = 0
}
func unlock2(l *mutex) {
- now, dt := timePair{nanotime: nanotime(), cputicks: cputicks()}, timePair{}
for {
v := atomic.Loaduintptr(&l.key)
if v == mutex_locked {
} else if v&mutex_locked == 0 {
throw("unlock of unlocked lock")
} else {
- if now != (timePair{}) {
- head := muintptr(v &^ (mutex_sleeping | mutex_locked))
- dt = claimMutexWaitTime(now, head)
- now = timePair{}
- }
-
// Other M's are waiting for the lock.
if atomic.Casuintptr(&l.key, v, v&^mutex_locked) {
futexwakeup(key32(&l.key), 1)
}
gp := getg()
- gp.m.mLockProfile.recordUnlock(dt)
+ gp.m.mLockProfile.recordUnlock(l)
gp.m.locks--
if gp.m.locks < 0 {
throw("runtime·unlock: lock count")
return head
}
-// claimMutexWaitTime advances the acquireTimes of the list of waiting Ms at
-// head to now, returning an estimate of the total wait time claimed by that
-// action.
-func claimMutexWaitTime(now timePair, head muintptr) timePair {
- fixMutexWaitList(head)
- hp := head.ptr()
- if hp == nil {
- return timePair{}
+// lockTimer assists with profiling contention on runtime-internal locks.
+//
+// There are several steps between the time that an M experiences contention and
+// when that contention may be added to the profile. This comes from our
+// constraints: We need to keep the critical section of each lock small,
+// especially when those locks are contended. The reporting code cannot acquire
+// new locks until the M has released all other locks, which means no memory
+// allocations and encourages use of (temporary) M-local storage.
+//
+// The M will have space for storing one call stack that caused contention, and
+// for the magnitude of that contention. It will also have space to store the
+// magnitude of additional contention the M caused, since it only has space to
+// remember one call stack and might encounter several contention events before
+// it releases all of its locks and is thus able to transfer the local buffer
+// into the profile.
+//
+// The M will collect the call stack when it unlocks the contended lock. That
+// minimizes the impact on the critical section of the contended lock, and
+// matches the mutex profile's behavior for contention in sync.Mutex: measured
+// at the Unlock method.
+//
+// The profile for contention on sync.Mutex blames the caller of Unlock for the
+// amount of contention experienced by the callers of Lock which had to wait.
+// When there are several critical sections, this allows identifying which of
+// them is responsible.
+//
+// Matching that behavior for runtime-internal locks will require identifying
+// which Ms are blocked on the mutex. The semaphore-based implementation is
+// ready to allow that, but the futex-based implementation will require a bit
+// more work. Until then, we report contention on runtime-internal locks with a
+// call stack taken from the unlock call (like the rest of the user-space
+// "mutex" profile), but assign it a duration value based on how long the
+// previous lock call took (like the user-space "block" profile).
+//
+// Thus, reporting the call stacks of runtime-internal lock contention is
+// guarded by GODEBUG for now. Set GODEBUG=runtimecontentionstacks=1 to enable.
+//
+// TODO(rhysh): plumb through the delay duration, remove GODEBUG, update comment
+//
+// The M will track this by storing a pointer to the lock; lock/unlock pairs for
+// runtime-internal locks are always on the same M.
+//
+// Together, that demands several steps for recording contention. First, when
+// finally acquiring a contended lock, the M decides whether it should plan to
+// profile that event by storing a pointer to the lock in its "to be profiled
+// upon unlock" field. If that field is already set, it uses the relative
+// magnitudes to weight a random choice between itself and the other lock, with
+// the loser's time being added to the "additional contention" field. Otherwise
+// if the M's call stack buffer is occupied, it does the comparison against that
+// sample's magnitude.
+//
+// Second, having unlocked a mutex the M checks to see if it should capture the
+// call stack into its local buffer. Finally, when the M unlocks its last mutex,
+// it transfers the local buffer into the profile. As part of that step, it also
+// transfers any "additional contention" time to the profile. Any lock
+// contention that it experiences while adding samples to the profile will be
+// recorded later as "additional contention" and not include a call stack, to
+// avoid an echo.
+type lockTimer struct {
+ lock *mutex
+ timeRate int64
+ timeStart int64
+ tickStart int64
+}
+
+func (lt *lockTimer) begin() {
+ rate := int64(atomic.Load64(&mutexprofilerate))
+
+ lt.timeRate = gTrackingPeriod
+ if rate != 0 && rate < lt.timeRate {
+ lt.timeRate = rate
+ }
+ if int64(cheaprand())%lt.timeRate == 0 {
+ lt.timeStart = nanotime()
}
- tp := hp.mWaitList.tail.ptr()
- waiters := hp.mWaitList.waiters
- headTimes := hp.mWaitList.acquireTimes
- tailTimes := tp.mWaitList.acquireTimes
- var dt timePair
- dt.nanotime = now.nanotime - headTimes.nanotime
- dt.cputicks = now.cputicks - headTimes.cputicks
- if waiters > 1 {
- dt.nanotime = int64(waiters) * (dt.nanotime + now.nanotime - tailTimes.nanotime) / 2
- dt.cputicks = int64(waiters) * (dt.cputicks + now.cputicks - tailTimes.cputicks) / 2
+ if rate > 0 && int64(cheaprand())%rate == 0 {
+ lt.tickStart = cputicks()
}
+}
- // When removeMutexWaitList removes a head or tail node, it's responsible
- // for applying these changes to the new head or tail.
- hp.mWaitList.acquireTimes = now
- tp.mWaitList.acquireTimes = now
+func (lt *lockTimer) end() {
+ gp := getg()
- return dt
+ if lt.timeStart != 0 {
+ nowTime := nanotime()
+ gp.m.mLockProfile.waitTime.Add((nowTime - lt.timeStart) * lt.timeRate)
+ }
+
+ if lt.tickStart != 0 {
+ nowTick := cputicks()
+ gp.m.mLockProfile.recordLock(nowTick-lt.tickStart, lt.lock)
+ }
}
// mLockProfile is part of the M struct to hold information relating to mutex
// not include a call stack, to avoid an echo.
type mLockProfile struct {
waitTime atomic.Int64 // total nanoseconds spent waiting in runtime.lockWithRank
- stack []uintptr // unlock stack that caused delay in other Ms' runtime.lockWithRank
- cycles int64 // cycles attributable to "stack"
+ stack []uintptr // stack that experienced contention in runtime.lockWithRank
+ 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
disabled bool // attribute all time to "lost"
}
-// recordUnlock considers the current unlock call (which caused a total of dt
-// delay in other Ms) for later inclusion in the mutex contention profile. If
-// this M holds no other locks, it transfers the buffered contention record to
-// the mutex contention profile.
-//
-// From unlock2, we might not be holding a p in this code.
-//
-//go:nowritebarrierrec
-func (prof *mLockProfile) recordUnlock(dt timePair) {
- if dt != (timePair{}) {
- // We could make a point of clearing out the local storage right before
- // this, to have a slightly better chance of being able to see the call
- // stack if the program has several (nested) contended locks. If apps
- // are seeing a lot of _LostContendedRuntimeLock samples, maybe that'll
- // be a worthwhile change.
- prof.proposeUnlock(dt)
- }
- if getg().m.locks == 1 && prof.cycles != 0 {
- prof.store()
- }
-}
-
-func (prof *mLockProfile) proposeUnlock(dt timePair) {
- if nanos := dt.nanotime; nanos > 0 {
- prof.waitTime.Add(nanos)
- }
-
- cycles := dt.cputicks
+func (prof *mLockProfile) recordLock(cycles int64, l *mutex) {
if cycles <= 0 {
return
}
- rate := int64(atomic.Load64(&mutexprofilerate))
- if rate <= 0 || int64(cheaprand())%rate != 0 {
- return
- }
-
if prof.disabled {
// We're experiencing contention while attempting to report contention.
// Make a note of its magnitude, but don't allow it to be the sole cause
return
}
+ if uintptr(unsafe.Pointer(l)) == prof.pending {
+ // Optimization: we'd already planned to profile this same lock (though
+ // possibly from a different unlock site).
+ prof.cycles += cycles
+ return
+ }
+
if prev := prof.cycles; prev > 0 {
// 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
prof.cyclesLost += prev
}
}
+ // Saving the *mutex as a uintptr is safe because:
+ // - lockrank_on.go does this too, which gives it regular exercise
+ // - the lock would only move if it's stack allocated, which means it
+ // cannot experience multi-M contention
+ prof.pending = uintptr(unsafe.Pointer(l))
prof.cycles = cycles
- prof.captureStack()
+}
+
+// From unlock2, we might not be holding a p in this code.
+//
+//go:nowritebarrierrec
+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 {
+ prof.store()
+ }
}
func (prof *mLockProfile) captureStack() {
return
}
- skip := 4 // runtime.(*mLockProfile).proposeUnlock runtime.(*mLockProfile).recordUnlock runtime.unlock2 runtime.unlockWithRank
+ skip := 3 // runtime.(*mLockProfile).recordUnlock runtime.unlock2 runtime.unlockWithRank
if staticLockRanking {
// When static lock ranking is enabled, we'll always be on the system
// stack at this point. There will be a runtime.unlockWithRank.func1
// "runtime.unlock".
skip += 1 // runtime.unlockWithRank.func1
}
+ prof.pending = 0
prof.stack[0] = logicalStackSentinel
if debug.runtimeContentionStacks.Load() == 0 {