Commit eb3182ef authored by Dapeng Mi's avatar Dapeng Mi Committed by Ingo Molnar
Browse files

perf/core: Fix system hang caused by cpu-clock usage 

cpu-clock usage by the async-profiler tool can trigger a system hang,
which got bisected back to the following commit by Octavia Togami:

  18dbcbfa ("perf: Fix the POLL_HUP delivery breakage") causes this issue

The root cause of the hang is that cpu-clock is a special type of SW
event which relies on hrtimers. The __perf_event_overflow() callback
is invoked from the hrtimer handler for cpu-clock events, and
__perf_event_overflow() tries to call cpu_clock_event_stop()
to stop the event, which calls htimer_cancel() to cancel the hrtimer.

But that's a recursion into the hrtimer code from a hrtimer handler,
which (unsurprisingly) deadlocks.

To fix this bug, use hrtimer_try_to_cancel() instead, and set
the PERF_HES_STOPPED flag, which causes perf_swevent_hrtimer()
to stop the event once it sees the PERF_HES_STOPPED flag.

[ mingo: Fixed the comments and improved the changelog. ]

Closes: https://lore.kernel.org/all/CAHPNGSQpXEopYreir+uDDEbtXTBvBvi8c6fYXJvceqtgTPao3Q@mail.gmail.com/


Fixes: 18dbcbfa ("perf: Fix the POLL_HUP delivery breakage")
Reported-by: default avatarOctavia Togami <octavia.togami@gmail.com>
Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarDapeng Mi <dapeng1.mi@linux.intel.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Tested-by: default avatarOctavia Togami <octavia.togami@gmail.com>
Cc: stable@vger.kernel.org
Link: https://github.com/lucko/spark/issues/530
Link: https://patch.msgid.link/20251015051828.12809-1-dapeng1.mi@linux.intel.com
parent 6146a0f1

kernel/events/core.c

+15 −5
Original line number Diff line number Diff line
@@ -11773,7 +11773,8 @@ static enum hrtimer_restart perf_swevent_hrtimer(struct hrtimer *hrtimer) 


	event = container_of(hrtimer, struct perf_event, hw.hrtimer);



	if (event->state != PERF_EVENT_STATE_ACTIVE)

	if (event->state != PERF_EVENT_STATE_ACTIVE ||

	    event->hw.state & PERF_HES_STOPPED)

		return HRTIMER_NORESTART;



	event->pmu->read(event);
@@ -11819,15 +11820,20 @@ static void perf_swevent_cancel_hrtimer(struct perf_event *event) 
	struct hw_perf_event *hwc = &event->hw;



	/*

	 * The throttle can be triggered in the hrtimer handler.

	 * The HRTIMER_NORESTART should be used to stop the timer,

	 * rather than hrtimer_cancel(). See perf_swevent_hrtimer()

	 * Careful: this function can be triggered in the hrtimer handler,

	 * for cpu-clock events, so hrtimer_cancel() would cause a

	 * deadlock.

	 *

	 * So use hrtimer_try_to_cancel() to try to stop the hrtimer,

	 * and the cpu-clock handler also sets the PERF_HES_STOPPED flag,

	 * which guarantees that perf_swevent_hrtimer() will stop the

	 * hrtimer once it sees the PERF_HES_STOPPED flag.

	 */

	if (is_sampling_event(event) && (hwc->interrupts != MAX_INTERRUPTS)) {

		ktime_t remaining = hrtimer_get_remaining(&hwc->hrtimer);

		local64_set(&hwc->period_left, ktime_to_ns(remaining));



		hrtimer_cancel(&hwc->hrtimer);

		hrtimer_try_to_cancel(&hwc->hrtimer);

	}

}
@@ -11871,12 +11877,14 @@ static void cpu_clock_event_update(struct perf_event *event) 


static void cpu_clock_event_start(struct perf_event *event, int flags)

{

	event->hw.state = 0;

	local64_set(&event->hw.prev_count, local_clock());

	perf_swevent_start_hrtimer(event);

}



static void cpu_clock_event_stop(struct perf_event *event, int flags)

{

	event->hw.state = PERF_HES_STOPPED;

	perf_swevent_cancel_hrtimer(event);

	if (flags & PERF_EF_UPDATE)

		cpu_clock_event_update(event);
@@ -11950,12 +11958,14 @@ static void task_clock_event_update(struct perf_event *event, u64 now) 


static void task_clock_event_start(struct perf_event *event, int flags)

{

	event->hw.state = 0;

	local64_set(&event->hw.prev_count, event->ctx->time);

	perf_swevent_start_hrtimer(event);

}



static void task_clock_event_stop(struct perf_event *event, int flags)

{

	event->hw.state = PERF_HES_STOPPED;

	perf_swevent_cancel_hrtimer(event);

	if (flags & PERF_EF_UPDATE)

		task_clock_event_update(event, event->ctx->time);