Re: [PATCH] perf: Fix system hang caused by cpu-clock

[Mi, Dapeng]

On 10/21/2025 10:47 PM, Peter Zijlstra wrote:
> On Wed, Oct 15, 2025 at 01:18:28PM +0800, Dapeng Mi wrote:
>> A system hang issue caused by cpu-clock is reported and bisection
>> indicates the commit 18dbcbfabfff ("perf: Fix the POLL_HUP delivery
>>  breakage") causes this issue.
>>
>> The root cause of the hang issue is that cpu-clock is a specific SW
>> event which relies on the hrtimer. The __perf_event_overflow()
>> is invoked from the hrtimer handler for cpu-clock event, and
>> __perf_event_overflow() tries to call event stop callback
>> (cpu_clock_event_stop()) to stop the event, and cpu_clock_event_stop()
>> calls htimer_cancel() to cancel the hrtimer. But unfortunately the
>> hrtimer callback is currently executing and then traps into deadlock.
>>
>> To avoid this deadlock, use hrtimer_try_to_cancel() instead of
>> hrtimer_cancel() to cancel the hrtimer, and set PERF_HES_STOPPED flag
>> for the stopping events. perf_swevent_hrtimer() would stop the event
>> hrtimer once it detects the PERF_HES_STOPPED flag.
>>
>> Reported-by: Octavia Togami <octavia.togami@xxxxxxxxx>
>> Closes: https://lore.kernel.org/all/CAHPNGSQpXEopYreir+uDDEbtXTBvBvi8c6fYXJvceqtgTPao3Q@xxxxxxxxxxxxxx/
>> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Fixes: 18dbcbfabfff ("perf: Fix the POLL_HUP delivery breakage")
>> Tested-by: Octavia Togami <octavia.togami@xxxxxxxxx>
>> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
>> ---
>>  kernel/events/core.c | 18 +++++++++++++-----
>>  1 file changed, 13 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 7541f6f85fcb..f90105d5f26a 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -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);
> I was wondering if we need a HES_STOPPED check after calling
> __perf_event_overflow(), but typically that will return 1 when it does
> the stop itself, which then already does NORESTART.

Yes.


>
> So yeah, I suppose this works. Let me go queue this up.

Thanks for reviewing this patch.


>
> Thanks!
>