Re: [PATCH V4 01/16] perf: Fix the throttle logic for a group

[Leo Yan <leo.yan@arm.com>]

Hi Kan,

[ + Aishwarya ]

On Tue, May 20, 2025 at 11:16:29AM -0700, kan.liang@linux.intel.com wrote:

[...]

> @@ -10331,8 +10358,7 @@ __perf_event_account_interrupt(struct perf_event *event, int throttle)
>  	if (unlikely(throttle && hwc->interrupts >= max_samples_per_tick)) {
>  		__this_cpu_inc(perf_throttled_count);
>  		tick_dep_set_cpu(smp_processor_id(), TICK_DEP_BIT_PERF_EVENTS);
> -		hwc->interrupts = MAX_INTERRUPTS;
> -		perf_log_throttle(event, 0);
> +		perf_event_throttle_group(event);
>  		ret = 1;
>  	}

Our (Arm) CI reports RCU stall that caused by this patch.  I can use a
simple command to trigger system stuck with cpu-clock:

  perf record -a -e cpu-clock -- sleep 2

I confirmed that if removing throttling code for cpu-clock event, then
the issue can be dimissed.  Based on reading code, the flow below:

  hrtimer interrupt:
   `> __perf_event_account_interrupt()
       `> perf_event_throttle_group()
           `> perf_event_throttle()
               `> cpu_clock_event_stop()
                   `> perf_swevent_cancel_hrtimer()
                       `> hrtimer_cancel()  -> Inifite loop.

In the hrtimer interrupt handler, it tries to cancel itself and causes
inifite loop.  Please consider to fix the issue.


Thanks,
Leo