Re: [PATCH 2/2] perf/core: Fix broken throttling when max_samples_per_tick=1

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Apr 05, 2025 at 10:16:35PM +0800, Qing Wong wrote:
> From: Qing Wang <wangqing7171@gmail.com>
> 
> According to the throttling mechanism, the pmu interrupts number can not
> exceed the max_samples_per_tick in one tick. But this mechanism is
> ineffective when max_samples_per_tick=1, because the throttling check is
> skipped during the first interrupt and only performed when the second
> interrupt arrives.
> 
> Perhaps this bug may cause little influence in one tick, but if in a
> larger time scale, the problem can not be underestimated.
> 
> When max_samples_per_tick = 1:
> Allowed-interrupts-per-second max-samples-per-second  default-HZ  ARCH
> 200                           100                     100         X86
> 500                           250                     250         ARM64
> ...
> Obviously, the pmu interrupt number far exceed the user's expect.
> 
> Fixes: e050e3f0a71b ("perf: Fix broken interrupt rate throttling")
> Signed-off-by: Qing Wang <wangqing7171@gmail.com>
> ---
>  kernel/events/core.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 29cdb240e104..4ac2ac988ddc 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -10047,16 +10047,15 @@ __perf_event_account_interrupt(struct perf_event *event, int throttle)
>  	if (seq != hwc->interrupts_seq) {
>  		hwc->interrupts_seq = seq;
>  		hwc->interrupts = 1;
> -	} else {
> +	} else
>  		hwc->interrupts++;
> -		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);
> -			ret = 1;
> -		}
> +
> +	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);
> +		ret = 1;
>  	}

Fair enough I suppose. I'll make this apply without that revert -- it
seems pointless to have that in between.