linux-kernel - Re: [PATCH] perf/core: Fix account interrupt throttle

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Date:   Sat, 28 Jan 2023 09:28:34 +0800
From:   Yang Jihong <yangjihong1@...wei.com>
To:     <peterz@...radead.org>, <mingo@...hat.com>, <acme@...nel.org>,
        <mark.rutland@....com>, <alexander.shishkin@...ux.intel.com>,
        <jolsa@...nel.org>, <namhyung@...nel.org>,
        <linux-perf-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf/core: Fix account interrupt throttle

Hello,

PING

Thanks,
Yang

On 2023/1/12 11:38, Yang Jihong wrote:
> In literal sense of "max_samples_per_tick", if hwc->interrupts ==
> max_samples_per_tick, it should not be throttled, therefore, the judgment
> condition should be changed to "hwc->interrupts > max_samples_per_tick".
> 
> In fact, this may cause the hardlockup to fail, The minimum value of
> max_samples_per_tick may be 1, in this case, the return value of
> __perf_event_account_interrupt function is 1.
> As a result, nmi_watchdog gets throttled, which would stop PMU (Use x86
> architecture as an example, see x86_pmu_handle_irq).
> 
> Signed-off-by: Yang Jihong <yangjihong1@...wei.com>
> ---
>   kernel/events/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index d56328e5080e..ced98e028d86 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9414,7 +9414,7 @@ __perf_event_account_interrupt(struct perf_event *event, int throttle)
>   	} else {
>   		hwc->interrupts++;
>   		if (unlikely(throttle
> -			     && hwc->interrupts >= max_samples_per_tick)) {
> +			     && 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;
>