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Date:   Fri, 14 Apr 2023 10:23:34 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yang Jihong <yangjihong1@huawei.com>
Cc:     mingo@redhat.com, acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, jolsa@kernel.org,
        namhyung@kernel.org, irogers@google.com, eranian@google.com,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND v3] perf/core: Fix hardlockup failure caused by
 perf throttle
Message-ID: <20230414082334.GU4253@hirez.programming.kicks-ass.net>
References: <20230227023508.102230-1-yangjihong1@huawei.com>
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On Mon, Feb 27, 2023 at 10:35:08AM +0800, Yang Jihong wrote:
> commit e050e3f0a71bf ("perf: Fix broken interrupt rate throttling")
> introduces a change in throttling threshold judgment. Before this,
> compare hwc->interrupts and max_samples_per_tick, then increase
> hwc->interrupts by 1, but this commit reverses order of these two
> behaviors, causing the semantics of max_samples_per_tick to change.
> 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).
> 
> Fixes: e050e3f0a71b ("perf: Fix broken interrupt rate throttling")
> Signed-off-by: Yang Jihong <yangjihong1@huawei.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 f79fd8b87f75..0540a8653906 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9434,7 +9434,7 @@ __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;

Thanks, I've made a slight edit to fix the && placement.