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Subject: Re: [PATCH RESEND v3] perf/core: Fix hardlockup failure caused by
 perf throttle
From:   Yang Jihong <yangjihong1@huawei.com>
To:     <peterz@infradead.org>, <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>
References: <20230227023508.102230-1-yangjihong1@huawei.com>
 <12782722-103e-88a9-df3e-6815b4734bc3@huawei.com>
 <37d019af-191c-9955-2417-b7cfa5e5efa3@huawei.com>
Message-ID: <56cc5b4f-3cbd-c265-36fc-fedab16b8cff@huawei.com>
Date:   Fri, 14 Apr 2023 15:04:11 +0800
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Hello,

PING again.
Look forward the review.

Thanks,
Yang.

On 2023/3/22 15:36, Yang Jihong wrote:
> Hello,
> 
> PING.
> 
> This patch has not been responded.
> Please take time to check whether the fix solution is OK.
> Look forward to reviewing the patch. Thanks :)
> 
> Thanks,
> Yang.
> 
> On 2023/3/6 9:14, Yang Jihong wrote:
>> Hello,
>>
>> PING.
>>
>> Thanks,
>> Yang.
>>
>> On 2023/2/27 10:35, 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>
>>> ---
>>>
>>> Changes since v2:
>>>    - Add fixed commit.
>>>
>>> Changes since v1:
>>>    - Modify commit title.
>>>
>>>   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;
>>>
>>
>> .
> 
> .