linux-kernel - Re: [PATCH] perf: fix perf_event

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Date:   Fri, 3 Mar 2023 01:21:51 +0000
From:   Song Liu <songliubraving@...a.com>
To:     Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Song Liu <songliubraving@...a.com>, Song Liu <song@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH] perf: fix perf_event_context->time



> On Mar 2, 2023, at 1:15 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> 
> On Wed, Mar 1, 2023 at 3:16 PM Song Liu <songliubraving@...a.com> wrote:
>> 
>> 
>> 
>>> On Mar 1, 2023, at 2:29 PM, Namhyung Kim <namhyung@...nel.org> wrote:
>>> 
>>> Hi Song,
>>> 
>>> On Tue, Feb 28, 2023 at 11:22 AM Song Liu <song@...nel.org> wrote:
>>>> 
>>>> Time readers rely on perf_event_context->[time|timestamp|timeoffset] to get
>>>> accurate time_enabled and time_running for an event. The difference between
>>>> ctx->timestamp and ctx->time is the among of time when the context is not
>>>> enabled. For cpuctx.ctx, time and timestamp should stay the same, however,
>>> 
>>> I'm not sure if it's correct.  The timestamp can go when the context is disabled
>>> for example, in ctx_resched() even if the NMI watchdog is enabled, right?
>> 
>> I think we do not disable EVENT_TIME for per cpu ctx?
> 
> I can see ctx_sched_out(ctx, EVENT_TIME) in some places.
> Also it'd reset EVENT_TIME if both _PINNED and _FLEXIBLE is
> cleared.

Yeah, you are right. I missed this part. 

Hi Peter, 

Does this fix look good do you?

Thanks,
Song