linux-kernel - Re: [PATCH v3] perf/core: Set event shadow time for inactive events too

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Message-ID: <7A415BC0-E6F2-4ED2-8996-8F5871ED8001@fb.com>
Date:   Sat, 18 Dec 2021 09:09:05 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Ian Rogers" <irogers@...gle.com>
Subject: Re: [PATCH v3] perf/core: Set event shadow time for inactive events
 too



> On Dec 17, 2021, at 8:35 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Fri, Dec 10, 2021 at 11:33:41AM +0100, Peter Zijlstra wrote:
> 
>> I'm thinking this is a cgroup specific thing. Normally the shadow_time
>> thing is simply a relative displacement between event-time and the
>> global clock. That displacement never changes, except when you do
>> IOC_DISABLE/IOC_ENABLE.
>> 
>> However, for cgroup things are different, since the cgroup events aren't
>> unconditionally runnable, that is, the enabled time should only count
>> when the cgroup is active, right?
>> 
>> So perhaps perf_event_read_local() should use a cgroup clock instead of
>> perf_clock() for cgroup events.
>> 
>> Let me think about that some more...
> 
> How's this then? Song, could you also please test and or better explain
> the problem f79256532682 pretends to cure? Because the below is
> reverting that, I *really* hate having to touch the events we're not
> scheduling.

Unfortunately, this change bring the bug back. For time_enabled in rdpmc
case to work properly, we have to touch all the enabled but not running 
events, right?

Thanks,
Song

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