linux-kernel - Re: [RFC 1/2] perf/core: Enable sched

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Date:   Fri, 6 Nov 2020 11:11:55 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>, Gabriel Marin <gmx@...gle.com>
Subject: Re: [RFC 1/2] perf/core: Enable sched_task callbacks if PMU has it



On 11/5/2020 7:53 PM, Namhyung Kim wrote:
> On Fri, Nov 6, 2020 at 4:01 AM Liang, Kan<kan.liang@...ux.intel.com>  wrote:
>>
>>
>> On 11/5/2020 10:45 AM, Namhyung Kim wrote:
>>> Hello,
>>>
>>> On Thu, Nov 5, 2020 at 11:47 PM Liang, Kan<kan.liang@...ux.intel.com>  wrote:
>>>>
>>>>
>>>> On 11/2/2020 9:52 AM, Namhyung Kim wrote:
>>>>> If an event associated with a PMU which has a sched_task callback,
>>>>> it should be called regardless of cpu/task context.  For example,
>>>>
>>>> I don't think it's necessary. We should call it when we have to.
>>>> Otherwise, it just waste cycles.
>>>> Shouldn't the patch 2 be enough?
>>> I'm not sure, without this patch __perf_event_task_sched_in/out
>>> cannot be called for per-cpu events (w/o cgroups)  IMHO.
>>> And I could not find any other place to check the
>>> perf_sched_cb_usages.
>>>
>> Yes, it should a bug for large PEBS, and it should has always been there
>> since the large PEBS was introduced. I just tried some older kernels
>> (before recent change). Large PEBS is not flushed with per-cpu events.
>>
>> But from your description, it looks like the issue is only found after
>> recent change. Could you please double check if the issue can also be
>> reproduced before the recent change?
> Yep, actually Gabriel reported this problem on v4.4 kernel.

Thanks for the confirm.

So large PEBS never works with per-cpu events. :(
I will send a new patch set to address the issue.

Thanks,
Kan