linux-kernel - Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups

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Date:   Tue, 20 Apr 2021 20:43:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     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>,
        Song Liu <songliubraving@...com>, Tejun Heo <tj@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups

On Wed, Apr 21, 2021 at 03:37:11AM +0900, Namhyung Kim wrote:
> On Tue, Apr 20, 2021 at 7:28 PM Peter Zijlstra <peterz@...radead.org> wrote:

> > You forgot NMI.
> 
> Thanks for your explanation.  Maybe I'm missing something but
> this event is basically for counting and doesn't allow sampling.
> Do you say it's affected by other sampling events?  Note that
> it's not reading from the PMU here, what it reads is a snapshot
> of last pmu->read(event) afaik.

Even !sampling events will trigger NMI to deal with short hardware
counters rolling over. But yes, !sampling can also be updated from NMI
by other events if they're in a group etc..

Basically, always assume NMI/PMI can happen.