linux-kernel - Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

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Message-ID: <CAP-5=fWXXi7Y=6Q0k8oLOZmYon+vvg-k4dNUSex_ijcL8ti9sQ@mail.gmail.com>
Date:   Tue, 3 Oct 2023 15:36:19 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Mingwei Zhang <mizhang@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yang Jihong <yangjihong1@...wei.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

On Tue, Oct 3, 2023 at 1:08 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Hello,
>
> On Wed, Sep 20, 2023 at 10:05 PM Mingwei Zhang <mizhang@...gle.com> wrote:
> >
> > On Mon, Sep 18, 2023 at 3:43 PM Ian Rogers <irogers@...gle.com> wrote:
> > >
> > > On Sat, Sep 16, 2023 at 5:46 PM Mingwei Zhang <mizhang@...gle.com> wrote:
> > > > Thank you very much for the change. I have one quick question about
> > > > the PMU unthrottling logic. When I am looking into the function
> > > > perf_adjust_freq_unthr_context(), I see the loop with PMU stop and
> > > > start in each iteration. Is there a good way to avoid this PMU reset
> > > > operation while quickly figuring out the event in frequency mode?
> > >
> > > Agreed. I think before the pmu_disable could be avoided for this condition:
> > > ```
> > > if (event->hw.interrupts != MAX_INTERRUPTS &&
> > >     (!event->attr.freq || !event->attr.sample_freq))
> > >         continue;
> > > ```
> > > Fixing up the event stop/start looks harder.
> > >
> >
> > Right, I think putting the check early before pmu_disable() is already
> > a great optimization. The only concern I initially had was whether
> > event->hw.interrupts can be accessed before we disable the pmu. But
> > after checking this field in other locations, I don't see any problem
> > at all.
>
> The event->hw.interrupts would be increased in the NMI handler
> so there is a race between the check and the NMI.  That's why
> I think it checks that after disabling the PMU.
>
> But I think we can skip non-sampling events for sure.  Then it
> would be better to set attr.sample_period = 0 rather than attr.freq.
>
>     if (!is_sampling_event(event))
>         continue;
>
>     perf_pmu_disable(event->pmu);
>     ...
>
> Thanks,
> Namhyung

With the PMU disabled, isn't there still a risk of an interrupt still
being in flight? In other words the disable doesn't prevent a race and
we'll catch this on the next timer call to
perf_adjust_freq_unthr_context. I think we can also improve the code
by just disabling a PMU once, we can take advantage of the
perf_event_pmu_context and disable that PMU, iterate its events and
then re-enable the PMU - i.e. no need for an enable and disable per
event. I'll put a patch together.

Thanks,
Ian