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

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Date:   Wed, 11 Oct 2023 09:14:42 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
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 4:02 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Tue, Oct 3, 2023 at 3:36 PM Ian Rogers <irogers@...gle.com> wrote:
> > 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, I was thinking about that too.  It's also a side effect of
> the context rewrite.  Maybe we could iterate pmu_ctx's active lists
> and skip pmus with PERF_PMU_CAP_NO_INTERRUPT and
> individual non-sampling events.

Or we can add pmu_ctx->nr_freq and check it before accessing
pmu MSRs.

Thanks,
Namhyung