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From:   Mingwei Zhang <mizhang@google.com>
Date:   Mon, 30 Oct 2023 12:04:25 -0700
Message-ID: <CAL715WLq+wMGC26ESviAwSFGCpCYbf8knRFo=6Lbwgk8qwB8xA@mail.gmail.com>
Subject: Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event
To:     gregkh@linuxfoundation.org
Cc:     Ian Rogers <irogers@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        Yang Jihong <yangjihong1@huawei.com>,
        Stephane Eranian <eranian@google.com>, stable@vger.linux.org
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On Fri, Sep 15, 2023 at 9:10=E2=80=AFPM Ian Rogers <irogers@google.com> wro=
te:
>
> Dummy events are created with an attribute where the period and freq
> are zero. evsel__config will then see the uninitialized values and
> initialize them in evsel__default_freq_period. As fequency mode is
> used by default the dummy event would be set to use frequency
> mode. However, this has no effect on the dummy event but does cause
> unnecessary timers/interrupts. Avoid this overhead by setting the
> period to 1 for dummy events.
>
> evlist__add_aux_dummy calls evlist__add_dummy then sets freq=3D0 and
> period=3D1. This isn't necessary after this change and so the setting is
> removed.
>
> From Stephane:
>
> The dummy event is not counting anything. It is used to collect mmap
> records and avoid a race condition during the synthesize mmap phase of
> perf record. As such, it should not cause any overhead during active
> profiling. Yet, it did. Because of a bug the dummy event was
> programmed as a sampling event in frequency mode. Events in that mode
> incur more kernel overheads because on timer tick, the kernel has to
> look at the number of samples for each event and potentially adjust
> the sampling period to achieve the desired frequency. The dummy event
> was therefore adding a frequency event to task and ctx contexts we may
> otherwise not have any, e.g., perf record -a -e
> cpu/event=3D0x3c,period=3D10000000/. On each timer tick the
> perf_adjust_freq_unthr_context() is invoked and if ctx->nr_freq is
> non-zero, then the kernel will loop over ALL the events of the context
> looking for frequency mode ones. In doing, so it locks the context,
> and enable/disable the PMU of each hw event. If all the events of the
> context are in period mode, the kernel will have to traverse the list for
> nothing incurring overhead. The overhead is multiplied by a very large
> factor when this happens in a guest kernel. There is no need for the
> dummy event to be in frequency mode, it does not count anything and
> therefore should not cause extra overhead for no reason.
>
> Fixes: 5bae0250237f ("perf evlist: Introduce perf_evlist__new_dummy const=
ructor")
> Reported-by: Stephane Eranian <eranian@google.com>
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/perf/util/evlist.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 25c3ebe2c2f5..e36da58522ef 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -251,6 +251,9 @@ static struct evsel *evlist__dummy_event(struct evlis=
t *evlist)
>                 .type   =3D PERF_TYPE_SOFTWARE,
>                 .config =3D PERF_COUNT_SW_DUMMY,
>                 .size   =3D sizeof(attr), /* to capture ABI version */
> +               /* Avoid frequency mode for dummy events to avoid associa=
ted timers. */
> +               .freq =3D 0,
> +               .sample_period =3D 1,
>         };
>
>         return evsel__new_idx(&attr, evlist->core.nr_entries);
> @@ -277,8 +280,6 @@ struct evsel *evlist__add_aux_dummy(struct evlist *ev=
list, bool system_wide)
>         evsel->core.attr.exclude_kernel =3D 1;
>         evsel->core.attr.exclude_guest =3D 1;
>         evsel->core.attr.exclude_hv =3D 1;
> -       evsel->core.attr.freq =3D 0;
> -       evsel->core.attr.sample_period =3D 1;
>         evsel->core.system_wide =3D system_wide;
>         evsel->no_aux_samples =3D true;
>         evsel->name =3D strdup("dummy:u");
> --
> 2.42.0.459.ge4e396fd5e-goog
>

Hi Greg,

This patch is a critical performance fix for perf and vPMU. Can you
help us dispatch the commit to all stable kernel versions?

Appreciate your help. Thanks.
-Mingwei