linux-kernel - Re: [PATCH 4/7] perf record: Record dropped sample count

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Date:   Tue, 14 Feb 2023 10:06:36 -0800
From:   Namhyung Kim <namhyung@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Song Liu <song@...nel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ravi Bangoria <ravi.bangoria@....com>,
        Leo Yan <leo.yan@...aro.org>,
        James Clark <james.clark@....com>, Hao Luo <haoluo@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 4/7] perf record: Record dropped sample count

On Tue, Feb 14, 2023 at 8:41 AM Ian Rogers <irogers@...gle.com> wrote:
>
> On Mon, Feb 13, 2023 at 9:05 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > When it uses bpf filters, event might drop some samples.  It'd be nice
> > if it can report how many samples it lost.  As LOST_SAMPLES event can
> > carry the similar information, let's use it for bpf filters.
> >
> > To indicate it's from BPF filters, add a new misc flag for that and
> > do not display cpu load warnings.
>
> Can you potentially have lost samples from being too slow to drain the
> ring buffer and dropped samples because of BPF? Is it possible to
> distinguish lost and dropped with this approach?

Yeah, the former is exactly what LOST_SAMPLES event gives you.
It should come from the kernel while BPF filters keep a separate
counter for dropped samples and inject LOST_SAMPLES events
with the new misc flag.  So we can differentiate them using the misc
flag and that's how I suppress the warning for BPF dropped ones.

Thanks,
Namhyung