MIME-Version: 1.0
References: <20230214050452.26390-1-namhyung@kernel.org> <20230214050452.26390-5-namhyung@kernel.org>
 <CAP-5=fWV+VCheBocBpXg-jRHr+vkNnKWbH4Rjma9imQRJpis+w@mail.gmail.com>
In-Reply-To: <CAP-5=fWV+VCheBocBpXg-jRHr+vkNnKWbH4Rjma9imQRJpis+w@mail.gmail.com>
From:   Namhyung Kim <namhyung@kernel.org>
Date:   Tue, 14 Feb 2023 10:06:36 -0800
Message-ID: <CAM9d7ch9pLB7ZvNjisASruLqaQTw_DdZDjJQFSir7zntQHNnhw@mail.gmail.com>
Subject: Re: [PATCH 4/7] perf record: Record dropped sample count
To:     Ian Rogers <irogers@google.com>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Song Liu <song@kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Ravi Bangoria <ravi.bangoria@amd.com>,
        Leo Yan <leo.yan@linaro.org>,
        James Clark <james.clark@arm.com>, Hao Luo <haoluo@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-perf-users@vger.kernel.org, bpf@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Feb 14, 2023 at 8:41 AM Ian Rogers <irogers@google.com> wrote:
>
> On Mon, Feb 13, 2023 at 9:05 PM Namhyung Kim <namhyung@kernel.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