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

[Namhyung Kim <namhyung@kernel.org>]

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