linux-kernel - Re: [PATCH v4] perf/core: Add a new read format to get a number of lost samples

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Date:   Fri, 17 Jun 2022 14:53:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH v4] perf/core: Add a new read format to get a number of
 lost samples

On Thu, Jun 16, 2022 at 11:06:23AM -0700, Namhyung Kim wrote:
> Sometimes we want to know an accurate number of samples even if it's
> lost.  Currenlty PERF_RECORD_LOST is generated for a ring-buffer which
> might be shared with other events.  So it's hard to know per-event
> lost count.
> 
> Add event->lost_samples field and PERF_FORMAT_LOST to retrieve it from
> userspace.
> 
> Original-patch-by: Jiri Olsa <jolsa@...hat.com>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> ---

Looks OK I suppose. Bit sad to get an unconditional atomic on the lost
path, but then again, we shouldn't be hitting that in the first place.

Thanks!