linux-kernel - Re: [PATCH] perf record: Fix "read LOST count failed" msg with sample read

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Date:   Wed, 1 Mar 2023 09:27:44 -0800
From:   Namhyung Kim <namhyung@...nel.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...nel.org, mingo@...hat.com, peterz@...radead.org,
        jolsa@...nel.org, irogers@...gle.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf record: Fix "read LOST count failed" msg with sample read

Hi Kan,

On Wed, Mar 1, 2023 at 7:04 AM <kan.liang@...ux.intel.com> wrote:
>
> From: Kan Liang <kan.liang@...ux.intel.com>
>
> Hundreds of "read LOST count failed" error messages may be displayed,
> when the below command is launched.
>
> perf record -e '{cpu/mem-loads-aux/,cpu/event=0xcd,umask=0x1/}:S' -a
>
> According to the commit 89e3106fa25f ("libperf: Handle read format in
> perf_evsel__read()"), the PERF_FORMAT_GROUP is only available for the
> leader. However, the record__read_lost_samples() goes through every
> entry of an evlist, which includes both leader and member. The member
> event errors out and triggers the error message. Since there may be
> hundreds of CPUs on a server, the message will be printed hundreds of
> times, which is very annoying.
>
> The message itself is correct, but the pr_err is a overkill. Other error
> messages in the record__read_lost_samples() are all pr_debug. To make
> the output format consistent, change the pr_err("read LOST count
> failed\n"); to pr_debug("read LOST count failed\n");.
> User can still get the message via -v option.
>
> Fixes: e3a23261ad06 ("perf record: Read and inject LOST_SAMPLES events")
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>

Acked-by: Namhyung Kim <namhyung@...nel.org>

Thanks,
Namhyung


> ---
>  tools/perf/builtin-record.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 8374117e66f6..be7c0c29d15b 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -1866,7 +1866,7 @@ static void __record__read_lost_samples(struct record *rec, struct evsel *evsel,
>         int id_hdr_size;
>
>         if (perf_evsel__read(&evsel->core, cpu_idx, thread_idx, &count) < 0) {
> -               pr_err("read LOST count failed\n");
> +               pr_debug("read LOST count failed\n");
>                 return;
>         }
>
> --
> 2.35.1
>