linux-kernel - Re: [PATCH] perf record: Make logs more readable for event open failures

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Message-ID: <CAP-5=fUQxuVK+6GObmqVdBmTLzu06FyNaLdLsW6tcgNv5=7gxw@mail.gmail.com>
Date: Wed, 4 Feb 2026 09:29:14 -0800
From: Ian Rogers <irogers@...gle.com>
To: Leo Yan <leo.yan@....com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, James Clark <james.clark@...aro.org>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf record: Make logs more readable for event open failures

On Wed, Feb 4, 2026 at 6:26 AM Leo Yan <leo.yan@....com> wrote:
>
> Since commit ee27476fa300 ("perf record: Skip don't fail for events that
> don't open"), if a user does not have permission to access a PMU event,
> perf reports:
>
>   perf record -e cs_etm// -C 3 -- ls
>   Error:
>   Failure to open event 'cs_etm//u' on PMU 'cs_etm' which will be removed.
>   No fallback found for 'cs_etm//u' for error 13
>   Error:
>   Failure to open event 'dummy:u' on PMU 'software' which will be removed.
>   No fallback found for 'dummy:u' for error 13
>   Error:
>   Failure to open any events for recording.
>
> The log is not very helpful, as no clear indication of what "error 13"
> means or how to address the issue.
>
> This commit restores evsel__open_strerror() to generate a readable error
> message and print it out:
>
>   perf record -e cs_etm// -C 3 -- ls
>   Error:
>   Failure to open event 'cs_etm//' on PMU 'cs_etm' which will be removed.
>   Access to performance monitoring and observability operations is limited.
>   Consider adjusting /proc/sys/kernel/perf_event_paranoid setting to open
>   access to performance monitoring and observability operations for processes
>   without CAP_PERFMON, CAP_SYS_PTRACE or CAP_SYS_ADMIN Linux capability.
>   More information can be found at 'Perf events and tool security' document:
>   https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html
>   perf_event_paranoid setting is 1:
>     -1: Allow use of (almost) all events by all users
>         Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK
>   >= 0: Disallow raw and ftrace function tracepoint access
>   >= 1: Disallow CPU event access
>   >= 2: Disallow kernel profiling
>   To make the adjusted perf_event_paranoid setting permanent preserve it
>   in /etc/sysctl.conf (e.g. kernel.perf_event_paranoid = <setting>)
>   Error:
>   Failure to open event 'dummy:u' on PMU 'software' which will be removed.
>   Access to performance monitoring and observability operations is limited.
>   Consider adjusting /proc/sys/kernel/perf_event_paranoid setting to open
>   access to performance monitoring and observability operations for processes
>   without CAP_PERFMON, CAP_SYS_PTRACE or CAP_SYS_ADMIN Linux capability.
>   More information can be found at 'Perf events and tool security' document:
>   https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html
>   perf_event_paranoid setting is 1:
>     -1: Allow use of (almost) all events by all users
>         Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK
>   >= 0: Disallow raw and ftrace function tracepoint access
>   >= 1: Disallow CPU event access
>   >= 2: Disallow kernel profiling
>   To make the adjusted perf_event_paranoid setting permanent preserve it
>   in /etc/sysctl.conf (e.g. kernel.perf_event_paranoid = <setting>)
>   Error:
>   Failure to open any events for recording.
>
> Signed-off-by: Leo Yan <leo.yan@....com>

Lgtm and sorry for making things worse - I believe it was motivated by
trying to avoid spammy warnings.

Reviewed-by: Ian Rogers <irogers@...gle.com>

Thanks,
Ian

> ---
>  tools/perf/builtin-record.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 2584d0d8bc820676da60bdf5f59d41371d85a93a..aa8dc3e18190dd7db1ed5e3c7673fde8d5785a26 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -1404,6 +1404,7 @@ static int record__open(struct record *rec)
>                         }
>  #endif
>                         if (report_error || verbose > 0) {
> +                               evsel__open_strerror(pos, &opts->target, errno, msg, sizeof(msg));
>                                 ui__error("Failure to open event '%s' on PMU '%s' which will be "
>                                           "removed.\n%s\n",
>                                           evsel__name(pos), evsel__pmu_name(pos), msg);
>
> ---
> base-commit: 5fd0a1df5d05ad066e5618ccdd3d0fa6cb686c27
> change-id: 20260204-perf_improve_log_for_open_event_failures-deda2c61b93d
>
> Best regards,
> --
> Leo Yan <leo.yan@....com>
>