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

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Message-ID: <aYUOowQ_CZf3BDKo@x1>
Date: Thu, 5 Feb 2026 18:41:55 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Leo Yan <leo.yan@....com>, 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 04, 2026 at 09:29:14AM -0800, Ian Rogers wrote:
> 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:

I'm applying this one, maybe we can at least suppress the one for dummy
when it is auto added?

- Arnaldo

> >   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>
> >