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

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Message-ID: <CAP-5=fUObi2ZoY60VF-WMD0=4=EB4e5s7SUAQyf3zoNA1my6Og@mail.gmail.com>
Date: Thu, 5 Feb 2026 14:33:04 -0800
From: Ian Rogers <irogers@...gle.com>
To: Arnaldo Carvalho de Melo <acme@...nel.org>
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 Thu, Feb 5, 2026 at 1:42 PM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
>
> 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?

IIRC it was left in as "perf record -e dummy" can be a way of just
tracking sideband data and you'd want errors opening dummy to be
warned upon in that case.

Thanks,
Ian

> - 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>
> > >