Re: [PATCH 7/8] perf tools: Check fallback error and order

[Ian Rogers]

On Tue, Oct 1, 2024 at 2:36 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> On Tue, Oct 01, 2024 at 11:00:20AM -0700, Ian Rogers wrote:
> > On Mon, Sep 30, 2024 at 5:20 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > >
> > > The perf_event_open might fail due to various reasons, so blindly
> > > reducing precise_ip level might not be the best way to deal with it.
> > >
> > > It seems the kernel return -EOPNOTSUPP when PMU doesn't support the
> > > given precise level.  Let's try again with the correct error code.
> > >
> > > This caused a problem on AMD, as it stops on precise_ip of 2 for IBS but
> > > user events with exclude_kernel=1 cannot make progress.  Let's add the
> > > evsel__handle_error_quirks() to this case specially.  I plan to work on
> > > the kernel side to improve this situation but it'd still need some
> > > special handling for IBS.
> > >
> > > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > > ---
> > >  tools/perf/util/evsel.c | 27 +++++++++++++++++++++------
> > >  1 file changed, 21 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > > index 32e30c293d0c6198..ef8356260eea54cd 100644
> > > --- a/tools/perf/util/evsel.c
> > > +++ b/tools/perf/util/evsel.c
> > > @@ -2419,6 +2419,20 @@ static bool evsel__detect_missing_features(struct evsel *evsel)
> > >         return false;
> > >  }
> > >
> > > +static bool evsel__handle_error_quirks(struct evsel *evsel, int error)
> > > +{
> > > +       /* AMD IBS doesn't support exclude_kernel, forward it to core PMU */
> >
> > Should the quirk handling be specific to evsels with the IBS PMU given
> > the comment above? ie this is a PMU specific workaround rather than an
> > AMD specific workaround, however, the PMU only exists on AMD :-)
> >
> > > +       if (error == -EINVAL && evsel->precise_max && evsel->core.attr.precise_ip &&
> > > +           evsel->core.attr.exclude_kernel && x86__is_amd_cpu()) {
> >
> > So here rather than x86__is_amd_cpu it would be
> > !strcmp(evsel->pmu->name, "ibs_...") . But it may be cleaner to move
> > the logic into pmu.c.
>
> I guess the problem is that AMD driver does implicit forwarding to IBS
> if the legacy hardware events have precise_ip.  So it might have just
> core pmu (or no pmu) in the evsel.

I think the no PMU case should probably have a PMU possibly similar to
the tool PMU in:
https://lore.kernel.org/all/20240912190341.919229-1-irogers@xxxxxxxxxx/
But that's not in place yet. You can always use
perf_pmus__scan_core(NULL) or
perf_pmus__find_by_type(evsel->core.attr.type or PERF_TYPE_RAW).
evsel->pmu->is_core && x86__is_amd_cpu() would identify an AMD core
PMU whereas the code above could fire for IBS or other PMUs.

Thanks,
Ian

> >
> > > +               evsel->core.attr.precise_ip = 0;
> > > +               pr_debug2_peo("removing precise_ip on AMD\n");
> > > +               display_attr(&evsel->core.attr);
> > > +               return true;
> > > +       }
> > > +
> > > +       return false;
> > > +}
> > > +
> > >  static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
> > >                 struct perf_thread_map *threads,
> > >                 int start_cpu_map_idx, int end_cpu_map_idx)
> > > @@ -2580,9 +2594,6 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
> > >         return 0;
> > >
> > >  try_fallback:
> > > -       if (evsel__precise_ip_fallback(evsel))
> > > -               goto retry_open;
> > > -
> > >         if (evsel__ignore_missing_thread(evsel, perf_cpu_map__nr(cpus),
> > >                                          idx, threads, thread, err)) {
> > >                 /* We just removed 1 thread, so lower the upper nthreads limit. */
> > > @@ -2599,11 +2610,15 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
> > >         if (err == -EMFILE && rlimit__increase_nofile(&set_rlimit))
> > >                 goto retry_open;
> > >
> > > -       if (err != -EINVAL || idx > 0 || thread > 0)
> > > -               goto out_close;
> > > +       if (err == -EOPNOTSUPP && evsel__precise_ip_fallback(evsel))
> > > +               goto retry_open;
> > >
> > > -       if (evsel__detect_missing_features(evsel))
> > > +       if (err == -EINVAL && evsel__detect_missing_features(evsel))
> > >                 goto fallback_missing_features;
> > > +
> > > +       if (evsel__handle_error_quirks(evsel, err))
> > > +               goto retry_open;
> > > +
> > >  out_close:
> > >         if (err)
> > >                 threads->err_thread = thread;
> > > --
> > > 2.46.1.824.gd892dcdcdd-goog
> > >