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

[Namhyung Kim]

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.
 
Thanks,
Namhyung

> 
> > +               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
> >