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

[Ian Rogers]

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.

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
>