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

[Ian Rogers]

On Tue, Sep 3, 2024 at 11:41 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.

We also have pmu's max_precise:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/pmu.h?h=perf-tools-next#n91
The reducing the precision approach was iirc taken for AMD who will
forward some precise events to IBS, but the max_precise on the cpu PMU
is 0. I think because of this, reducing the precision below
evsel->pmu->max_precise shouldn't be necessary and another fallback
may help better.

Thanks,
Ian

> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/util/evsel.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 8c4d70f7b2f5b880..0133c9ad3ce07a24 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -2565,9 +2565,6 @@ 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. */
> @@ -2584,11 +2581,12 @@ 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;
> +
>  out_close:
>         if (err)
>                 threads->err_thread = thread;
> --
> 2.46.0.469.g59c65b2a67-goog
>