Re: [RFC PATCH 1/1] libperf: evlist: Fix --cpu argument on hybrid platform

[Ian Rogers]

On Tue, Oct 15, 2024 at 7:54 AM James Clark <james.clark@xxxxxxxxxx> wrote:
>
> Since the linked fixes: commit, specifying a CPU on hybrid platforms
> results in an error because Perf tries to open an extended type event
> on "any" CPU which isn't valid. Extended type events can only be opened
> on CPUs that match the type.
>
> Before (working):
>
>   $ perf record --cpu 1 -- true
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 2.385 MB perf.data (7 samples) ]
>
> After (not working):
>
>   $ perf record -C 1 -- true
>   WARNING: A requested CPU in '1' is not supported by PMU 'cpu_atom' (CPUs 16-27) for event 'cycles:P'
>   Error:
>   The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (cpu_atom/cycles:P/).
>   /bin/dmesg | grep -i perf may provide additional information.
>
> (Ignore the warning message, that's expected and not particularly
> relevant to this issue).
>
> This is because perf_cpu_map__intersect() of the user specified CPU (1)
> and one of the PMU's CPUs (16-27) correctly results in an empty (NULL)
> CPU map. However for the purposes of opening an event, libperf converts
> empty CPU maps into an any CPU (-1) which the kernel rejects.

Ugh. The cpumap API tries its best to confuse NULL == empty but empty
can give you dummy, dummy is also known as 'any' or -1, 'any' sounds a
lot like 'all' but sometimes 'all' is only online CPUs. I tried to
tidy up the naming a while ago, but there is still a mess.

> Fix it by deleting evsels with empty CPU maps in the specific case where
> user requested CPU maps are evaluated.

If we delete evsels than the indices can be broken for certain things.
I'm guessing asan testing is clean but the large number of side data
structures that are indexed by things in another data structure makes
the whole code base brittle and I am nervous around this change.

> Fixes: 251aa040244a ("perf parse-events: Wildcard most "numeric" events")
> Signed-off-by: James Clark <james.clark@xxxxxxxxxx>

Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks,
Ian

> ---
>  tools/lib/perf/evlist.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/perf/evlist.c b/tools/lib/perf/evlist.c
> index c6d67fc9e57e..8fae9a157a91 100644
> --- a/tools/lib/perf/evlist.c
> +++ b/tools/lib/perf/evlist.c
> @@ -47,6 +47,13 @@ static void __perf_evlist__propagate_maps(struct perf_evlist *evlist,
>                  */
>                 perf_cpu_map__put(evsel->cpus);
>                 evsel->cpus = perf_cpu_map__intersect(evlist->user_requested_cpus, evsel->own_cpus);
> +
> +               /*
> +                * Empty cpu lists would eventually get opened as "any" so remove
> +                * genuinely empty ones before they're opened in the wrong place.
> +                */
> +               if (perf_cpu_map__is_empty(evsel->cpus))
> +                       perf_evlist__remove(evlist, evsel);
>         } else if (!evsel->own_cpus || evlist->has_user_cpus ||
>                 (!evsel->requires_cpu && perf_cpu_map__has_any_cpu(evlist->user_requested_cpus))) {
>                 /*
> @@ -80,11 +87,11 @@ static void __perf_evlist__propagate_maps(struct perf_evlist *evlist,
>
>  static void perf_evlist__propagate_maps(struct perf_evlist *evlist)
>  {
> -       struct perf_evsel *evsel;
> +       struct perf_evsel *evsel, *n;
>
>         evlist->needs_map_propagation = true;
>
> -       perf_evlist__for_each_evsel(evlist, evsel)
> +       list_for_each_entry_safe(evsel, n, &evlist->entries, node)
>                 __perf_evlist__propagate_maps(evlist, evsel);
>  }
>
> --
> 2.34.1
>