Re: [PATCH v2] perf print-events: make is_event_supported() more robust

[Ian Rogers]

On Fri, Jan 26, 2024 at 6:56 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> Currently the perf tool doesn't detect support for extended event types
> on Apple M1/M2 systems, and will not auto-expand plain PERF_EVENT_TYPE
> hardware events into per-PMU events. This is due to the detection of
> extended event types not handling mandatory filters required by the
> M1/M2 PMU driver.
>
> PMU drivers and the core perf_events code can require that
> perf_event_attr::exclude_* filters are configured in a specific way and
> may reject certain configurations of filters, for example:
>
> (a) Many PMUs lack support for any event filtering, and require all
>     perf_event_attr::exclude_* bits to be clear. This includes Alpha's
>     CPU PMU, and ARM CPU PMUs prior to the introduction of PMUv2 in
>     ARMv7,
>
> (b) When /proc/sys/kernel/perf_event_paranoid >= 2, the perf core
>     requires that perf_event_attr::exclude_kernel is set.
>
> (c) The Apple M1/M2 PMU requires that perf_event_attr::exclude_guest is
>     set as the hardware PMU does not count while a guest is running (but
>     might be extended in future to do so).
>
> In is_event_supported(), we try to account for cases (a) and (b), first
> attempting to open an event without any filters, and if this fails,
> retrying with perf_event_attr::exclude_kernel set. We do not account for
> case (c), or any other filters that drivers could theoretically require
> to be set.
>
> Thus is_event_supported() will fail to detect support for any events
> targeting an Apple M1/M2 PMU, even where events would be supported with
> perf_event_attr:::exclude_guest set.
>
> Since commit:
>
>   82fe2e45cdb00de4 ("perf pmus: Check if we can encode the PMU number in perf_event_attr.type")
>
> ... we use is_event_supported() to detect support for extended types,
> with the PMU ID encoded into the perf_event_attr::type. As above, on an
> Apple M1/M2 system this will always fail to detect that the event is
> supported, and consequently we fail to detect support for extended types
> even when these are supported, as they have been since commit:
>
>   5c816728651ae425 ("arm_pmu: Add PERF_PMU_CAP_EXTENDED_HW_TYPE capability")
>
> Due to this, the perf tool will not automatically expand plain
> PERF_TYPE_HARDWARE events into per-PMU events, even when all the
> necessary kernel support is present.
>
> This patch updates is_event_supported() to additionally try opening
> events with perf_event_attr::exclude_guest set, allowing support for
> events to be detected on Apple M1/M2 systems. I believe that this is
> sufficient for all contemporary CPU PMU drivers, though in future it may
> be necessary to check for other combinations of filter bits.
>
> I've deliberately changed the check to not expect a specific error code
> for missing filters, as today ;the kernel may return a number of
> different error codes for missing filters (e.g. -EACCESS, -EINVAL, or
> -EOPNOTSUPP) depending on why and where the filter configuration is
> rejected, and retrying for any error is more robust.
>
> Note that this does not remove the need for commit:
>
>   a24d9d9dc096fc0d ("perf parse-events: Make legacy events lower priority than sysfs/JSON")
>
> ... which is still necessary so that named-pmu/event/ events work on
> kernels without extended type support, even if the event name happens to
> be the same as a PERF_EVENT_TYPE_HARDWARE event (e.g. as is the case for
> the M1/M2 PMU's 'cycles' and 'instructions' events).
>
> Fixes: 82fe2e45cdb00de4 ("perf pmus: Check if we can encode the PMU number in perf_event_attr.type")
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Tested-by: Ian Rogers <irogers@xxxxxxxxxx>
> Tested-by: James Clark <james.clark@xxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Hector Martin <marcan@xxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: James Clark <james.clark@xxxxxxx>
> Cc: John Garry <john.g.garry@xxxxxxxxxx>
> Cc: Leo Yan <leo.yan@xxxxxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Mike Leach <mike.leach@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Cc: Thomas Richter <tmricht@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>

Ping. Could we land this one?

Thanks,
Ian

> ---
>  tools/perf/util/print-events.c | 27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)
>
> Since v1 [1]:
> * Fix typos in commit message
> * Accumulate tags
>
> [1] https://lore.kernel.org/lkml/20240116170348.463479-1-mark.rutland@armcom/
>
> Mark.
>
> diff --git a/tools/perf/util/print-events.c b/tools/perf/util/print-events.c
> index b0fc48be623f3..4f67e8f00a4d6 100644
> --- a/tools/perf/util/print-events.c
> +++ b/tools/perf/util/print-events.c
> @@ -232,7 +232,6 @@ void print_sdt_events(const struct print_callbacks *print_cb, void *print_state)
>  bool is_event_supported(u8 type, u64 config)
>  {
>         bool ret = true;
> -       int open_return;
>         struct evsel *evsel;
>         struct perf_event_attr attr = {
>                 .type = type,
> @@ -246,20 +245,32 @@ bool is_event_supported(u8 type, u64 config)
>
>         evsel = evsel__new(&attr);
>         if (evsel) {
> -               open_return = evsel__open(evsel, NULL, tmap);
> -               ret = open_return >= 0;
> +               ret = evsel__open(evsel, NULL, tmap) >= 0;
>
> -               if (open_return == -EACCES) {
> +               if (!ret) {
>                         /*
> -                        * This happens if the paranoid value
> +                        * The event may fail to open if the paranoid value
>                          * /proc/sys/kernel/perf_event_paranoid is set to 2
> -                        * Re-run with exclude_kernel set; we don't do that
> -                        * by default as some ARM machines do not support it.
> -                        *
> +                        * Re-run with exclude_kernel set; we don't do that by
> +                        * default as some ARM machines do not support it.
>                          */
>                         evsel->core.attr.exclude_kernel = 1;
>                         ret = evsel__open(evsel, NULL, tmap) >= 0;
>                 }
> +
> +               if (!ret) {
> +                       /*
> +                        * The event may fail to open if the PMU requires
> +                        * exclude_guest to be set (e.g. as the Apple M1 PMU
> +                        * requires).
> +                        * Re-run with exclude_guest set; we don't do that by
> +                        * default as it's equally legitimate for another PMU
> +                        * driver to require that exclude_guest is clear.
> +                        */
> +                       evsel->core.attr.exclude_guest = 1;
> +                       ret = evsel__open(evsel, NULL, tmap) >= 0;
> +               }
> +
>                 evsel__delete(evsel);
>         }
>
> --
> 2.30.2
>