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

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jan 22, 2024 at 10:43:27AM +0000, James Clark wrote:
> 
> 
> On 16/01/2024 17:03, Mark Rutland wrote:
> > Currently the perf tool doesn't deteect support for extneded 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
> > targetting 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 beleive 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@arm.com>
> > Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Cc: Hector Martin <marcan@marcan.st>
> > Cc: Ian Rogers <irogers@google.com>
> > Cc: James Clark <james.clark@arm.com>
> > Cc: John Garry <john.g.garry@oracle.com>
> > Cc: Leo Yan <leo.yan@linaro.org>
> > Cc: Marc Zyngier <maz@kernel.org>
> > Cc: Mike Leach <mike.leach@linaro.org>
> > Cc: Namhyung Kim <namhyung@kernel.org>
> > Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
> > Cc: Thomas Richter <tmricht@linux.ibm.com>
> > Cc: Will Deacon <will@kernel.org>
> > ---
> >  tools/perf/util/print-events.c | 27 +++++++++++++++++++--------
> >  1 file changed, 19 insertions(+), 8 deletions(-)
> > 
> 
> Tested-by: James Clark <james.clark@arm.com>
> 
> Tested on Juno and N1SDP, although I wouldn't have expected it to make a
> difference on those platforms because they support exclude_guest=0.

It's good to be certain, anyhow!

I've folded that tag in for v2.

> Although I do see an interaction with the test "Session topology" if I
> hack the driver to behave like M1. The test has been failing (on
> big.LITTLE) since commit 251aa040244a ("perf parse-events: Wildcard most
> "numeric" events") but the result is that the test actually starts
> passing with this change. I don't think that should really block this
> though, it's likely going to require a separate fix which I will look into.

IIUC that fix is:

  https://lore.kernel.org/lkml/20240124094358.489372-1-james.clark@arm.com/

... right?

Mark.

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