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

On Mon, Jan 22, 2024 at 10:43:27AM +0000, James Clark wrote:

On 16/01/2024 17:03, Mark Rutland wrote:

quoted

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 <redacted>
Cc: Hector Martin <redacted>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <redacted>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Leo Yan <redacted>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mike Leach <redacted>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Thomas Richter <redacted>
Cc: Will Deacon <will@kernel.org>
---
 tools/perf/util/print-events.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

Tested-by: James Clark <redacted>

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/ (local)

... right?

Mark.

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