Re: [PATCH v5 3/4] perf record: Skip don't fail for events that don't open

[Ian Rogers <irogers@google.com>]

On Thu, Jan 9, 2025 at 5:25 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> On Thu, Jan 09, 2025 at 02:21:08PM -0800, Ian Rogers wrote:
> > Whilst for many tools it is an expected behavior that failure to open
> > a perf event is a failure, ARM decided to name PMU events the same as
> > legacy events and then failed to rename such events on a server uncore
> > SLC PMU. As perf's default behavior when no PMU is specified is to
> > open the event on all PMUs that advertise/"have" the event, this
> > yielded failures when trying to make the priority of legacy and
> > sysfs/json events uniform - something requested by RISC-V and ARM. A
> > legacy event user on ARM hardware may find their event opened on an
> > uncore PMU which for perf record will fail. Arnaldo suggested skipping
> > such events which this patch implements. Rather than have the skipping
> > conditional on running on ARM, the skipping is done on all
> > architectures as such a fundamental behavioral difference could lead
> > to problems with tools built/depending on perf.
> >
> > An example of perf record failing to open events on x86 is:
> > ```
> > $ perf record -e data_read,cycles,LLC-prefetch-read -a sleep 0.1
> > Error:
> > Failure to open event 'data_read' on PMU 'uncore_imc_free_running_0' which will be removed.
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (data_read).
> > "dmesg | grep -i perf" may provide additional information.
> >
> > Error:
> > Failure to open event 'data_read' on PMU 'uncore_imc_free_running_1' which will be removed.
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (data_read).
> > "dmesg | grep -i perf" may provide additional information.
> >
> > Error:
> > Failure to open event 'LLC-prefetch-read' on PMU 'cpu' which will be removed.
> > The LLC-prefetch-read event is not supported.
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 2.188 MB perf.data (87 samples) ]
>
> I'm afraid this can be too noisy.

The intention is to be noisy:
1) it matches the existing behavior, anything else is potentially a regression;
2) it only happens if trying to record on a PMU/event that doesn't
support recording, something that is currently an error and so we're
not motivated to change the behavior as no-one should be using it;
3) for the wildcard case the only offender is ARM's SLC PMU and the
appropriate fix there has always been to make the CPU cycle's event
name match the bus_cycles event name by calling it cpu_cycles -
something that doesn't conflict with a core PMU event name, the thing
that has introduced all these problems, patches, long email exchanges,
unfixed inconsistencies, etc.. If the errors aren't noisy then there
is little motivation for the ARM SLC PMU's event name to be fixed.

Thanks,
Ian