Re: [PATCH v1] perf evlist: Force adding default events only to core PMUs

[Namhyung Kim]

On Thu, May 30, 2024 at 06:46:08AM -0700, Ian Rogers wrote:
> On Thu, May 30, 2024 at 5:48 AM James Clark <james.clark@xxxxxxx> wrote:
> >
> > On 30/05/2024 06:35, Namhyung Kim wrote:
> > > On Wed, May 29, 2024 at 12:25 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> > >> We can fix the arm_dsu bug by renaming cycles there. If that's too
> > >> hard to land, clearing up ambiguity by adding a PMU name has always
> > >> been the way to do this. My preference for v6.10 is revert the revert,
> > >> then add either a rename of the arm_dsu event and/or the change here.
> > >>
> > >> We can make perf record tolerant and ignore opening events on PMUs
> > >> that don't support sampling, but I think it is too big a thing to do
> > >> for v6.10.
> > >
> > > How about adding a flag to parse_event_option_args so that we
> > > can check if it's for sampling events.  And then we might skip
> > > uncore PMUs easily (assuming arm_dsu PMU is uncore).
> >
> > It's uncore yes.
> >
> > Couldn't we theoretically have a core PMU that still doesn't support
> > sampling though? And then we'd end up in the same situation. Attempting
> > to open the event is the only sure way of knowing, rather than trying to
> > guess with some heuristic in userspace?
> >
> > Maybe a bit too hypothetical but still worth considering.

Then I think it's a real problem and perf should report it like we do
now.

> >
> > >
> > > It might not be a perfect solution but it could be a simple one.
> > > Ideally I think it'd be nice if the kernel exports more information
> > > about the PMUs like sampling and exclude capabilities.
> > > > Thanks,
> > > Namhyung
> >
> > That seems like a much better suggestion. Especially with the ever
> > expanding retry/fallback mechanism that can never really take into
> > account every combination of event attributes that can fail.
> 
> I think this approach can work but we may break PMUs.
> 
> Rather than use `is_core` on `struct pmu` we could have say a
> `supports_sampling` and we pass to parse_events an option to exclude
> any PMU that doesn't have that flag. Now obviously more than just core
> PMUs support sampling. All software PMUs, tracepoints, probes. We have
> an imprecise list of these in perf_pmu__is_software. So we can set
> supports_sampling for perf_pmu__is_software and is_core.

Yep, we can do that if the kernel provides the info.  But before that
I think it's practical to skip uncore PMUs and hope other PMUs don't
have event aliases clashing with the legacy names. :)

> 
> I think the problem comes for things like the AMD IBS PMUs, intel_bts
> and intel_pt. Often these only support sampling but aren't core. There
> may be IBM S390 PMUs or other vendor PMUs that are similar. If we can
> make a list of all these PMU names then we can use that to set
> supports_sampling and not break event parsing for these PMUs.
> 
> The name list sounds somewhat impractical, let's say we lazily compute
> the supports_sampling on a PMU. We need the sampling equivalent of
> is_event_supported:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/print-events.c?h=perf-tools-next#n242
> is_event_supported has had bugs, look at the exclude_guest workaround
> for Apple PMUs. It also isn't clear to me how we choose the event
> config that we're going to probe to determine whether sampling works.
> The perf_event_open may reject the test because of a bad config and
> not because sampling isn't supported.
> 
> So I think we can make the approach work if we had either:
> 1) a list of PMUs that support sampling,
> 2) a reliable "is_sampling_supported" test.
> 
> I'm not sure of the advantages of doing (2) rather than just creating
> the set of evsels and ignoring those that fail to open. Ignoring
> evsels that fail to open seems more unlikely to break anything as the
> user is giving the events/config values for the PMUs they care about.

Yep, that's also possible.  I'm ok if you want to go that direction.

Thanks,
Namhyung