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

[Ian Rogers]

On Wed, Jun 5, 2024 at 1:29 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> On Thu, May 30, 2024 at 3:52 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > 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:
> > > > > 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.
>
> Hmm.. I thought about this again.  But it can be a problem if we ignore
> any failures as it can be a real error due to other reason - e.g. not
> supported configuration or other user mistakes.

Right, we have two not good choices:

1) Try to detect whether sampling is supported, but any test doing
this needs to guess at a configuration and we'll need to deflake this
on off platforms like those that don't allow things like exclude
guest.
2) Ignore failures, possibly hiding user errors.

I would prefer for (2) the errors were pr_err rather than pr_debug,
something the user can clean up by getting rid of warned about PMUs.
This will avoid hiding the error, but then on Neoverse cycles will
warn about the arm_dsu PMU's cycles event for exactly Linus' test
case. My understanding is that this is deemed a regression, hence
Arnaldo proposing pr_debug to hide it.

Thanks,
Ian