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

[Ian Rogers]

On Tue, May 28, 2024 at 12:44 PM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> On Mon, May 27, 2024 at 10:36:45PM -0700, Ian Rogers wrote:
> > On Mon, May 27, 2024 at 3:58 AM Leo Yan <leo.yan@xxxxxxxxx> wrote:
> > > On Sat, May 25, 2024 at 02:14:26PM -0700, Linus Torvalds wrote:
> > > > On Sat, 25 May 2024 at 09:43, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> > > > > This makes 'perf record' work for me again.
>
> > > > Oh, wait, no it doesn't.
>
> > > > It makes just the plain "perf record" without any arguments work,
> > > > which was what I was testing because I was lazy.
>
> > > > So now
>
> > > >     $ perf record sleep 1
>
> > > > works fine. But
>
> > > >     $ perf record -e cycles:pp sleep 1
>
> > > > is still completely broken (with or without ":p" and ":pp").
>
> > > Seems to me that this patch fails to check if a PMU is a core-attached
> > > PMU that can support common hardware events. Therefore, we should
> > > consider adding the following check.
>
> > > +++ b/tools/perf/util/parse-events.c
> > > @@ -1594,6 +1594,9 @@ int parse_events_multi_pmu_add(struct parse_events_state *parse_state,
> > >         while ((pmu = perf_pmus__scan(pmu)) != NULL) {
> > >                 bool auto_merge_stats;
> > >
> > > +               if (hw_config != PERF_COUNT_HW_MAX && !pmu->is_core)
> > > +                       continue;
> > > +
> > >                 if (parse_events__filter_pmu(parse_state, pmu))
> > >                         continue;
>
> > > To be clear, I only compiled this change but I have no chance to test
> > > it. @Ian, could you confirm this?
>
> > Hi Leo,
>
> > so the code is working as intended. I believe it also agrees with what
> > Arnaldo thinks.
>
> > If you do:
>
> > $ perf stat -e cycles ...
>
> > and you have
>
> > /sys/devices/pmu1/events/cycles
> > /sys/devices/pmu2/events/cycles
>
> > The output of perf stat should contain counts for pmu1 and pmu2. Were
> > the event 'data_read' or 'inst_retired.any' we wouldn't be having the
>
> Sure, what is being asked is to count events and if those two events in
> those two PMUs can count, then do what the user asked.
>
> For 'perf record' we're asking for sampling, if the event has the name
> specified and can't be sampled, skip it, warn the user and even so
> only if verbose mode is asked, something like:
>
>   root@x1:~# perf record -e cycles -a sleep 1
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 1.998 MB perf.data (4472 samples) ]
>   root@x1:~# perf evlist
>   cpu_atom/cycles/
>   cpu_core/cycles/
>   dummy:u
>   root@x1:~#
>
> Cool, there are two 'cycles' events, one in a PMU named 'cpu_atom',
> another in a 'cpu_core' one, both can be sampled, my workload may
> run/use resources on then, I'm interested, sample both.
>
> But if we had some other PMU, to use a name Jiri uses in tests/fake
> PMUs, the 'krava' PMU and it has a 'cycles' event, so 'krava/cycles/'
> and for some reason it doesn't support sampling, skip it, then the
> result should be the same as above.
>
> If the user finds it strange after looking at sysfs that 'krava/cycles/'
> isn't being sampled, the usual workflow is to ask perf for more
> verbosity, using -v (or multiple 'v' letters to get increasing levels of
> verbosity), in which case the user would see:
>
>   root@x1:~# perf record -v -e cycles -a sleep 1
>   WARNING: skipping 'krava/cycles/' event, it doesn't support sampling.
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 1.998 MB perf.data (4472 samples) ]
>   root@x1:~# perf evlist

The problem here is that we're hiding a problem rather than reporting
it. Typically we report the issue and more than that we ask the user
to work around the issue. That would be analogous to wanting the user
to specify what PMU they want the event to apply to, which has always
been perf's behavior.

Thanks,
Ian

> - Arnaldo