linux-kernel - Re: [PATCH V2 21/23] libperf evsel: Add comments for booleans

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Message-ID: <CAP-5=fW4k0A_iYPYq5pS59GmvUa8uUkzS2c2C5ruyKd40wjU8A@mail.gmail.com>
Date:   Wed, 11 May 2022 22:34:24 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexey Bayduraev <alexey.v.bayduraev@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Leo Yan <leo.yan@...aro.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 21/23] libperf evsel: Add comments for booleans

On Wed, May 11, 2022 at 12:03 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>
> On 6/05/22 23:51, Ian Rogers wrote:
> > On Fri, May 6, 2022 at 5:26 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
> >>
> >> Add comments for 'system_wide' and 'requires_cpu' booleans
> >>
> >> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> >> ---
> >>  tools/lib/perf/include/internal/evsel.h | 9 +++++++++
> >>  1 file changed, 9 insertions(+)
> >>
> >> diff --git a/tools/lib/perf/include/internal/evsel.h b/tools/lib/perf/include/internal/evsel.h
> >> index 77fbb8b97e5c..cc8f1ba790bd 100644
> >> --- a/tools/lib/perf/include/internal/evsel.h
> >> +++ b/tools/lib/perf/include/internal/evsel.h
> >> @@ -49,7 +49,16 @@ struct perf_evsel {
> >>
> >>         /* parse modifier helper */
> >>         int                      nr_members;
> >> +       /*
> >> +        * system_wide is for events that need to be on every CPU, irrespective
> >> +        * of user requested CPUs or threads. Map propagation will not override
> >> +        * this events own_cpus, which will contribute to evlist all_cpus.
> >
> > So this muddies my understanding of cpus and own_cpus (sigh, again
> > undocumented). I believe cpus to be the set of CPUs (or any CPU, aka
> > dummy) that perf_event_open happens on. all_cpus in evlist is the
> > union of all the evsels cpus (ie not related to own_cpus as described
> > here). Own_cpus I believe to be what's computed at parse-event time
> > from sysfs. Is this a typo in the comment or quite likely my error?
>
> A 'system_wide' event is not created by the parser, so own_cpus is set
> by the code adding the event.

I think I may be misreading the comment. Perhaps it should read:

system_wide is for events that need to be on every CPU, irrespective
of user requested CPUs or threads. Map propagation will not override
this event's own_cpus and own_cpus don't contribute to the evlist
all_cpus.

It would be nice if there were a comment on own_cpus to explain its
relationship to cpus and more broadly when it is used. Fwiw, the parse
time copy is made here:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/parse-events.c?h=perf/core#n367

I worry that if own_cpus should be used in place of cpus then
iterators like evlist__for_each_cpu may need to handle this, and so
they'd be broken currently.

Thanks,
Ian

> >
> > Thanks,
> > Ian
> >
> >> +        */
> >>         bool                     system_wide;
> >> +       /*
> >> +        * Some events, for example uncore events, require a CPU.
> >> +        * i.e. it cannot be the 'any CPU' value of -1.
> >> +        */
> >>         bool                     requires_cpu;
> >>         int                      idx;
> >>  };
> >> --
> >> 2.25.1
> >>
>