linux-kernel - Re: [PATCH 2/5] libperf: Propagate maps only if necessary

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Date:   Wed, 28 Sep 2022 19:07:43 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Leo Yan <leo.yan@...aro.org>
Subject: Re: [PATCH 2/5] libperf: Propagate maps only if necessary

On Wed, Sep 28, 2022 at 4:46 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Wed, Sep 28, 2022 at 12:54 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
> >
> > On 27/09/22 20:28, Namhyung Kim wrote:
> > > Hi Adrian,
> > >
> > > On Tue, Sep 27, 2022 at 12:06 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
> > >>
> > >> On 24/09/22 19:57, Namhyung Kim wrote:
> > >>> The current code propagate evsel's cpu map settings to evlist when it's
> > >>> added to an evlist.  But the evlist->all_cpus and each evsel's cpus will
> > >>> be updated in perf_evlist__set_maps() later.  No need to do it before
> > >>> evlist's cpus are set actually.
> > >>>
> > >>> Actually we discarded this intermediate all_cpus maps at the beginning
> > >>> of perf_evlist__set_maps().  Let's not do this.  It's only needed when
> > >>> an evsel is added after the evlist cpu maps are set.
> > >>
> > >> That might not be true.  Consider evlist__fix_hybrid_cpus() which fiddles
> > >> with evsel->core.cpus and evsel->core.own_cpus after the evsel has been
> > >> added to the evlist.  It can also remove an evsel from the evlist.
> > >
> > > Thanks for your review.  I think it's fine to change evsel cpus or to remove
> > > an evsel from evlist before calling evlist__create_maps().  The function
> > > will take care of setting evlist's all_cpus from the evsels in the evlist.
> > > So previous changes in evsel/cpus wouldn't be any special.
> > >
> > > After this point, adding a new evsel needs to update evlist all cpus by
> > > propagating cpu maps.  So I think hybrid cpus should be fine.
> > > Did I miss something?
> >
> > I wondered how it might play out if evlist__fix_hybrid_cpus() reduced the
> > cpus from the target->cpu_list (using perf record -C) , since after this
> > patch all_cpus always starts with the target->cpu_list instead of an empty
> > list.  But then, in the hybrid case, it puts a dummy event that uses the
> > target cpu list anyway, so the result is the same.
> >
> > I don't know if there are any cases where all_cpus would actually need to
> > exclude some of the cpus from target->cpu_list.
>
> I'm not aware of other cases to reduce cpu list.  I think it'd be fine
> if it has a cpu in the evlist->all_cpus even if it's not used.  The evsel
> should have a correct list anyway and we mostly use the evsel cpus
> to do the real work.
>
> Thanks,
> Namhyung

The affinity changes made it so that we use all_cpus probably more
often than the evsel CPU maps for real work. The reason being we want
to avoid IPIs so we do all the work on 1 CPU and then move to the next
CPU in evlist all_cpus. evsel CPU maps are used to make sure the
indices are kept accurate - for example, if an uncore event is
measured with a CPU event:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/evlist.h?h=perf/core#n366
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/evlist.c?h=perf/core#n404

Thanks,
Ian