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

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Sep 30, 2022 at 5:50 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 29/09/22 23:42, Namhyung Kim wrote:
> > On Wed, Sep 28, 2022 at 10:19 PM Adrian Hunter <adrian.hunter@intel.com> wrote:
> >>
> >> On 29/09/22 08:09, Namhyung Kim wrote:
> >>> On Wed, Sep 28, 2022 at 7:08 PM Ian Rogers <irogers@google.com> wrote:
> >>>>
> >>>> On Wed, Sep 28, 2022 at 4:46 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >>>>>
> >>>>> On Wed, Sep 28, 2022 at 12:54 AM Adrian Hunter <adrian.hunter@intel.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@intel.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
> >>>
> >>> Right, I meant it'd check the evsel cpus eventually even if it iterates
> >>> on the evlist all_cpus.  The evlist_cpu_iterator__next() will skip a
> >>> CPU if it's not in the evsel cpus.
> >>>
> >>> Thanks,
> >>> Namhyung
> >>
> >> Perhaps an alternative is to be explicit about deferring map
> >> propagation e.g.
> >
> > Thanks for your patch.  Yeah, we can use this.
> >
> > But I still think it'd be better doing it unconditionally
> > since any propagation before perf_evlist__set_maps
> > will be discarded anyway.  With this change, other
> > than perf record will collect all cpus before _set_maps
> > and then discard it.  It seems like a waste, no?
> >
> > Or else, we can have allow_map_propagation initialized
> > to false and set it to true in perf_evlist__set_maps().
> >
>
> That sounds fine.

Thanks!

Arnaldo, how do you want to handle it?  I can send v2 for the
whole series, but I think you already applied it.  Then do you
want me to send this change on top?

Namhyung