Messages in this thread | | | Date | Fri, 30 Sep 2022 15:49:25 +0300 | Subject | Re: [PATCH 2/5] libperf: Propagate maps only if necessary | From | Adrian Hunter <> |
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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.
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