linux-kernel - Re: [PATCH v2] perf/core: Set event shadow time for inactive events too

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Date:   Sun, 5 Dec 2021 12:56:14 -0800
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Namhyung Kim <namhyung@...gle.com>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH v2] perf/core: Set event shadow time for inactive events too

Hi Peter,

On Sun, Dec 5, 2021 at 6:16 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Nov 30, 2021 at 08:58:07PM -0800, Namhyung Kim wrote:
> > From: Namhyung Kim <namhyung@...gle.com>
> >
> > While f79256532682 ("perf/core: fix userpage->time_enabled of inactive
> > events") fixed this problem for user rdpmc usage, bperf (perf stat
> > with BPF) still has the same problem that accessing inactive perf
> > events from BPF using bpf_perf_event_read_value().
> >
> > +static inline void group_update_event_time(struct perf_event *group_event)
> >  {
> > +     struct perf_event *event;
> > +     struct perf_event_context *ctx = group_event->ctx;
>
> :-( surely you're aware of the reverse xmas tree thing by now?

Will change the order.

>
> >
> > +     perf_event_update_time(group_event);
> > +     perf_set_shadow_time(group_event, ctx);
> >
> > +     for_each_sibling_event(event, group_event) {
> > +             perf_event_update_time(event);
> > +             perf_set_shadow_time(event, ctx);
> > +     }
> >
> > +     if (likely(!atomic_read(&group_event->mmap_count)))
> >               return;
> >
> > +     perf_event_update_userpage(group_event);
> > +
> >       for_each_sibling_event(event, group_event)
> > +             perf_event_update_userpage(event);
>
> How does it make sense to chase those pointers twice?

OK, then I think I can use the existing code to update
event time before checking the mmap count.

Thanks,
Namhyung

>
> >  }