linux-kernel - Re: [PATCH V3 0/5] bugs fix for large PEBS mmap read and rdpmc read

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Date:   Tue, 30 Jan 2018 11:56:33 -0800
From:   Stephane Eranian <eranian@...gle.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH V3 0/5] bugs fix for large PEBS mmap read and rdpmc read

On Tue, Jan 30, 2018 at 10:52 AM, Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Tue, Jan 30, 2018 at 11:48:18AM -0500, Liang, Kan wrote:
>
> SNIP
>
> > > >
> > > > The events in fixed mode could enable large PEBS. Events in freq mode should
> > > > not enable large PEBS.
> > > > I think that could be a problem if some events try to enable large PEBS,
> > > > while others not.
> > > >
> > > You only enable large PEBS if 100% of the events use fixed periods,
> > > either via -c period
> > > or because they all use individual period=p. The --no-period could
> > > also be used to remove
> > > the period for measurements where the period is not needed.
> >
> >
> > Oh, right, the kernel has already guaranteed that.
> >       if (cpuc->n_pebs == cpuc->n_large_pebs) {
> >               threshold = ds->pebs_absolute_maximum -
> >                       x86_pmu.max_pebs_events * x86_pmu.pebs_record_size;
> >       } else {
> >
> > Sorry for the noise.
> >
> > jirka's patch looks good to me.
>
> cool, I'll post it later this week
>
Still, the part I am missing here, is why asking for
PERF_SAMPLE_PERIOD voids large PEBS.