Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

[Ian Rogers]

On Tue, Oct 3, 2023 at 1:08 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Wed, Sep 20, 2023 at 10:05 PM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
> >
> > On Mon, Sep 18, 2023 at 3:43 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> > >
> > > On Sat, Sep 16, 2023 at 5:46 PM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
> > > > Thank you very much for the change. I have one quick question about
> > > > the PMU unthrottling logic. When I am looking into the function
> > > > perf_adjust_freq_unthr_context(), I see the loop with PMU stop and
> > > > start in each iteration. Is there a good way to avoid this PMU reset
> > > > operation while quickly figuring out the event in frequency mode?
> > >
> > > Agreed. I think before the pmu_disable could be avoided for this condition:
> > > ```
> > > if (event->hw.interrupts != MAX_INTERRUPTS &&
> > >     (!event->attr.freq || !event->attr.sample_freq))
> > >         continue;
> > > ```
> > > Fixing up the event stop/start looks harder.
> > >
> >
> > Right, I think putting the check early before pmu_disable() is already
> > a great optimization. The only concern I initially had was whether
> > event->hw.interrupts can be accessed before we disable the pmu. But
> > after checking this field in other locations, I don't see any problem
> > at all.
>
> The event->hw.interrupts would be increased in the NMI handler
> so there is a race between the check and the NMI.  That's why
> I think it checks that after disabling the PMU.
>
> But I think we can skip non-sampling events for sure.  Then it
> would be better to set attr.sample_period = 0 rather than attr.freq.
>
>     if (!is_sampling_event(event))
>         continue;
>
>     perf_pmu_disable(event->pmu);
>     ...
>
> Thanks,
> Namhyung

With the PMU disabled, isn't there still a risk of an interrupt still
being in flight? In other words the disable doesn't prevent a race and
we'll catch this on the next timer call to
perf_adjust_freq_unthr_context. I think we can also improve the code
by just disabling a PMU once, we can take advantage of the
perf_event_pmu_context and disable that PMU, iterate its events and
then re-enable the PMU - i.e. no need for an enable and disable per
event. I'll put a patch together.

Thanks,
Ian