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

[Namhyung Kim]

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