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

[Namhyung Kim]

On Tue, Oct 3, 2023 at 3:36 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> 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, I was thinking about that too.  It's also a side effect of
the context rewrite.  Maybe we could iterate pmu_ctx's active lists
and skip pmus with PERF_PMU_CAP_NO_INTERRUPT and
individual non-sampling events.

Thanks,
Namhyung