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

[Namhyung Kim]

Hi Ian,

On Tue, Oct 3, 2023 at 4:20 PM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
>
> 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,
> > Ian
>
> +Jim Mattson
>
> I initially thought this idea was just an alternative, or a more
> professional fix in the perf subsystem. I was wrong...
>
> This would be way better than just skipping frequency events in the
> loop. Since if we just skip by event, we may still suffer from huge
> overhead if the event list contains many sampling events in frequency
> mode. Unfortunately, that is the general case when we do perf record
> -e 'eventlist' (IIUC all events in eventlist are in frequency mode if
> we don't specify period=). So the problem actually remains whenever we
> do perf sampling unless we use something like Intel vtune.
>
> On the other hand, since all of the events are presumably CPU core
> events, with the fix we pay only once for the PMU reset per hrtimer
> regardless of how many events are in frequency mode.
>
> Looking forward to the patch! Please keep us posted if possible.

Any updates?

Thanks,
Namhyung