Re: [PATCH 3/3] perf/x86: Add CAP_NO_INTERRUPT for uncore PMUs

[Namhyung Kim]

Hi Kan,

On Tue, Nov 21, 2023 at 7:59 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2023-11-20 5:19 p.m., Namhyung Kim wrote:
> > It doesn't support sampling in uncore PMU events.  While it's
> > technically possible to generate interrupts, let's treat it as if it
> > has no interrupt in order to skip the freq adjust/unthrottling logic
> > in the timer handler which is only meaningful to sampling events.
> >
> > Also remove the sampling event check because it'd be done in the general
> > code in the perf_event_open syscall.
> >
> > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > ---
> >  arch/x86/events/intel/uncore.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/events/intel/uncore.c b/arch/x86/events/intel/uncore.c
> > index 69043e02e8a7..f7e6228bd1b1 100644
> > --- a/arch/x86/events/intel/uncore.c
> > +++ b/arch/x86/events/intel/uncore.c
> > @@ -744,10 +744,6 @@ static int uncore_pmu_event_init(struct perf_event *event)
> >       if (pmu->func_id < 0)
> >               return -ENOENT;
> >
> > -     /* Sampling not supported yet */
> > -     if (hwc->sample_period)
> > -             return -EINVAL;
> > -
> >       /*
> >        * Place all uncore events for a particular physical package
> >        * onto a single cpu
> > @@ -919,7 +915,12 @@ static int uncore_pmu_register(struct intel_uncore_pmu *pmu)
> >                       .stop           = uncore_pmu_event_stop,
> >                       .read           = uncore_pmu_event_read,
> >                       .module         = THIS_MODULE,
> > -                     .capabilities   = PERF_PMU_CAP_NO_EXCLUDE,
> > +                     /*
> > +                      * It doesn't allow sampling for uncore events, let's
> > +                      * treat the PMU has no interrupts to skip them in the
> > +                      * perf_adjust_freq_unthr_context().
> > +                      */
> > +                     .capabilities   = PERF_PMU_CAP_NO_EXCLUDE | PERF_PMU_CAP_NO_INTERRUPT,
> >                       .attr_update    = pmu->type->attr_update,
> >               };
>
>
> There is a special customized uncore PMU which needs the flag as well.

Ok, I will add that too.

Btw, during the work I noticed many PMU drivers didn't set the
CAP_NO_INTERRUPT flag even if they didn't support sampling and
rejected the sampling events manually in the ->event_init() callback.

I guess it's because the name of the flag is somewhat misleading.
As the PMU drivers handle IRQ (for overflows), they thought they had
interrupts and didn't set the flag.  I think it'd be better to rename it to
CAP_NO_SAMPLING to reveal the intention.  And then we could just set
the flag in the pmu.capabilities and remove the manual checks.

The benefit is it can skip the PMUs in the timer tick handler even if
it needs to unthrottle some events.  What do you think?

Thanks,
Namhyung

>
> diff --git a/arch/x86/events/intel/uncore_snb.c
> b/arch/x86/events/intel/uncore_snb.c
> index 7fd4334e12a1..46a63e291975 100644
> --- a/arch/x86/events/intel/uncore_snb.c
> +++ b/arch/x86/events/intel/uncore_snb.c
> @@ -1013,7 +1013,7 @@ static struct pmu snb_uncore_imc_pmu = {
>         .start          = uncore_pmu_event_start,
>         .stop           = uncore_pmu_event_stop,
>         .read           = uncore_pmu_event_read,
> -       .capabilities   = PERF_PMU_CAP_NO_EXCLUDE,
> +       .capabilities   = PERF_PMU_CAP_NO_EXCLUDE |
> PERF_PMU_CAP_NO_INTERRUPT,
>  };
>
>  static struct intel_uncore_ops snb_uncore_imc_ops = {
>
>
> Thanks,
> Kan
> >       } else {