Re: [PATCH RESEND 1/2] perf/core: Update perf_adjust_freq_unthr_context()

[Namhyung Kim]

Hi Mark,

On Wed, Jan 10, 2024 at 6:49 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Tue, Jan 09, 2024 at 01:36:22PM -0800, Namhyung Kim wrote:
> > It was unnecessarily disabling and enabling PMUs for each event.  It
> > should be done at PMU level.  Add pmu_ctx->nr_freq counter to check it
> > at each PMU.  As pmu context has separate active lists for pinned group
> > and flexible group, factor out a new function to do the job.
> >
> > Another minor optimization is that it can skip PMUs w/ CAP_NO_INTERRUPT
> > even if it needs to unthrottle sampling events.
> >
> > Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> > Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > Tested-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
>
> Hi,
>
> I've taken a quick look and I don't think this is quite right for
> hybrid/big.LITTLE, but I think that should be relatively simple to fix (more on
> that below).

Thanks for your review!

>
> This seems to be a bunch of optimizations; was that based on inspection alone,
> or have you found a workload where this has a measureable impact?

It's from a code inspection but I think Mingwei reported some excessive
MSR accesses for KVM use cases.  Anyway it'd increase the interrupt \
latency if you have slow (uncore) PMUs and lots of events on those PMUs.

>
> > ---
> >  include/linux/perf_event.h |  1 +
> >  kernel/events/core.c       | 68 +++++++++++++++++++++++---------------
> >  2 files changed, 43 insertions(+), 26 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index d2a15c0c6f8a..b2ff60fa487e 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -883,6 +883,7 @@ struct perf_event_pmu_context {
> >
> >       unsigned int                    nr_events;
> >       unsigned int                    nr_cgroups;
> > +     unsigned int                    nr_freq;
> >
> >       atomic_t                        refcount; /* event <-> epc */
> >       struct rcu_head                 rcu_head;
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 59b332cce9e7..ce9db9dbfd4c 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -2277,8 +2277,10 @@ event_sched_out(struct perf_event *event, struct perf_event_context *ctx)
> >
> >       if (!is_software_event(event))
> >               cpc->active_oncpu--;
> > -     if (event->attr.freq && event->attr.sample_freq)
> > +     if (event->attr.freq && event->attr.sample_freq) {
> >               ctx->nr_freq--;
> > +             epc->nr_freq--;
> > +     }
> >       if (event->attr.exclusive || !cpc->active_oncpu)
> >               cpc->exclusive = 0;
> >
> > @@ -2533,9 +2535,10 @@ event_sched_in(struct perf_event *event, struct perf_event_context *ctx)
> >
> >       if (!is_software_event(event))
> >               cpc->active_oncpu++;
> > -     if (event->attr.freq && event->attr.sample_freq)
> > +     if (event->attr.freq && event->attr.sample_freq) {
> >               ctx->nr_freq++;
> > -
> > +             epc->nr_freq++;
> > +     }
> >       if (event->attr.exclusive)
> >               cpc->exclusive = 1;
> >
> > @@ -4098,30 +4101,14 @@ static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count, bo
> >       }
> >  }
> >
> > -/*
> > - * combine freq adjustment with unthrottling to avoid two passes over the
> > - * events. At the same time, make sure, having freq events does not change
> > - * the rate of unthrottling as that would introduce bias.
> > - */
> > -static void
> > -perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> > +static void perf_adjust_freq_unthr_events(struct list_head *event_list)
> >  {
> >       struct perf_event *event;
> >       struct hw_perf_event *hwc;
> >       u64 now, period = TICK_NSEC;
> >       s64 delta;
> >
> > -     /*
> > -      * only need to iterate over all events iff:
> > -      * - context have events in frequency mode (needs freq adjust)
> > -      * - there are events to unthrottle on this cpu
> > -      */
> > -     if (!(ctx->nr_freq || unthrottle))
> > -             return;
> > -
> > -     raw_spin_lock(&ctx->lock);
> > -
> > -     list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
> > +     list_for_each_entry(event, event_list, active_list) {
> >               if (event->state != PERF_EVENT_STATE_ACTIVE)
> >                       continue;
> >
> > @@ -4129,8 +4116,6 @@ perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> >               if (!event_filter_match(event))
> >                       continue;
> >
> > -             perf_pmu_disable(event->pmu);
> > -
> >               hwc = &event->hw;
> >
> >               if (hwc->interrupts == MAX_INTERRUPTS) {
> > @@ -4140,7 +4125,7 @@ perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> >               }
> >
> >               if (!event->attr.freq || !event->attr.sample_freq)
> > -                     goto next;
> > +                     continue;
> >
> >               /*
> >                * stop the event and update event->count
> > @@ -4162,8 +4147,39 @@ perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> >                       perf_adjust_period(event, period, delta, false);
> >
> >               event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
> > -     next:
> > -             perf_pmu_enable(event->pmu);
> > +     }
> > +}
> > +
> > +/*
> > + * combine freq adjustment with unthrottling to avoid two passes over the
> > + * events. At the same time, make sure, having freq events does not change
> > + * the rate of unthrottling as that would introduce bias.
> > + */
> > +static void
> > +perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> > +{
> > +     struct perf_event_pmu_context *pmu_ctx;
> > +
> > +     /*
> > +      * only need to iterate over all events iff:
> > +      * - context have events in frequency mode (needs freq adjust)
> > +      * - there are events to unthrottle on this cpu
> > +      */
> > +     if (!(ctx->nr_freq || unthrottle))
> > +             return;
> > +
> > +     raw_spin_lock(&ctx->lock);
> > +
> > +     list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) {
> > +             if (!(pmu_ctx->nr_freq || unthrottle))
> > +                     continue;
> > +             if (pmu_ctx->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT)
> > +                     continue;
> > +
> > +             perf_pmu_disable(pmu_ctx->pmu);
> > +             perf_adjust_freq_unthr_events(&pmu_ctx->pinned_active);
> > +             perf_adjust_freq_unthr_events(&pmu_ctx->flexible_active);
> > +             perf_pmu_enable(pmu_ctx->pmu);
> >       }
>
> I don't think this is correct for big.LITTLE/hybrid systems.
>
> Imagine a system where CPUs 0-1 have pmu_a, CPUs 2-3 have pmu_b, and a task has
> events for both pmu_a and pmu_b. The perf_event_context for that task will have
> a perf_event_pmu_context for each PMU in its pmu_ctx_list.
>
> Say that task is run on CPU0, and perf_event_task_tick() is called. That will
> call perf_adjust_freq_unthr_context(), and it will iterate over the
> pmu_ctx_list. Note that regardless of pmu_ctx->nr_freq, if 'unthottle' is true,
> we'll go ahead and call the following for all of the pmu contexts in the
> pmu_ctx_list:
>
>         perf_pmu_disable(pmu_ctx->pmu);
>         perf_adjust_freq_unthr_events(&pmu_ctx->pinned_active);
>         perf_adjust_freq_unthr_events(&pmu_ctx->flexible_active);
>         perf_pmu_enable(pmu_ctx->pmu);
>
> ... and that means we might call that for pmu_b, even though it's not
> associated with CPU0. That could be fatal depending on what those callbacks do.

Thanks for pointing that out.  I missed the hybrid cases.

>
> The old logic avoided that possibility implicitly, since the events for pmu_b
> couldn't be active, and so the check at the start of the look would skip all of
> pmu_b's events:
>
>         if (event->state != PERF_EVENT_STATE_ACTIVE)
>                 continue;
>
> We could do similar by keeping track of how many active events each
> perf_event_pmu_context has, which'd allow us to do something like:
>
>         if (pmu_ctx->nr_active == 0)
>                 continue;
>
> How does that sound to you?

Sounds good.  Maybe we can just test if both active lists are empty.

Thanks,
Namhyung