linux-kernel - Re: [PATCH 11/13] perf: Simplify perf_adjust_freq_unthr

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Date:   Fri, 3 Nov 2023 18:14:32 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com
Subject: Re: [PATCH 11/13] perf: Simplify perf_adjust_freq_unthr_context()

On Thu, Nov 2, 2023 at 8:32 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/events/core.c |   51 +++++++++++++++++++++++----------------------------
>  1 file changed, 23 insertions(+), 28 deletions(-)
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4090,7 +4090,7 @@ perf_adjust_freq_unthr_context(struct pe
>         if (!(ctx->nr_freq || unthrottle))
>                 return;
>
> -       raw_spin_lock(&ctx->lock);
> +       guard(raw_spinlock)(&ctx->lock);
>
>         list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
>                 if (event->state != PERF_EVENT_STATE_ACTIVE)
> @@ -4100,7 +4100,7 @@ perf_adjust_freq_unthr_context(struct pe
>                 if (!event_filter_match(event))
>                         continue;
>
> -               perf_pmu_disable(event->pmu);
> +               guard(perf_pmu_disable)(event->pmu);
>
>                 hwc = &event->hw;
>
> @@ -4110,34 +4110,29 @@ perf_adjust_freq_unthr_context(struct pe
>                         event->pmu->start(event, 0);
>                 }
>
> -               if (!event->attr.freq || !event->attr.sample_freq)
> -                       goto next;
> +               if (event->attr.freq && event->attr.sample_freq) {

Any reason for this change?  I think we can just change the
'goto next' to 'continue', no?

Also I think this code needs changes to optimize the access.
A similar reason for the cgroup switch, it accesses all the
pmu/events in the context even before checking the sampling
frequency.  This is bad for uncore PMUs (and KVM too).

But this is a different issue..

Thanks,
Namhyung


> +                       /*
> +                        * stop the event and update event->count
> +                        */
> +                       event->pmu->stop(event, PERF_EF_UPDATE);
> +
> +                       now = local64_read(&event->count);
> +                       delta = now - hwc->freq_count_stamp;
> +                       hwc->freq_count_stamp = now;
> +
> +                       /*
> +                        * restart the event
> +                        * reload only if value has changed
> +                        * we have stopped the event so tell that
> +                        * to perf_adjust_period() to avoid stopping it
> +                        * twice.
> +                        */
> +                       if (delta > 0)
> +                               perf_adjust_period(event, period, delta, false);
>
> -               /*
> -                * stop the event and update event->count
> -                */
> -               event->pmu->stop(event, PERF_EF_UPDATE);
> -
> -               now = local64_read(&event->count);
> -               delta = now - hwc->freq_count_stamp;
> -               hwc->freq_count_stamp = now;
> -
> -               /*
> -                * restart the event
> -                * reload only if value has changed
> -                * we have stopped the event so tell that
> -                * to perf_adjust_period() to avoid stopping it
> -                * twice.
> -                */
> -               if (delta > 0)
> -                       perf_adjust_period(event, period, delta, false);
> -
> -               event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
> -       next:
> -               perf_pmu_enable(event->pmu);
> +                       event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
> +               }
>         }
> -
> -       raw_spin_unlock(&ctx->lock);
>  }
>
>  /*
>
>