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

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Message-ID: <20231115103129.GC3818@noisy.programming.kicks-ass.net>
Date:   Wed, 15 Nov 2023 11:31:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.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 Fri, Nov 03, 2023 at 06:14:32PM -0700, Namhyung Kim wrote:
> 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?

Linus initially got confused about the life-time of for-loop scopes, but
yeah, this could be continue just fine.

> 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..

Right, lets do that in another patch. Also, there seems to be a problem
with the cgroup thing :/