linux-kernel - Re: [RFC][PATCH 6/8] perf/x86: Clear ->event

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Message-ID: <CABPqkBReEzMKUDJVAUArwbzxtL5KTNRot2coW4MUOvy0qzgC-w@mail.gmail.com>
Date:   Tue, 19 Mar 2019 14:50:21 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, tonyj@...e.com,
        nelson.dsouza@...el.com
Subject: Re: [RFC][PATCH 6/8] perf/x86: Clear ->event_constraint[] on put

On Thu, Mar 14, 2019 at 6:11 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> The current code unconditionally clears cpuc->event_constraint[i]
> before calling get_event_constraints(.idx=i). The only site that cares
> is intel_get_event_constraints() where the c1 load will always be
> NULL.
>
> However, always calling get_event_constraints() on all events is
> wastefull, most times it will return the exact same result. Therefore
> retain the logic in intel_get_event_constraints() and change the
> generic code to only clear the constraint on put.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

I thought there was caching of the constraint in case it was static
(or unconstrained)
to avoid walking the constraint table each time between invocations
on the same group_sched_in() call. But the way the c1 vs. c2 logic
is written I don't see it. In which case, this could be another opportunity.

Looks good to me.

Reviewed-by: Stephane Eranian <eranian@...gle.com>

> ---
>  arch/x86/events/core.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -858,7 +858,6 @@ int x86_schedule_events(struct cpu_hw_ev
>                 x86_pmu.start_scheduling(cpuc);
>
>         for (i = 0, wmin = X86_PMC_IDX_MAX, wmax = 0; i < n; i++) {
> -               cpuc->event_constraint[i] = NULL;
>                 c = x86_pmu.get_event_constraints(cpuc, i, cpuc->event_list[i]);
>                 cpuc->event_constraint[i] = c;
>
> @@ -941,6 +940,8 @@ int x86_schedule_events(struct cpu_hw_ev
>                          */
>                         if (x86_pmu.put_event_constraints)
>                                 x86_pmu.put_event_constraints(cpuc, e);
> +
> +                       cpuc->event_constraint[i] = NULL;
>                 }
>         }
>
> @@ -1404,6 +1405,7 @@ static void x86_pmu_del(struct perf_even
>                 cpuc->event_list[i-1] = cpuc->event_list[i];
>                 cpuc->event_constraint[i-1] = cpuc->event_constraint[i];
>         }
> +       cpuc->event_constraint[i-1] = NULL;
>         --cpuc->n_events;
>
>         perf_event_update_userpage(event);
>
>