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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 8 Dec 2020 16:19:40 +0100
Message-ID: <CAKfTPtAUb50SHtDQs0xeehrPGqkyA6HqFQ3=nUbtOCGj62Fc9A@mail.gmail.com>
Subject: Re: [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Ziljstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Linux-ARM <linux-arm-kernel@lists.infradead.org>
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On Tue, 8 Dec 2020 at 16:12, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Tue, Dec 08, 2020 at 03:47:40PM +0100, Vincent Guittot wrote:
> > > I considered it but made the choice to exclude the cost of cpumask_and()
> > > from the avg_scan_cost instead. It's minor but when doing the original
> >
> > At the cost of a less readable code
> >
>
> Slightly less readable, yes.
>
> > > prototype, I didn't think it was appropriate to count the cpumask
> > > clearing as part of the scan cost as it's not directly related.
> >
> > hmm... I think it is because the number of loop is directly related to
> > the allowed cpus
> >
>
> While that is true, the cost of initialising the map is constant and
> what is most important is tracking the scan cost which is variable.
> Without SIS_AVG_CPU, the cpumask init can go before SIS_PROP without any
> penalty so is this version preferable?

yes looks good to me

>
> --8<--
> sched/fair: Move avg_scan_cost calculations under SIS_PROP
>
> As noted by Vincent Guittot, avg_scan_costs are calculated for SIS_PROP
> even if SIS_PROP is disabled. Move the time calculations under a SIS_PROP
> check and while we are at it, exclude the cost of initialising the CPU
> mask from the average scan cost.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
>  kernel/sched/fair.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ac7b34e7372b..5c41875aec23 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6153,6 +6153,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>         if (!this_sd)
>                 return -1;
>
> +       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +
>         if (sched_feat(SIS_PROP)) {
>                 u64 avg_cost, avg_idle, span_avg;
>
> @@ -6168,11 +6170,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>                         nr = div_u64(span_avg, avg_cost);
>                 else
>                         nr = 4;
> -       }
> -
> -       time = cpu_clock(this);
>
> -       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +               time = cpu_clock(this);
> +       }
>
>         for_each_cpu_wrap(cpu, cpus, target) {
>                 if (!--nr)
> @@ -6181,8 +6181,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>                         break;
>         }
>
> -       time = cpu_clock(this) - time;
> -       update_avg(&this_sd->avg_scan_cost, time);
> +       if (sched_feat(SIS_PROP)) {
> +               time = cpu_clock(this) - time;
> +               update_avg(&this_sd->avg_scan_cost, time);
> +       }
>
>         return cpu;
>  }