linux-kernel - Re: [PATCH 1/4] sched/fair: Remove SIS_AVG

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Date:   Tue, 8 Dec 2020 17:13:01 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Ziljstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU

On Tue, 8 Dec 2020 at 16:35, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> SIS_AVG_CPU was introduced as a means of avoiding a search when the
> average search cost indicated that the search would likely fail. It
> was a blunt instrument and disabled by 4c77b18cf8b7 ("sched/fair: Make
> select_idle_cpu() more aggressive") and later replaced with a proportional
> search depth by 1ad3aaf3fcd2 ("sched/core: Implement new approach to
> scale select_idle_cpu()").
>
> While there are corner cases where SIS_AVG_CPU is better, it has now been
> disabled for almost three years. As the intent of SIS_PROP is to reduce
> the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
> on SIS_PROP as a throttling mechanism.
>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c     | 20 +++++++++-----------
>  kernel/sched/features.h |  1 -
>  2 files changed, 9 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 98075f9ea9a8..ac7b34e7372b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6145,7 +6145,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  {
>         struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
>         struct sched_domain *this_sd;
> -       u64 avg_cost, avg_idle;
>         u64 time;
>         int this = smp_processor_id();
>         int cpu, nr = INT_MAX;
> @@ -6154,18 +6153,17 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>         if (!this_sd)
>                 return -1;
>
> -       /*
> -        * Due to large variance we need a large fuzz factor; hackbench in
> -        * particularly is sensitive here.
> -        */
> -       avg_idle = this_rq()->avg_idle / 512;
> -       avg_cost = this_sd->avg_scan_cost + 1;
> +       if (sched_feat(SIS_PROP)) {
> +               u64 avg_cost, avg_idle, span_avg;
>
> -       if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
> -               return -1;
> +               /*
> +                * Due to large variance we need a large fuzz factor;
> +                * hackbench in particularly is sensitive here.
> +                */
> +               avg_idle = this_rq()->avg_idle / 512;
> +               avg_cost = this_sd->avg_scan_cost + 1;
>
> -       if (sched_feat(SIS_PROP)) {
> -               u64 span_avg = sd->span_weight * avg_idle;
> +               span_avg = sd->span_weight * avg_idle;
>                 if (span_avg > 4*avg_cost)
>                         nr = div_u64(span_avg, avg_cost);
>                 else
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 68d369cba9e4..e875eabb6600 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -54,7 +54,6 @@ SCHED_FEAT(TTWU_QUEUE, true)
>  /*
>   * When doing wakeups, attempt to limit superfluous scans of the LLC domain.
>   */
> -SCHED_FEAT(SIS_AVG_CPU, false)
>  SCHED_FEAT(SIS_PROP, true)
>
>  /*
> --
> 2.26.2
>