linux-kernel - Re: [PATCH v2 1/8] sched/fair: Update affine statistics when needed

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Message-ID: <87sg2yil1q.mognet@arm.com>
Date:   Fri, 07 May 2021 17:08:17 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Parth Shah <parth@...ux.ibm.com>
Subject: Re: [PATCH v2 1/8] sched/fair: Update affine statistics when needed

On 06/05/21 22:15, Srikar Dronamraju wrote:
> wake_affine_idle() can return prev_cpu. Even in such a scenario,
> scheduler was going ahead and updating schedstats related to wake
> affine. i.e even if the task is not moved across LLC domains,
> schedstats would have accounted.
>
> Hence add a check before updating schedstats.
>

I briefly glanced at the git history but didn't find any proper description
of that stat. As it stands, it counts the number of times wake_affine()
purposedly steered a task towards a particular CPU (waker or wakee's prev),
so nr_wakeups_affine / nr_wakeups_affine_attempts is your wake_affine()
"success rate" - how often could it make a choice with the available data.

I could see a point in only incrementing the count if wake_affine() steers
towards the waker rather than the wakee (i.e. don't increment if choice is
prev), but then that has no link with LLC spans

> Cc: LKML <linux-kernel@...r.kernel.org>
> Cc: Gautham R Shenoy <ego@...ux.vnet.ibm.com>
> Cc: Parth Shah <parth@...ux.ibm.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Valentin Schneider <valentin.schneider@....com>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Rik van Riel <riel@...riel.com>
> Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> ---
>  kernel/sched/fair.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..a258a84cfdfd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5884,8 +5884,10 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
>       if (target == nr_cpumask_bits)
>               return prev_cpu;
>
> -	schedstat_inc(sd->ttwu_move_affine);
> -	schedstat_inc(p->se.statistics.nr_wakeups_affine);
> +	if (!cpus_share_cache(prev_cpu, target)) {

Per the above, why? Why not just if(target == this_cpu) ?

> +		schedstat_inc(sd->ttwu_move_affine);
> +		schedstat_inc(p->se.statistics.nr_wakeups_affine);
> +	}
>       return target;
>  }
>
> --
> 2.18.2