linux-kernel - Re: [PATCH] sched/fair: Use this_sd->weight to calculate span

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Date:   Mon, 11 Oct 2021 10:58:02 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tao Zhou <tao.zhou@...ux.dev>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] sched/fair: Use this_sd->weight to calculate span_avg

On Sun, 10 Oct 2021 02:10:55 +0800
Tao Zhou <tao.zhou@...ux.dev> wrote:

> avg_idle, avg_cost got from this_rq and this_sd. I think
> use this_sd->weight to calculate and estimate the number
> of loop cpus in the target domain.

If that's the case, then shouldn't the CPUs to be checked come from this_sd
as well? I mean, at the beginning of the function we have:

	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
	if (!this_sd)
		return -1;

	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);

Where "cpus" comes from sd, and not this_sd.


> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f6a05d9b5443..7fab7b70814c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6300,7 +6300,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  		avg_idle = this_rq->wake_avg_idle;
>  		avg_cost = this_sd->avg_scan_cost + 1;
>  
> -		span_avg = sd->span_weight * avg_idle;
> +		span_avg = this_sd->span_weight * avg_idle;
>  		if (span_avg > 4*avg_cost)
>  			nr = div_u64(span_avg, avg_cost);
>  		else


And after this code, the nr that is determined from the above, is for
limiting the looping over those CPUs from sd, not this_sd:

	for_each_cpu_wrap(cpu, cpus, target + 1) {
		if (has_idle_core) {
			i = select_idle_core(p, cpu, cpus, &idle_cpu);
			if ((unsigned int)i < nr_cpumask_bits)
				return i;

		} else {
			if (!--nr)
				return -1;
			idle_cpu = __select_idle_cpu(cpu, p);
			if ((unsigned int)idle_cpu < nr_cpumask_bits)
				break;
		}
	}

I'm guessing there's nothing wrong here. But, I don't fully understand it
myself.

-- Steve