Re: [PATCH 2/4] sched/fair: Do not replace recent_used_cpu with the new target

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 07/12/2020 10:15, Mel Gorman wrote:
> After select_idle_sibling, p->recent_used_cpu is set to the
> new target. However on the next wakeup, prev will be the same as

I'm confused here. Isn't current->recent_used_cpu set to 'cpu =
smp_processor_id()' after sis()? Looking at v5.10-rc6.

[...]

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 23934dbac635..01b38fc17bca 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6274,6 +6274,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  
>  	/* Check a recently used CPU as a potential idle candidate: */
>  	recent_used_cpu = p->recent_used_cpu;
> +	p->recent_used_cpu = prev;
>  	if (recent_used_cpu != prev &&
>  	    recent_used_cpu != target &&
>  	    cpus_share_cache(recent_used_cpu, target) &&

p->recent_used_cpu is already set to prev in this if condition.

    asym_fits_capacity(task_util, recent_used_cpu)) {
        /*
         * Replace recent_used_cpu with prev as it is a potential
         * candidate for the next wake:
         */
        p->recent_used_cpu = prev;
        return recent_used_cpu;

> @@ -6765,9 +6766,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	} else if (wake_flags & WF_TTWU) { /* XXX always ? */
>  		/* Fast path */
>  		new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> -
> -		if (want_affine)
> -			current->recent_used_cpu = cpu;
>  	}
>  	rcu_read_unlock();