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Date:   Tue, 27 Jul 2021 15:33:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Christian Borntraeger <borntraeger@de.ibm.com>, bristot@redhat.com,
        bsegall@google.com, dietmar.eggemann@arm.com, joshdon@google.com,
        juri.lelli@redhat.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        linux@rasmusvillemoes.dk, mgorman@suse.de, mingo@kernel.org,
        rostedt@goodmis.org, valentin.schneider@arm.com,
        vincent.guittot@linaro.org
Subject: Re: [PATCH 1/1] sched/fair: improve yield_to vs fairness
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References: <YIlXQ43b6+7sUl+f@hirez.programming.kicks-ass.net>
 <20210707123402.13999-1-borntraeger@de.ibm.com>
 <20210707123402.13999-2-borntraeger@de.ibm.com>
 <20210723093523.GX3809@techsingularity.net>
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On Fri, Jul 23, 2021 at 10:35:23AM +0100, Mel Gorman wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 44c452072a1b..ddc0212d520f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4522,7 +4522,8 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>  			se = second;
>  	}
>  
> -	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
> +	if (cfs_rq->next &&
> +	    (cfs_rq->skip == left || wakeup_preempt_entity(cfs_rq->next, left) < 1)) {
>  		/*
>  		 * Someone really wants this to run. If it's not unfair, run it.
>  		 */

With a little more context this function reads like:

	se = left;

	if (cfs_rq->skip && cfs_rq->skip == se) {
		...
+		if (cfs_rq->next && (cfs_rq->skip == left || ...))

If '...' doesn't change @left (afaict it doesn't), then your change (+)
is equivalent to '&& true', or am I reading things wrong?