linux-kernel - Re: [PATCH v3 6/6] sched/rt: Fix pushing unfit tasks to a better CPU

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Date:   Wed, 11 Mar 2020 10:00:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/6] sched/rt: Fix pushing unfit tasks to a better
 CPU

On Mon,  2 Mar 2020 13:27:21 +0000
Qais Yousef <qais.yousef@....com> wrote:

> +			 * Don't bother moving it if the destination CPU is
> +			 * not running a lower priority task.
> +			 */
> +			if (p->prio < cpu_rq(target)->rt.highest_prio.curr) {
> +
> +				cpu = target;
> +
> +			} else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) {
> +
> +				/*
> +				 * If the priority is the same and the new CPU
> +				 * is a better fit, then move, otherwise don't
> +				 * bother here either.
> +				 */
> +				if (fit_target)
> +					cpu = target;
> +			}

BTW, A little better algorithm would be to test fit_target first:

	target_prio = cpu_rq(target)->rt.hightest_prio.curr;
	if (p->prio < target_prio) {
		cpu = target;

	} else if (fit_target && p->prio == target_prio) {
		cpu = target;
	}

Which can also just be a single if statement:

	if (p->prio < target_prio ||
	    (fit_target && p->prio == target_prio)
		cpu = target;

-- Steve