linux-kernel - Re: [RFC PATCH v7 09/23] sched/fair: Fix forced idle sibling starvation corner case

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Date:   Fri, 28 Aug 2020 23:25:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Desfossez <jdesfossez@...italocean.com>
Cc:     Vineeth Pillai <viremana@...ux.microsoft.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Chris Hyser <chris.hyser@...cle.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        mingo@...nel.org, tglx@...utronix.de, pjt@...gle.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, joel@...lfernandes.org,
        vineeth@...byteword.org, Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        rostedt@...dmis.org, derkling@...gle.com, benbjiang@...cent.com,
        Vineeth Remanan Pillai <vpillai@...italocean.com>
Subject: Re: [RFC PATCH v7 09/23] sched/fair: Fix forced idle sibling
 starvation corner case

On Fri, Aug 28, 2020 at 03:51:10PM -0400, Julien Desfossez wrote:
> +/*
> + * If runqueue has only one task which used up its slice and if the sibling
> + * is forced idle, then trigger schedule to give forced idle task a chance.
> + */
> +static void resched_forceidle_sibling(struct rq *rq, struct sched_entity *se)
> +{
> +	int cpu = cpu_of(rq), sibling_cpu;
> +
> +	if (rq->cfs.nr_running > 1 || !__entity_slice_used(se))
> +		return;
> +
> +	for_each_cpu(sibling_cpu, cpu_smt_mask(cpu)) {
> +		struct rq *sibling_rq;
> +		if (sibling_cpu == cpu)
> +			continue;
> +		if (cpu_is_offline(sibling_cpu))
> +			continue;
> +
> +		sibling_rq = cpu_rq(sibling_cpu);
> +		if (sibling_rq->core_forceidle) {
> +			resched_curr(sibling_rq);
> +		}
> +	}

The only pupose of this loop seem to be to find if we have a forceidle;
surely we can avoid that by storing this during the pick.

> +}

static void task_tick_core(struct rq *rq)
{
	if (sched_core_enabled(rq))
		resched_forceidle_sibling(rq, &rq->curr->se);
}

#else

static void task_tick_core(struct rq *rq) { }

> +#endif
> +
>  /*
>   * scheduler tick hitting a task of our scheduling class.
>   *
> @@ -10654,6 +10688,11 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
>  
>  	update_misfit_status(curr, rq);
>  	update_overutilized_status(task_rq(curr));
> +
> +#ifdef CONFIG_SCHED_CORE
> +	if (sched_core_enabled(rq))
> +		resched_forceidle_sibling(rq, &curr->se);
> +#endif

Then you can ditch the #ifdef here

>  }
>  
>  /*
> -- 
> 2.17.1
>