linux-kernel - Re: [PATCH v10 5/9] sched/fair: Take into account latency priority at wakeup

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Date:   Tue, 21 Feb 2023 14:04:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        cgroups@...r.kernel.org, qyousef@...alina.io,
        chris.hyser@...cle.com, patrick.bellasi@...bug.net,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        tj@...nel.org, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
        yu.c.chen@...el.com, youssefesmat@...omium.org,
        joel@...lfernandes.org
Subject: Re: [PATCH v10 5/9] sched/fair: Take into account latency priority
 at wakeup

On Fri, Jan 13, 2023 at 03:12:30PM +0100, Vincent Guittot wrote:
> @@ -6155,6 +6159,35 @@ static int sched_idle_cpu(int cpu)
>  }
>  #endif
>  
> +static void set_next_buddy(struct sched_entity *se);
> +
> +static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
> +{
> +	struct sched_entity *next;
> +
> +	if (se->latency_offset >= 0)
> +		return;
> +
> +	if (cfs->nr_running <= 1)
> +		return;
> +	/*
> +	 * When waking from another class, we don't need to check to preempt at
> +	 * wakeup and don't set next buddy as a candidate for being picked in
> +	 * priority.
> +	 * In case of simultaneous wakeup when current is another class, the
> +	 * latency sensitive tasks lost opportunity to preempt non sensitive
> +	 * tasks which woke up simultaneously.
> +	 */
> +
> +	if (cfs->next)
> +		next = cfs->next;
> +	else
> +		next = __pick_first_entity(cfs);
> +
> +	if (next && wakeup_preempt_entity(next, se) == 1)
> +		set_next_buddy(se);
> +}
> +
>  /*
>   * The enqueue_task method is called before nr_running is
>   * increased. Here we update the fair scheduling stats and
> @@ -6241,14 +6274,15 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	if (!task_new)
>  		update_overutilized_status(rq);
>  
> +	if (rq->curr->sched_class != &fair_sched_class)
> +		check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
> +
>  enqueue_throttle:
>  	assert_list_leaf_cfs_rq(rq);
>  
>  	hrtick_update(rq);
>  }

Hmm.. This sets a next selection when the task gets enqueued while not
running a fair task -- and looses a wakeup preemption opportunity.

Should we perhaps also do this for latency_nice == 0?, in any case I
think this can be moved to its own patch to avoid doing too much in the
one patch. It seems fairly self contained.