linux-kernel - Re: [RFC 5/6] sched/fair: Take into account latency nice at wakeup

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Date:   Mon, 28 Mar 2022 11:24:09 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, linux-kernel@...r.kernel.org,
        parth@...ux.ibm.com
Cc:     qais.yousef@....com, chris.hyser@...cle.com,
        pkondeti@...eaurora.org, valentin.schneider@....com,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        pjt@...gle.com, pavel@....cz, tj@...nel.org,
        dhaval.giani@...cle.com, qperret@...gle.com,
        tim.c.chen@...ux.intel.com
Subject: Re: [RFC 5/6] sched/fair: Take into account latency nice at wakeup

On 11/03/2022 17:14, Vincent Guittot wrote:

[...]

> @@ -4412,7 +4417,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>  		p->prio = p->normal_prio = p->static_prio;
>  		set_load_weight(p, false);
>  
> -		p->latency_nice = DEFAULT_LATENCY_NICE;
> +		p->latency_prio = NICE_TO_LATENCY(0);
>  		/*
>  		 * We don't need the reset flag anymore after the fork. It has
>  		 * fulfilled its duty:
> @@ -4420,6 +4425,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>  		p->sched_reset_on_fork = 0;
>  	}
>  
> +	/* Once latency_prio is set, update the latency weight */
> +	set_latency_weight(p);

I thought we only have to do this in the `sched_reset_on_fork` case?
Like we do with set_load_weight(). Can we not rely on dup_task_struct()
in the other case?

[...]

> @@ -5648,6 +5677,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	if (!task_new)
>  		update_overutilized_status(rq);
>  
> +	if (rq->curr == rq->idle)
> +		check_preempt_from_idle(cfs_rq_of(&p->se), &p->se);

This is done here (1) because check_preempt_wakeup() (2) is only called
if p and rq->curr have CFS sched class?


ttwu_do_activate()
  activate_task()
    enqueue_task <-- (1)
  ttwu_do_wakeup()
    check_preempt_curr()
      if (p->sched_class == rq->curr->sched_class)
        rq->curr->sched_class->check_preempt_curr() <-- (2)

[...]

> @@ -7008,6 +7059,10 @@ static int
>  wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>  {
>  	s64 gran, vdiff = curr->vruntime - se->vruntime;
> +	int latency_weight = se->latency_weight - curr->latency_weight;
> +
> +	latency_weight = min(latency_weight, se->latency_weight);

Why the min out of latency_weight_diff(se, curr) and se->latency_weight
here?

[...]