linux-kernel - Re: [RFC PATCH 2/6] sched/dl: Capacity-aware migrations

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Date:   Thu, 4 Jul 2019 14:05:22 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Luca Abeni <luca.abeni@...tannapisa.it>,
        linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Quentin Perret <quentin.perret@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC PATCH 2/6] sched/dl: Capacity-aware migrations

On 5/6/19 6:48 AM, Luca Abeni wrote:

[...]

> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 5b981eeeb944..3436f3d8fa8f 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1584,6 +1584,9 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
>  	if (sd_flag != SD_BALANCE_WAKE)
>  		goto out;
>  
> +	if (dl_entity_is_special(&p->dl))
> +		goto out;

I wonder if this is really required. The if condition

1591         if (unlikely(dl_task(curr)) &&
1592             (curr->nr_cpus_allowed < 2 ||
1593              !dl_entity_preempt(&p->dl, &curr->dl)) &&
1594             (p->nr_cpus_allowed > 1)) {

further below uses '!dl_entity_preempt(&p->dl, &curr->dl))' which
returns 'dl_entity_is_special(a) || ...'

A BUG_ON(dl_entity_is_special(&p->dl)) in this if condition hasn't
triggered on my platform yet.

[...]