linux-kernel - Re: [PATCH 3/5] sched/deadline: Use __sub_running_bw() throughout dl_change

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Date:   Mon, 29 Jul 2019 18:47:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <Valentin.Schneider@....com>,
        Qais Yousef <Qais.Yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] sched/deadline: Use __sub_running_bw() throughout
 dl_change_utilization()

On Fri, Jul 26, 2019 at 09:27:54AM +0100, Dietmar Eggemann wrote:
> dl_change_utilization() has a BUG_ON() to check that no schedutil
> kthread (sugov) is entering this function. So instead of calling
> sub_running_bw() which checks for the special entity related to a
> sugov thread, call the underlying function __sub_running_bw().
> 
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> ---
>  kernel/sched/deadline.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 99d4c24a8637..1fa005f79307 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -164,7 +164,7 @@ void dl_change_utilization(struct task_struct *p, u64 new_bw)
>  
>  	rq = task_rq(p);
>  	if (p->dl.dl_non_contending) {
> -		sub_running_bw(&p->dl, &rq->dl);
> +		__sub_running_bw(p->dl.dl_bw, &rq->dl);
>  		p->dl.dl_non_contending = 0;
>  		/*
>  		 * If the timer handler is currently running and the

I'm confused; the only called of dl_change_utilization() is
sched_dl_overflow(), and that already checks FLAG_SUGOV and exits before
calling.

So how can this matter?