linux-kernel - Re: [RFC PATCH V3 4/6] sched/deadline: Introduce deadline servers

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Date:   Fri, 23 Jun 2023 17:47:05 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joel Fernandes <joel@...lfernandes.org>,
        Vineeth Pillai <vineeth@...byteword.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: Re: [RFC PATCH V3 4/6] sched/deadline: Introduce deadline servers

On 08/06/23 17:58, Daniel Bristot de Oliveira wrote:
> @@ -2033,9 +2147,20 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
>  	struct task_struct *p;
>  
>  	p = pick_task_dl(rq);
> -	if (p)
> +	if (!p)
> +		return p;
> +
> +	/*
> +	 * XXX: re-check !dl_server, changed from v2 because of
> +	 * pick_next_task_dl change
> +	 */
> +	if (!dl_server(&p->dl))
>  		set_next_task_dl(rq, p, true);
>

Should this be

       if (!p->server)

instead? AFAICT dl_server(&p->dl) can never be true since there's no
pi_se-like link to the server via the dl_se, only via the task_struct, and
the server pick cannot return the server itself (as it's a pure sched_entity).

> +	/* XXX not quite right */
> +	if (hrtick_enabled(rq))
> +		start_hrtick_dl(rq, &p->dl);
> +

IIUC that got hauled out of set_next_task_dl() to cover the case where we
pick the server (+ the server pick) and want to more thoroughly enforce the
server's bandwidth. If so, what's the issue with starting the hrtick here?

>  	return p;
>  }
>