linux-kernel - Re: [PATCH v3 2/5] sched/deadline: Fix reclaim inaccuracy with SMP

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Date:   Mon, 15 May 2023 10:06:16 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Vineeth Pillai <vineeth@...byteword.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 2/5] sched/deadline: Fix reclaim inaccuracy with SMP

Hi,

this patch is giving me some headaches:

On Sun, 14 May 2023 22:57:13 -0400
Vineeth Pillai <vineeth@...byteword.org> wrote:
[...]
>   *	Uextra:		Extra bandwidth not reserved:
> - *			= Umax - \Sum(u_i / #cpus in the root domain)
> + *			= Umax - this_bw

While I agree that this setting should be OK, it ends up with
	dq = -Uact / Umax * dt
which I remember I originally tried, and gave some issues
(I do not remember the details, but I think if you try N
identical reclaiming tasks, with N > M, the reclaimed time
is not distributed equally among them?)

I need to think a little bit more about this...


		Luca

>   *	u_i:		Bandwidth of an admitted dl task in the
>   *			root domain.
>   *
> @@ -1286,22 +1286,14 @@ int dl_runtime_exceeded(struct
> sched_dl_entity *dl_se) */
>  static u64 grub_reclaim(u64 delta, struct rq *rq, struct
> sched_dl_entity *dl_se) {
> -	u64 u_act;
> -	u64 u_inact = rq->dl.this_bw - rq->dl.running_bw; /* Utot -
> Uact */ -
>  	/*
> -	 * Instead of computing max{u, (rq->dl.max_bw - u_inact -
> u_extra)},
> -	 * we compare u_inact + rq->dl.extra_bw with
> -	 * rq->dl.max_bw - u, because u_inact + rq->dl.extra_bw can
> be larger
> -	 * than rq->dl.max_bw (so, rq->dl.max_bw - u_inact -
> rq->dl.extra_bw
> -	 * would be negative leading to wrong results)
> +	 * max{u, Umax - Uinact - Uextra}
> +	 * = max{u, max_bw - (this_bw - running_bw) + (this_bw -
> running_bw)}
> +	 * = max{u, running_bw} = running_bw
> +	 * So dq = -(max{u, Umax - Uinact - Uextra} / Umax) dt
> +	 *       = -(running_bw / max_bw) dt
>  	 */
> -	if (u_inact + rq->dl.extra_bw > rq->dl.max_bw - dl_se->dl_bw)
> -		u_act = dl_se->dl_bw;
> -	else
> -		u_act = rq->dl.max_bw - u_inact - rq->dl.extra_bw;
> -
> -	return div64_u64(delta * u_act, rq->dl.max_bw);
> +	return div64_u64(delta * rq->dl.running_bw, rq->dl.max_bw);
>  }
>  
>  /*