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Date:   Fri, 26 May 2023 17:29:59 +0200
From:   luca abeni <luca.abeni@santannapisa.it>
To:     Vineeth Pillai <vineeth@bitbyteword.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        youssefesmat@google.com,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org
Subject: Re: [PATCH v4 1/2] sched/deadline: Fix bandwidth reclaim equation
 in GRUB
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In-Reply-To: <20230526145519.2282062-1-vineeth@bitbyteword.org>
References: <20230526145519.2282062-1-vineeth@bitbyteword.org>
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Hi,

I think the code changes look good. I only see a small issue in the
comments.

On Fri, 26 May 2023 10:55:18 -0400
Vineeth Pillai <vineeth@bitbyteword.org> wrote:

> According to the GRUB[1] rule, the runtime is depreciated as:
>   "dq = -max{u, (1 - Uinact - Uextra)} dt" (1)
> 
> To guarantee that deadline tasks doesn't starve lower class tasks,
> we do not allocate the full bandwidth of the cpu to deadline tasks.
> Maximum bandwidth usable by deadline tasks is denoted by "Umax".
> Considering Umax, equation (1) becomes:
>   "dq = -(max{u, (Umax - Uinact - Uextra)} / Umax) dt" (2)

This is correct...

[...]
>  /*
> - * This function implements the GRUB accounting rule:
> - * according to the GRUB reclaiming algorithm, the runtime is
> - * not decreased as "dq = -dt", but as
> - * "dq = -max{u / Umax, (1 - Uinact - Uextra)} dt",
> + * This function implements the GRUB accounting rule. According to
> the
> + * GRUB reclaiming algorithm, the runtime is not decreased as "dq =
> -dt",
> + * but as "dq = -(max{u, (1 - Uinact - Uextra)} / Umax) dt",

...But I think this is wrong (should be "Umax - ...", not "1 - ...").
I think patch 2/2 has the same issue.

[...]
> +	if (u_inact + rq->dl.extra_bw > rq->dl.max_bw - dl_se->dl_bw)
> +		u_act = dl_se->dl_bw;
>  	else
> -		u_act = BW_UNIT - u_inact - rq->dl.extra_bw;
> +		u_act = rq->dl.max_bw - u_inact - rq->dl.extra_bw;

This again is IMHO OK


			Thanks,
				Luca