linux-kernel - Re: [RFC v5 8/9] sched/deadline: base GRUB reclaiming on the inactive utilization

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Date:   Mon, 8 May 2017 09:41:08 +0200
From:   Luca Abeni <luca.abeni@...tannapisa.it>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [RFC v5 8/9] sched/deadline: base GRUB reclaiming on the
 inactive utilization

Hi Peter,

sorry for the delay; anyway, I am working on fixing the patchset
according to the comments I received....

When working on one of your comments, I have a doubt:

On Mon, 27 Mar 2017 16:26:33 +0200
Peter Zijlstra <peterz@...radead.org> wrote:
[...]
> 
> 
> #define BW_SHIFT	20
> #define BW_UNIT		(1 << BW_SHIFT)
> 
> static inline
> u64 grub_reclaim(u64 delta, struct rq *rq, struct sched_dl_entity
> *dl_se) {
> u64 u_inact = rq->dl.this_bw - rq->dl.running_bw; /* Utot -
> Uact */ u64 u_act;
[...]

I think introducing the BW_SHIFT and BW_UNIT defines can be more useful
in a previous patch (patch 4, where I introduce the "grub_reclaim()"
function, and use ">> 20" for the first time.

Moreover, the "20" magic number is already used in core.c... Should I
introduce the defines in sched/sched.h, and change the existing core.c
code too? Is it ok to embed this change in patch 4 (sched/deadline:
implement GRUB accounting), or should it go in a separate patch?

			Thanks,
				Luca

> 
> 	/*
>          * What we want to write is:
> 	 *
> 	 *   max(BW_UNIT - u_inact, dl_se->dl_bw)
> 	 *
> 	 * but we cannot do that since Utot can be larger than 1,
> 	 * which means u_inact can be larger than 1, which would
> 	 * have the above result in negative values.
> 	 */
> 	if (u_inact > (BW_UNIT - dl_se->dl_bw))
> 		u_act = dl_se->dl_bw;
> 	else
> 		u_act = BW_UNIT - u_inact;
> 
> 	return (delta * u_act) >> BW_SHIFT;
> }
> 
> Hmm?