linux-kernel - Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

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Message-ID: <1335183740.28150.135.camel@twins>
Date:	Mon, 23 Apr 2012 14:22:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juri Lelli <juri.lelli@...il.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, rostedt@...dmis.org,
	cfriesen@...tel.com, oleg@...hat.com, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com
Subject: Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

On Mon, 2012-04-23 at 14:13 +0200, Juri Lelli wrote:
> On 04/23/2012 01:32 PM, Peter Zijlstra wrote:
> > On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
> >> +       /*
> >> +        * We Keep moving the deadline away until we get some
> >> +        * available runtime for the entity. This ensures correct
> >> +        * handling of situations where the runtime overrun is
> >> +        * arbitrary large.
> >> +        */
> >> +       while (dl_se->runtime<= 0) {
> >> +               dl_se->deadline += dl_se->dl_deadline;
> >> +               dl_se->runtime += dl_se->dl_runtime;
> >> +       }
> >
> > Does gcc 'optimize' that into a division? If so, it might need special
> > glue to make it not do that.
> 
> I got two adds and a jle, no div here..

Gcc is known to change such loops to something like:

 if (runtime <= 0) {
   tmp = 1 - runtime / dl_runtime;
   deadline += tmp * dl_deadline;
   runtime += tmp * dl_runtime;
 }


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