linux-kernel - Re: [PATCH] sched/deadline: Add sched

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Date:	Tue, 28 Jan 2014 10:31:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Cc:	Luca Abeni <luca.abeni@...tn.it>, Henrik Austad <henrik@...tad.us>,
	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, rostedt@...dmis.org, 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, nicola.manica@...i.unitn.it,
	dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com,
	harald.gustafsson@...csson.com, vincent.guittot@...aro.org,
	bruce.ashfield@...driver.com, rob@...dley.net
Subject: Re: [PATCH] sched/deadline: Add sched_dl documentation

On Fri, Jan 24, 2014 at 10:08:35AM +0000, Tommaso Cucinotta wrote:
> > We should also again talk about what it would take to allow
> > unprivileged access to SCHED_DEADLINE. The things I can remember are the
> > obvious cap on utilization and a minimum period -- the latter so that we
> > don't DoS the system with a metric ton of tiny tasks.
> > 
> > But I seem to remember there were a few other issues.
> 
> Exactly. I can remember also a huge period might be harmful, because with it
> even a tiny utilization may lead to a big runtime that starves the whole non
> RT tasks for a significant time. 

Another way to solve that is with explicit slack time scheduling, where
slack here means the time/utilization not allocated to dl tasks (a quick
google shows various different usages of slack, including laxity).

So suppose we have 50% utilization but with a huge period, we could
schedule the other 50% at a fixed but smaller period and have that run
the rt/fair/etc tasks.

So effectively we'll always fill up the utilization to 100% and use the
'slack' time as a server for the other classes.

> Just in case, I had this paper
> 
>   http://retis.sssup.it/~tommaso/papers/rtas08.php
> 
> discussing the issues I had found, and how they were tackled in the AQuoSA
> supervisor. See Section 3.1.

/me *click*
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