linux-kernel - Re: [RFC][PATCH 09/22] sched: add period support for -deadline tasks

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Date:	Fri, 12 Nov 2010 14:45:33 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Raistlin <raistlin@...ux.it>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chris Friesen <cfriesen@...tel.com>, oleg@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Claudio Scordino <claudio@...dence.eu.com>,
	michael trimarchi <trimarchi@...is.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Tommaso Cucinotta <cucinotta@...up.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Nicola Manica <nicola.manica@...i.unitn.it>,
	Luca Abeni <luca.abeni@...tn.it>,
	Dhaval Giani <dhaval@...is.sssup.it>,
	Harald Gustafsson <hgu1972@...il.com>,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 09/22] sched: add period support for -deadline
 tasks

On Fri, 2010-11-12 at 14:33 +0100, Raistlin wrote:
> On Thu, 2010-11-11 at 20:43 +0100, Peter Zijlstra wrote:
> > > Since you spotted it... The biggest issue here is admission control
> > > test. Right now this is done against task's bandwidth, i.e.,
> > > sum_i(runtime_i/period_i)<=threshold, but it is unfortunately wrong...
> > > Or at least very, very loose, to the point of being almost useless! :-(
> > 
> > Right, I have some recollection on that.
> > 
> :-)
> 
> > So sufficient (but not necessary) means its still a pessimistic approach
> > but better than the one currently employed, or does it mean its
> > optimistic and allows for unschedulable sets to be allowed in?
> > 
> Tommaso already gave the best possible explanation of this! :-P
> 
> So, trying to recap:
>  - using runtime/min(deadline,period) _does_ guarantee schedulability,
>    but also rejects schedulable situations in UP/partitioning. Quite
>    sure it _does_not_ guarantee schedulability in SMP/global, but
>    *should* enable bounded tardiness;
>  - using runtime/period _does_not_ guarantee schedulability nor in
>    UP/partitioning neither in SMP/global, but *should* enable bounded
>    tardiness for _both_.

> Thus, all this being said, what do you want me to do? :-D

runtime/min(deadline,period) sounds fine, as its more useful than
runtime/period.
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