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

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Message-ID: <1289568827.6525.411.camel@Palantir>
Date:	Fri, 12 Nov 2010 14:33:47 +0100
From:	Raistlin <raistlin@...ux.it>
To:	Peter Zijlstra <peterz@...radead.org>
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 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_.

The *should*-s come from the fact that I feel like I read it somewhere,
but right now I can't find the paper(s), not even following the
references indicated by Bjorn and Jim in previous e-mails and threads
(i.e., I can't find anything _explicitly_ considering deadline!=period,
but it might be my fault)... :-(

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

Since we care about bounded tardiness more than 100%-guaranteed
schedulability (which, BTW, neither min{} could give us, at least for
SMPs), should we stay with runtime/period? Tommaso, Luca, do you think
it would be so bad?

Thanks and Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

http://blog.linux.it/raistlin / raistlin@...ga.net /
dario.faggioli@...ber.org

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