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

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Date:	Thu, 11 Nov 2010 20:31:23 +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 20:17 +0100, Peter Zijlstra wrote:
> On Fri, 2010-10-29 at 08:34 +0200, Raistlin wrote:
> > Make it possible to specify a period (different or equal than
> > deadline) for -deadline tasks.
> > 
> I would expect it to be:
> 
> runtime <= deadline <= period
> 
Well, apart from that really unhappy comment/changelog, it should be
like that in the code, and if it's not, it is what I meant and I'll
change to that as soon as I can! :-)

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! :-(

The more correct --in the sense that it at least yield a sufficient (not
necessary!) condition-- thing to do would be
sum_i(runtime_i/min{deadline_i,period_i})<=threshold.

So, what you think we should do? Can I go for this latter option?

Thanks,
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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