linux-kernel - Re: [RFC][PATCH 18/22] sched: add reclaiming logic to -deadline tasks

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Date:	Fri, 12 Nov 2010 18:54:35 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Raistlin <raistlin@...ux.it>, 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>,
	Dhaval Giani <dhaval@...is.sssup.it>,
	Harald Gustafsson <hgu1972@...il.com>,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 18/22] sched: add reclaiming logic to -deadline tasks

On 12/11/10 18:51, Peter Zijlstra wrote:
> On Fri, 2010-11-12 at 18:41 +0100, Luca Abeni wrote:
>>> The problem the stochastic execution time model tries to address is the
>>> WCET computation mess, WCET computation is hard and often overly
>>> pessimistic, resulting in under-utilized systems.
>> [...]
>> BTW, sorry for the shameless plug, but even with the current
>> SCHED_DEADLINE you are not forced to dimension the runtime using the
>> WCET.
>
> Yes you are, it pushes the deadline back on overrun.
I think in case of overrun it postpones the deadline (by a period P), 
avoiding to execute the task until the end of the current period, right?

> The idea it to
> maintain the deadline despite overrunning your budget (up to a point).
>
> The paper we're all talking about is:
>
> A. Mills and J. Anderson, " A Stochastic Framework for Multiprocessor
> Soft Real-Time Scheduling", Proceedings of the 16th IEEE Real-Time and
> Embedded Technology and Applications Symposium, pp. 311-320, April 2010.
> http://www.cs.unc.edu/~anderson/papers/rtas10brevised.pdf
I see... This is a different approach to stochastic analysis, which 
requires modifications to the scheduler.

In the analysis I mentioned, you still enforce a maximum runtime C every 
period P, but C can be smaller than the WCET of the task. If C is larger 
than the average execution time, you can use queuing theory to find the 
probability to miss a deadline (or, the probability to finish a job in a 
time x * P).


				Luca
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