linux-kernel - Re: periods and deadlines in SCHED

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Date:	Sat, 10 Jul 2010 12:36:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Luca Abeni <lucabe72@...il.it>
Cc:	Raistlin <raistlin@...ux.it>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Song Yuan <song.yuan@...csson.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nicola Manica <nicola.manica@...i.unitn.it>,
	Claudio Scordino <claudio@...dence.eu.com>,
	Harald Gustafsson <harald.gustafsson@...csson.com>,
	Bjoern Brandenburg <bbb@...il.unc.edu>, bastoni@...unc.edu,
	Giuseppe Lipari <lipari@...is.sssup.it>
Subject: Re: periods and deadlines in SCHED_DEADLINE

On Sat, 2010-07-10 at 09:11 +0200, Luca Abeni wrote:
> On Fri, 2010-07-09 at 16:24 +0200, Peter Zijlstra wrote:
> > On Fri, 2010-07-09 at 15:38 +0200, Raistlin wrote:
> > > Basically, from the scheduling point of view, what it could happen is
> > > that I'm still _NOT_ going to allow a task with runtime Q_i, deadline
> > > D_i and period P_i to use more bandwidth than Q_i/P_i, I'm still using D
> > > for scheduling but the passing of the simple in-kernel admission test
> > > Sum_i(Q_i/P_i)<1 won't guarantee that the task will always finish its
> > > jobs before D. 
> > 
> > But the tardiness would still be bounded, right? So its a valid Soft-RT
> > model?

> I think that if Sum_i(Q_i/P_i)<1 but Sum_i(Q_i/min{P_i,D_i})>=1 then you
> can have sporadic deadline misses, but it should still be possible to
> compute an upper bound for the tardiness.
> But this is just a feeling, I have no proof... :)

The paper referenced by Bjoern yesterday mentioned that Baruah et al.
did that proof.

        "For the considered case di < pi , Baruah et al. [7] showed
        that the complexity increases considerably. However, as-
        suming the processor utilization to be strictly less than 1,
        Baruah et al. [6, 7] proved that if a deadline is missed,
        this happens within a maximum time upper bound which
        can be computed."


[6] S. Baruah, A. Mok, and L. Rosier. Preemptively scheduling
hard-real-time sporadic tasks on one processor. Proceedings
of the 11th IEEE Real-Time Systems Symposium, December 1990.

[7] S. Baruah, L. Rosier, and R. Howell. Algorithms and
complexity concerning the preemptive scheduling of peri-
odic real-time tasks on one processor. Real-Time Systems,
2(4):301–324, November 1990.


It also jives well with that I remember from reading through Jim's
papers on Soft-RT G-EDF.
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