Date: Mon, 17 Oct 2016 10:25:03 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: luca abeni <luca.abeni@unitn.it>
Cc: linux-kernel@vger.kernel.org,
        Tommaso Cucinotta <tommaso.cucinotta@sssup.it>,
        Juri Lelli <juri.lelli@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrea Parri <parri.andrea@gmail.com>,
        giuseppe lipari <giuseppe.lipari@lsv.ens-cachan.fr>,
        Claudio Scordino <claudio@evidence.eu.com>
Subject: Re: About group scheduling for SCHED_DEADLINE
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On Mon, Oct 17, 2016 at 08:38:57AM +0200, luca abeni wrote:

> > Yes, there currently is no existing schedulability analysis for
> > multi-processor EDF with random affinities (as far as I know)
> Correction: it looks like I was wrong, and the schedulability of
> multi-processor EDF with arbitrary affinities has already been analysed
> in
> A. Gujarati, F. Cerqueira, and B. Brandenburg, “Multiprocessor
> Real-Time Scheduling with Arbitrary Processor Affinities: From Practice
> to Theory”, Real- Time Systems, Volume 51, Issue 4, pp. 440–483.
> Springer Verlag, 2015
> (see https://www.mpi-sws.org/~bbb/papers/).
> 
> Thanks to Giuseppe Lipari for pointing me to this paper.

Ooh, shiny. Let me go read that.

> So, having DL tasks with arbitrary affinities is not a big problem from
> the theoretical point of view... The only issue is that the
> utilisation-based admission test that is currently implemented in the
> kernel does not work (and given the complexity of the analysis I think
> it is better not to perform it in the kernel :)

So the problem with doing admission control out of the kernel is that
then all the guarantees and resource control the kernel should provide
are out the window, and we're back to all the reasons why SCHED_FIFO is
a horrible horrible scheduling policy.