Subject: Re: [RFC][PATCH 09/22] sched: add period support for -deadline
 tasks
From: Peter Zijlstra <peterz@infradead.org>
To: Raistlin <raistlin@linux.it>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Chris Friesen <cfriesen@nortel.com>, oleg@redhat.com,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Darren Hart <darren@dvhart.com>, Johan Eker <johan.eker@ericsson.com>,
        "p.faure" <p.faure@akatech.ch>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Claudio Scordino <claudio@evidence.eu.com>,
        michael trimarchi <trimarchi@retis.sssup.it>,
        Fabio Checconi <fabio@gandalf.sssup.it>,
        Tommaso Cucinotta <cucinotta@sssup.it>,
        Juri Lelli <juri.lelli@gmail.com>,
        Nicola Manica <nicola.manica@disi.unitn.it>,
        Luca Abeni <luca.abeni@unitn.it>, Dhaval Giani <dhaval@retis.sssup.it>,
        Harald Gustafsson <hgu1972@gmail.com>,
        paulmck <paulmck@linux.vnet.ibm.com>
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Date: Fri, 12 Nov 2010 14:45:33 +0100
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On Fri, 2010-11-12 at 14:33 +0100, Raistlin wrote:
> 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_.

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

runtime/min(deadline,period) sounds fine, as its more useful than
runtime/period.
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