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Subject: Re: [RFC][PATCH 09/22] sched: add period support for -deadline
 tasks
From: Peter Zijlstra <peterz@infradead.org>
To: Raistlin <raistlin@linux.it>
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Date: Thu, 11 Nov 2010 20:43:55 +0100
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On Thu, 2010-11-11 at 20:31 +0100, Raistlin wrote:
> 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! :-(

Right, I have some recollection on that.

> 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?

I remember we visited this subject last time, but I seem to have
forgotten most details.

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?