linux-kernel - Re: Bug in AC?

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Message-ID: <20160517103300.2cf6e28e@gandalf.local.home>
Date:	Tue, 17 May 2016 10:33:00 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	luca abeni <luca.abeni@...tn.it>
Cc:	Juri Lelli <juri.lelli@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Bug in AC?

On Tue, 17 May 2016 16:07:49 +0200
luca abeni <luca.abeni@...tn.it> wrote:

> > As I
> > mentioned on IRC, what about the case with two CPUs and this:
> > 
> > Two tasks with:       R:10us D: 15us P:100us
> > and two tasks with:   R:6us  D: 14us P:14us
> > 
> > If the period of the first two tasks line up on two different CPUs
> > then there's no way the other two tasks will make their deadlines.  
> I agree this taskset is not schedulable on 2 CPUs. The problem is that
> it is possible to generate tasksets with sum of densities < 2 that are
> not schedulable on 2 CPUs.

Makes sense. Thus the case is that we can't guarantee it on SMP anyway,
so why put the extra effort to use deadline instead of period, where on
UP we can make those guarantees.

I was under the impression that it appeared everyone was saying that SMP
we can guarantee it and UP we could not, which is where I was confused.

-- Steve