Re: [RFC 7/8] Make GRUB a task's flag

From: Peter Zijlstra
Date: Fri Jan 15 2016 - 03:42:01 EST

On Fri, Jan 15, 2016 at 09:15:39AM +0100, Luca Abeni wrote:
> On 01/14/2016 08:56 PM, Peter Zijlstra wrote:

> >Something with an average runtime/budget that also puts limits on the
> >max (say 2*avg) would be far more amenable to be exposed to unpriv
> >tasks, except since that would directly result in an average tardiness
> >bound this might be non-trivial to combine with tasks not opting for
> >this.

> I'll try to think about this... The advantage of GRUB is that a theoretically
> sound algorithm already existed; here, we would need to design the algorithm
> so that it does not break the SCHED_DEADLINE guarantees. Anyway, this is an
> interesting challenge, I'll work on it :)

Didn't Baruah and Jim do the whole theory on statistical EDF? Which
shows that if you use a statistical budget the combined distribution
transfers to the tardiness. With stdev=0 for the budgets this trivially
collapses to the regular EDF, since then the combined distribution is
also stdev=0 and you get 0 tardiness (on UP).

But yes, combining the two into one scheduler is 'interesting'. I was
thinking it would be possible with least-laxity-first, since you can
assign the hard (stdev=0) tasks a tighter laxity bound.

But LLF is horrendously painful to implement IIRC.