Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 14, 2015 at 04:56:17PM +0100, Vincent Guittot wrote:
> I agree that if the WCET is far from reality, we will underestimate
> available capacity for CFS. Have you got some use case in mind which
> overestimates the WCET ?

Pretty much any 'correct' WCET is pessimistic. There's heaps of smart
people working on improving WCET bounds, but they're still out there.
This is mostly because of the .00001% tail cases that 'never' happen but
would make your tokamak burn a hole just when you're outside.

> If we can't rely on this parameters to evaluate the amount of capacity
> used by deadline scheduler on a core, this will imply that we can't
> also use it for requesting capacity to cpufreq and we should fallback
> on a monitoring mechanism which reacts to a change instead of
> anticipating it.

No, since the WCET can and _will_ happen, its the best you can do with
cpufreq. If you were to set it lower you could not be able to execute
correctly in your 'never' tail cases.

There 'might' be smart pants ways around this, where you run part of the
execution at lower speed and switch to a higher speed to 'catch' up if
you exceed some boundary, such that, on average, you run at the same
speed the WCET mandates, but I'm not sure that's worth it. Juri/Luca
might know.