Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 11, 2016 at 07:23:55PM +0100, Vincent Guittot wrote:
> I agree that using rt_avg is not the best choice to evaluate the
> capacity that is used by RT tasks but it has the advantage of been
> already there. Do you mean that we should use another way to compute
> the capacity that is used by rt tasks to then select the frequency  ?

Nope, RR/FIFO simply do not contain enough information to compute
anything from.

> Or do you mean that we can't do anything else than asking for max
> frequency ?

Yep.

> Trying to set max frequency just before scheduling RT task is not
> really doable on a lot of platform because the sequence that changes
> the frequency can sleep and takes more time than the run time of the
> task.

So what people do today is shoot cpufreq in the head and not use it,
maybe that's the 'right' thing on these platforms.

> At the end, we will have set max frequency once the task has
> finished to run. There is no other solution than increasing the
> min_freq of cpufreq to a level that will ensure enough compute
> capacity for RT task with such high constraints that cpufreq can't
> react.

But you cannot a priori tell how much time RR/FIFO tasks will require,
that's the entire problem with them. We can compute a hysterical
average, but that _will_ mis predict the future and get you
underruns/deadline misses.

> For other RT tasks, we can probably found a way to set a
> frequency that can fit both RT constraints and power consumption.

You cannot, not without adding a lot more information about what these
tasks are doing, and that is not captured in the task model.