Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity

Peter Zijlstra
2016-08-31T14:39:33Z

On Fri, Aug 26, 2016 at 11:40:48AM -0700, Steve Muckle wrote:
> A policy of going to fmax on any RT activity will be detrimental
> for power on many platforms. Often RT accounts for only a small amount
> of CPU activity so sending the CPU frequency to fmax is overkill. Worse
> still, some platforms may not be able to even complete the CPU frequency
> change before the RT activity has already completed.
>
> Cpufreq governors have not treated RT activity this way in the past so
> it is not part of the expected semantics of the RT scheduling class. The
> DL class offers guarantees about task completion and could be used for
> this purpose.

Not entirely true. People have simply disabled cpufreq because of this.

Yes, RR/FIFO are a pain, but they should still be deterministic, and
variable cpufreq destroys that.

I realize that the fmax thing is annoying, but I'm not seeing how rt_avg
is much better.