Re: [RFC][PATCH 4/7] sched: power: Remove power capacity hints for kworker threads

[Morten Rasmussen <morten.rasmussen@arm.com>]

On Thu, Oct 17, 2013 at 06:18:38PM +0100, Arjan van de Ven wrote:
> >>
> >> cpufreq has pre- and post-change notifiers so the current TC2 clock driver
> 
> yeah those are EVIL ;-)
> 
> >> waits (yields) in its clk_set_rate() implementation until the change has
> >> happened to ensure that the post-change notifier happens at the right
> >> time. Since clk_set_rate() is allowed to sleep other tasks may be
> >> running while waiting for the change to complete. This may be true for
> >> other clock drivers as well.
> >>
> >> AFAICT, there is no way to reuse the existing cpufreq drivers in a
> >> sensible way for scheduler driven frequency scaling.
> 
> that's the conclusion we came to as well about a year ago (and is also
> why we're no longer using cpufreq core for the Intel pstate driver.
> the locking/sleeping/callback/cpuhotplug/sysfs/etc stuff is just a MESS
> for something that ends up being extremely simple if you just code the
> sequence... for us it's just one register write to change... which shows this as an
> extreme obviously)

We should be able to boil it down to a sequence on ARM as well. But it
means dropping cpufreq and looking at the clock framework.

Are you still using the pre- and post-change notifiers on Intel, or can
they be ignored safely?

> 
> 
> >
> > Note that you still have preemption disabled in your late callback from
> > finish_task_switch(). There's no way you can wait/yield/whatever from
> > there. Nor is that really sane.
> 
> the other fun one with this could be that if you have a series of scheduleable tasks
> for changing stuff.... somehow you want to keep ordering in the requests, and only do
> the last one/etc.
> Not Fun(tm)

Agreed, I don't want to go there. Also, the overhead will probably kill any
benefit that there might be.