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

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Date:	Fri, 18 Oct 2013 09:47:37 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"pjt@...gle.com" <pjt@...gle.com>, "rjw@...k.pl" <rjw@...k.pl>,
	"dirk.j.brandewie@...el.com" <dirk.j.brandewie@...el.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"alex.shi@...aro.org" <alex.shi@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"efault@....de" <efault@....de>, "corbet@....net" <corbet@....net>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC][PATCH 4/7] sched: power: Remove power capacity hints for
 kworker threads

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.

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