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

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Message-ID: <20131017165416.GW10651@twins.programming.kicks-ass.net>
Date:	Thu, 17 Oct 2013 18:54:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	"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 05:40:38PM +0100, Morten Rasmussen wrote:
> On Mon, Oct 14, 2013 at 04:14:25PM +0100, Arjan van de Ven wrote:
> > On 10/14/2013 6:33 AM, Peter Zijlstra wrote:
> > > On Fri, Oct 11, 2013 at 06:19:14PM +0100, Morten Rasmussen wrote:
> > >> Removing power hints for kworker threads enables easier use of
> > >> workqueues in the power driver late callback. That would otherwise
> > >> lead to an endless loop unless it is prevented in the power driver.
> > >
> > > There's many kworker users; some of them actually consume lots of
> > > cputime. Therefore how did you come to the conclusion that excepting all
> > > users was the better choice of a little added complexity in the one
> > > place where it actually matters?
> > 
> > .. and likely only for a very few architectures
> > 
> > x86, and I suspect modern ARM, can change frequency synchronously.
> > (using an instruction or maybe two or three for ARM)
> 
> It should be possible to implement synchronous frequency changes on most
> modern ARM platforms. It is a bit more than a few instructions to change
> frequency though particularly for the current cpufreq drivers.
> 
> cpufreq drivers, like the one for ARM TC2, uses the clock framework to
> manage clocks. clk_set_rate() is allowed to sleep which won't work if we
> call it from scheduler context. The clock framework will need a look if
> it doesn't provide a very fast synchronous alternative to clk_set_rate()
> to change frequency and we want to use it for scheduler driven frequency
> scaling.
> 
> cpufreq has pre- and post-change notifiers so the current TC2 clock driver
> 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. It should be
> possible to have very fast frequency changes on ARM but it is not the
> way it is currently done.


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.

Just say no to the existing cruft ?
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