linux-kernel - Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose number of tasks running on cpu

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Date:	Tue, 15 Jul 2014 14:59:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S.Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Chandramouli Narayanan <mouli@...ux.intel.com>,
	Vinodh Gopal <vinodh.gopal@...el.com>,
	James Guilford <james.guilford@...el.com>,
	Wajdi Feghali <wajdi.k.feghali@...el.com>,
	Jussi Kivilinna <jussi.kivilinna@....fi>,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose
 number of tasks running on cpu

On Tue, 15 Jul 2014, Peter Zijlstra wrote:

> On Tue, Jul 15, 2014 at 11:50:45AM +0200, Peter Zijlstra wrote:
> > So you already have an idle notifier (which is x86 only, we should fix
> > that I suppose), and you then double check there really isn't anything
> > else running.
> 
> Note that we've already done a large part of the expense of going idle
> by the time we call that idle notifier -- in specific, we've
> reprogrammed the clock to stop the tick.
> 
> Its really wasteful to then generate work again, which means we have to
> again reprogram the clock etc.

Doing anything which is not related to idle itself in the idle
notifier is just plain wrong.

If that stuff wants to utilize idle slots, we really need to come up
with a generic and general solution. Otherwise we'll grow those warts
all over the architecture space, with slightly different ways of
wreckaging the world an some more.

This whole attidute of people thinking that they need their own
specialized scheduling around the real scheduler is a PITA. All this
stuff is just damanging any sensible approach of power saving, load
balancing, etc.

What we really want is infrastructure, which allows the scheduler to
actively query the async work situation and based on the results
actively decide when to process it and where.

Thanks,

	tglx

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