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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Message-ID: <20140715120728.GR3588@twins.programming.kicks-ass.net>
Date:	Tue, 15 Jul 2014 14:07:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	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, 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.

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