linux-kernel - Re: [PATCH] tick/powerclamp: Remove tick_nohz

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Date:	Fri, 19 Dec 2014 20:56:23 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
cc:	"Pan, Jacob jun" <jacob.jun.pan@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Viresh Kumar <viresh.kumar@...aro.org>, LKP <lkp@...org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Zhang, Rui" <rui.zhang@...el.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Eduardo Valentin <edubezval@...il.com>,
	"Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
Subject: Re: [PATCH] tick/powerclamp: Remove tick_nohz_idle abuse

On Fri, 19 Dec 2014, Jacob Pan wrote:

> On Thu, 18 Dec 2014 22:12:57 +0100 (CET)
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > On Thu, 18 Dec 2014, Jacob Pan wrote:
> > > OK I agree, also as I mentioned earlier, Peter already has a patch
> > > for consolidated idle loop and remove tick_nohz_idle_enter/exit
> > > call from powerclamp driver. I have been working on a few tweaks to
> > > maintain the functionality and efficiency with the consolidated
> > > idle loop. We can apply the patches on top of yours.
> > 
> > No. This is equally wrong as I pointed out before. The 'unified' idle
> > loop is still fake and just pretending to be idle.
> > 
> In terms of efficiency, the consolidated idle loop will allow turning
> off sched tick during idle injection period. If we just take out the
> tick_nohz_idle_xxx call, the effectiveness of powerclamp is going down
> significantly. I am not arguing the design but from fixing regression
> perspective or short term solution.

There is no perspective. Period.

Its violates every rightful assumption of the nohz_IDLE_* code and
just ever worked by chance. There is so much subtle wreckage lurking
there that the only sane solution is to forbid it. End of story.

Thanks,

	tglx
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