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

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Date:	Thu, 18 Dec 2014 22:12:57 +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>
Subject: Re: [PATCH] tick/powerclamp: Remove tick_nohz_idle abuse

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.

If simple standard interfaces like cpu_idle() are not working from
idle code anymore then this simply stinks. And that's what any fake
idle thread will do.

The whole approach is wrong. Implement a sched fair throttler and you
can avoid the whoile trainwreck.
 
> > 1. Queue a deferable periodic timer whose handler checks if idle
> > needs to be injected. If so, it sets rq->need_throttle for the cpu.
> > If its already in the fake idle period, it clears rq->need_throttle
> > and sets need_resched.
> > 
> The key to powerclamp driver is to achieve package level idle
> states, which implies synchronized idle injection. From
> power/performance standpoint, only package level idle states is worth
> injection.

Then use a synchronized non deferrable timer on all cpus. It's simple
enough.
 
Thanks,

	tglx
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