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

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Message-ID: <5494D160.8060808@linux.vnet.ibm.com>
Date:	Sat, 20 Dec 2014 07:01:12 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:	"Pan, Jacob jun" <jacob.jun.pan@...el.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 12/20/2014 01:26 AM, Thomas Gleixner wrote:
> 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
> 
Hi Jacob,

Like Thomas pointed out, we can design a sane solution for powerclamp.
Idle injection is nothing but throttling of runqueue. If the runqueue is
throttled, no fair tasks will be selected and the natural choice in the
absence of tasks from any other sched class is the idle task.

The idle loop will automatically be called and the nohz state will also
fall in place. The cpu is really idle now: the runqueue has no tasks and
the task running on the cpu is the idle thread. The throttled tasks are
on a separate list.

When the period of idle injection is over, we unthrottle the runqueue.
All this being taken care of my a non-deferrable timer. This design
ensures that the intention of powerclamp is not hampered while at the
same time maintaining a sane state for nohz; you will get the efficiency
you want.

Of course there may be corner cases and challenges around
synchronization of package idle, which I am sure we can work around with
a better design such as the above. I am working on that patchset and
will post out in a day. You can take a look and let us know the pieces
we are missing.

I find that implementing the above design is not too hard.

Regards
Preeti U Murthy

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