linux-kernel - Re: [RFC PATCH 0/3] CFS idle injection

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Message-ID: <9hhbnb38jy4.fsf@e105922-lin.cambridge.arm.com>
Date:	Mon, 09 Nov 2015 11:56:51 +0000
From:	Punit Agrawal <punit.agrawal@....com>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Eduardo Valentin <edubezval@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Paul Turner <pjt@...gle.com>, Len Brown <len.brown@...el.com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <andi.kleen@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>
Subject: Re: [RFC PATCH 0/3] CFS idle injection

Jacob Pan <jacob.jun.pan@...ux.intel.com> writes:

> On Fri, 06 Nov 2015 16:50:15 +0000
> Punit Agrawal <punit.agrawal@....com> wrote:
>
>> * idle injection once frequencies have been capped to the lowest
>>   feasible values (as suggested in the cover letter)
>> 
> actually, I was suggesting to start considering idle injection once
> frequency capped to the energy efficient point, which can be much
> higher than the lowest frequency. The idea being, deep idle power is
> negligible compared to running power which allows near linear
> power-perf scaling for balanced workload.
> Below energy efficient frequency, continuous lowering frequency may
> lose disproportion performance vs. power. i.e. worse than linear.
>

I agree. I was making that assumption that with the ability to inject
idle states, there wouldn't be a need to expose the inefficient
frequency states.

Do you still see a reason to do that?

>> One question about the implementation in these patches - should the
>> implementation hook into pick_next_task in core instead of CFS? Higher
>> priority tasks might get in the way of idle injection.
> My take is that RT and throttling will never go well together since they
> are conflicting in principle.

I am not sure I follow. If RT (or other higher priority classes) can't
be throttled then the CPUs are not able to contribute towards
constraining power consumption and hence temperature.

This is especially true in certain platforms where tasks belong to the
RT class to maintain user experience, e.g., audio and video. 

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