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

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Date:	Tue, 10 Nov 2015 10:07:35 +0000
From:	Juri Lelli <juri.lelli@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Punit Agrawal <punit.agrawal@....com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	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

Hi,

On 9 November 2015 at 14:15, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Nov 09, 2015 at 11:56:51AM +0000, Punit Agrawal wrote:
>> Jacob Pan <jacob.jun.pan@...ux.intel.com> writes:
>> > 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.
>
> Audio/Video playback generally doesn't take a _lot_ of time these days.
> What is important though is _when_ it happens.
>
> And media playback typically already has a very well defined and stable
> cadence (24Hz or whatnot).  What you want is for your idle injector to
> sync up with that, not disrupt it.
>

Do you think that using SCHED_DEADLINE here would be completely
foolish? I mean, we would have the duty_cycle/period thing for free, it
would be know to the scheduler (as to maybe address Thomas' concerns)
and we could think to make idle injection part of system analysis (for
the soft-RT use cases).

Thanks,

- Juri

> For other workloads, missing a deadline is about as bad as destroying
> the chip, complete system shutdown might be safer than getting delayed.
> (The very tired scenario of a saw, a laser and your finger; you want to
> shut down the entire machine rather than just cut off your finger.)
>
>
>
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