linux-kernel - Re: [PATCH 3/4] sched: introduce synchronized idle injection

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Date:	Fri, 20 Nov 2015 10:45:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Morten Rasmussen <morten.rasmussen@....com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Len Brown <len.brown@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 3/4] sched: introduce synchronized idle injection

On Thu, 19 Nov 2015, Peter Zijlstra wrote:
> On Thu, Nov 19, 2015 at 05:24:07PM +0000, Morten Rasmussen wrote:
> > I would consider it an
> > emergency-only mechanism (as in emergency brake) that isn't really
> > suitable for normal thermal management. In which case: Does this sort of
> > mechanism belong in the scheduler code?
> 
> I would prefer it not to be, but Thomas is very much opposed to teaching
> the nohz code to 'work' on !idle threads.

The whole concept of faking idle is simply crap.

If you want to avoid that stuff in the scheduler, then create a
mechanism which just defers the next timer interrupt for X
milliseconds and does not any fiddling with NOHZ state and such.

That might hurt RT tasks, but if someone really cares about real-time
and deterministic behaviour, then running the machine on its thermal
limits is simply stupid. In fact any sensible RT system will bring
itself into a safe state way before the machine runs into that
condition.

Thanks,

	tglx
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