linux-kernel - Re: [RFC PATCH v3 3/6] sched: pack small tasks

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Date:	Wed, 27 Mar 2013 09:46:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, mingo@...nel.org,
	linux@....linux.org.uk, pjt@...gle.com, santosh.shilimkar@...com,
	morten.rasmussen@....com, chander.kashyap@...aro.org,
	cmetcalf@...era.com, tony.luck@...el.com, alex.shi@...el.com,
	preeti@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, len.brown@...el.com, amit.kucheria@...aro.org,
	corbet@....net
Subject: Re: [RFC PATCH v3 3/6] sched: pack small tasks

On Tue, 2013-03-26 at 08:29 -0700, Arjan van de Ven wrote:
> > Isn't this basically related to picking the NO_HZ cpu; if the system
> > isn't fully symmetric with its power gates you want the NO_HZ cpu to be
> > the 'special' cpu. If it is symmetric we really don't care which core
> > is left 'running' and we can even select a new pack cpu from the idle
> > cores once the old one is fully utilized.
> 
> you don't really care much sure, but there's some advantages for sorting "all the way left",
> e.g. to linux cpu 0.
> Some tasks only run there, and interrupts tend to be favored to that cpu as well on x86.

Right, and I suspect all the big-little nonsense will have the little
cores on low numbers as well (is this architected or can a creative
licensee screw us over?)

So find_new_ilb() already does cpumask_first(), so it has a strong
leftmost preference. We just need to make sure it indeed does the right
thing and doesn't have some unintended side effect.

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