linux-kernel - Re: [PATCH 2/5] sched: factor out code to should_we

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Date:	Tue, 02 Apr 2013 10:10:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
	Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 2/5] sched: factor out code to should_we_balance()

On Thu, 2013-03-28 at 16:58 +0900, Joonsoo Kim wrote:
> Now checking that this cpu is appropriate to balance is embedded into
> update_sg_lb_stats() and this checking has no direct relationship to
> this
> function.
> 
> There is not enough reason to place this checking at
> update_sg_lb_stats(),
> except saving one iteration for sched_group_cpus.

Its only one iteration if there's only 2 groups, but there can be more
than 2, take any desktop Intel i7, it will have 4-8 cores, each with
HT; thus the CPU domain will have 4-8 groups.

And note that local_group is always the first group of a domain, so
we'd stop the balance at the first group and avoid touching the other
3-7, avoiding touching cachelines on 6-14 cpus.

So this short-cut does make sense.. its not pretty, granted, but
killing it doesn't seem right.

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