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