netdev - Re: [PATCH v2 3/5] change number_of

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Message-ID: <alpine.DEB.2.00.1204241120130.26005@router.home>
Date:	Tue, 24 Apr 2012 11:24:14 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Glauber Costa <glommer@...allels.com>
cc:	Tejun Heo <tj@...nel.org>, netdev@...r.kernel.org,
	cgroups@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	kamezawa.hiroyu@...fujitsu.com, David Miller <davem@...emloft.net>,
	devel@...nvz.org
Subject: Re: [PATCH v2 3/5] change number_of_cpusets to an atomic

On Tue, 24 Apr 2012, Glauber Costa wrote:

> > Would this not also be a good case to introduce static branching?
> >
> > number_of_cpusets is used to avoid going through unnecessary processing
> > should there be no cpusets in use.
>
> static branches comes with a set of problems themselves, so I usually prefer
> to use them only in places where we don't want to pay even a cache miss if we
> can avoid, or a function call, or anything like that - like the slub cache
> alloc as you may have seen in my kmem memcg series.
>
> It doesn't seem to be the case here.

How did you figure that? number_of_cpusets was introduced exactly because
the functions are used in places where we do not pay the cost of calling
__cpuset_node_allowed_soft/hardwall. Have a look at these. They may take
locks etc etc in critical allocation paths
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