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

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Message-ID: <4F96D1A8.6040604@parallels.com>
Date:	Tue, 24 Apr 2012 13:15:36 -0300
From:	Glauber Costa <glommer@...allels.com>
To:	Christoph Lameter <cl@...ux.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 04/24/2012 12:02 PM, Christoph Lameter wrote:
> On Mon, 23 Apr 2012, Glauber Costa wrote:
>
>> This will allow us to call destroy() without holding the
>> cgroup_mutex(). Other important updates inside update_flags()
>> are protected by the callback_mutex.
>>
>> We could protect this variable with the callback_mutex as well,
>> as suggested by Li Zefan, but we need to make sure we are protected
>> by that mutex at all times, and some of its updates happen inside the
>> cgroup_mutex - which means we would deadlock.
>
> 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.
>
Well,

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.

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