linux-kernel - Re: [PATCHSET cgroup/for-3.8] cpuset: decouple cpuset locking from cgroup core

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Date:	Fri, 30 Nov 2012 06:59:24 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	lizefan@...wei.com, paul@...lmenage.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	peterz@...radead.org, bsingharora@...il.com, hannes@...xchg.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET cgroup/for-3.8] cpuset: decouple cpuset locking from
 cgroup core

Hello,

On Fri, Nov 30, 2012 at 02:00:21PM +0400, Glauber Costa wrote:
> Now, what I am actually seeing with cgroup creation, is that the
> children will copy a lot of the values from the parent, like swappiness,
> hierarchy, etc. Once the child copies it, we should no longer be able to
> change those values in the parent: otherwise we'll get funny things like
> parent.use_hierarchy = 1, child.use_hierarchy = 0.

So, the best way to do this is from ->css_online().  If memcg
synchronizes and inherits from ->css_online(), it can guarantee that
the new cgroup will be visible in any following iterations.  Just have
an online flag which is turned on and off from ->css_on/offline() and
ignore any cgroups w/o online set.

> One option is to take a global lock in memcg_alloc_css(), and keep it
> locked until we did all the cgroup bookkeeping, and then unlock it in
> css_online. But I am guessing Tejun won't like it very much.

No, please *NEVER* *EVER* do that.  You'll be creating a bunch of
locking dependencies as cgroup walks through different controllers.

memcg should be able to synchornize fully both css on/offlining and
task attachments in memcg proper.  Let's please be boring about
locking.

Thanks.

-- 
tejun
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