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

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Date:	Fri, 30 Nov 2012 14:00:21 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>, <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

On 11/30/2012 01:49 PM, Michal Hocko wrote:
> On Fri 30-11-12 13:42:28, Glauber Costa wrote:
> [...]
>> Speaking of it: Tejun's tree still lacks the kmem bits. How hard would
>> it be for you to merge his branch into a temporary branch of your tree?
> 
> review-cpuset-locking is based on a post merge window merges so I cannot
> merge it as is. I could cherry-pick the series after it is settled. I
> have no idea how much conflicts this would bring, though.
> 
Ok.

I believe the task problem only exist for us for kmem. So I could come
up with a patchset that only deals with child cgroup creation, and
ignore attach for now. So long as we have a mechanism that will work for
it, and don't get lost and forget to patch it when the trees are merged.

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.

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.

What do you think about a children counter? If we are going to do things
similar to the attach_in_progress of cpuset, we might very well turn it
into a direct counter so we don't have to iterate at all.

The code would look like: (simplified example for use_hierarchy)

memcg_lock();
if (memcg->nr_children != 0)
    return -EINVAL;
else
    memcg->use_hierarchy = val
memcg_unlock()

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