linux-kernel - Re: [RFC][PATCH 0/7] memcg: make memcg's life cycle the same as cgroup

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Date:	Sun, 7 Apr 2013 14:00:24 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	<linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Glauber Costa <glommer@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC][PATCH 0/7] memcg: make memcg's life cycle the same as cgroup

On 2013/4/4 20:00, Michal Hocko wrote:
> On Wed 03-04-13 17:11:15, Li Zefan wrote:
>> (I'll be off from my office soon, and I won't be responsive in the following
>> 3 days.)
>>
>> I'm working on converting memcg to use cgroup->id, and then we can kill css_id.
>>
>> Now memcg has its own refcnt, so when a cgroup is destroyed, the memcg can
>> still be alive. This patchset converts memcg to always use css_get/put, so
>> memcg will have the same life cycle as its corresponding cgroup, and then
>> it's always safe for memcg to use cgroup->id.
>>
>> The historical reason that memcg didn't use css_get in some cases, is that
>> cgroup couldn't be removed if there're still css refs. The situation has
>> changed so that rmdir a cgroup will succeed regardless css refs, but won't
>> be freed until css refs goes down to 0.
>>
>> This is an early post, and it's NOT TESTED. I just want to see if the changes
>> are fine in general.
> 
> yes, I like the approach and it looks correct as well (some minor things
> mentioned in the patches). Thanks a lot Li! This will make our lifes much
> easier. The separate ref counting was PITA especially after
> introduction of kmem accounting which made its usage even more trickier.
> 
>> btw, after this patchset I think we don't need to free memcg via RCU, because
>> cgroup is already freed in RCU callback.
> 
> But this depends on changes waiting in for-3.10 branch, right?

What changes? memcg changes or cgroup core changes? I don't think this depends
on anything in cgroup 3.10 branch.

> Anyway, I think we should be safe with the workqueue based releasing as
> well once mem_cgroup_{get,put} are gone, right?
> 

cgroup calls mem_cgroup_css_free() in a work function, so seems memcg doesn't
need to use RCU or workqueue in mem_cgroup_css_free().

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