linux-kernel - Re: [patch v2 3/6] memcg: rework mem_cgroup

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Date:	Wed, 28 Nov 2012 13:23:57 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>, Tejun Heo <htejun@...il.com>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators

On 11/28/2012 01:17 PM, Michal Hocko wrote:
> On Wed 28-11-12 17:47:59, KAMEZAWA Hiroyuki wrote:
>> (2012/11/27 3:47), Michal Hocko wrote:
> [...]
>>> +		/*
>>> +		 * Even if we found a group we have to make sure it is alive.
>>> +		 * css && !memcg means that the groups should be skipped and
>>> +		 * we should continue the tree walk.
>>> +		 * last_visited css is safe to use because it is protected by
>>> +		 * css_get and the tree walk is rcu safe.
>>> +		 */
>>> +		if (css == &root->css || (css && css_tryget(css)))
>>> +			memcg = mem_cgroup_from_css(css);
>>
>> Could you note that this iterator will never visit dangling(removed)
>> memcg, somewhere ?
> 
> OK, I can add it to the function comment but the behavior hasn't changed
> so I wouldn't like to confuse anybody.
> 
>> Hmm, I'm not sure but it may be trouble at shrkinking dangling
>> kmem_cache(slab).
> 
> We do not shrink slab at all. 

yet. However...

> Those objects that are in a dead memcg
> wait for their owner tho release them which will make the dangling group
> eventually go away
> 
>>
>> Costa, how do you think ?
>>

In general, I particularly believe it is a good idea to skip dead memcgs
in the iterator. I don't anticipate any problems with shrinking at all.

Basically, we will only ever actively shrink when the memcg is dying, at
which point it is still alive.

After this, it's better to just leave it to vmscan. Whenever there is
pressure, it will go away.

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