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

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Message-ID: <50B5DAEE.6060009@parallels.com>
Date:	Wed, 28 Nov 2012 13:35:42 +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 01:33 PM, Michal Hocko wrote:
> On Wed 28-11-12 13:23:57, Glauber Costa wrote:
>> 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.
> 
> We even cannot iterate dead ones because their cgroups are gone and so
> you do not have any way to iterate. So either make them alive by css_get
> or we cannot iterate them.
> 

We are in full agreement.


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