linux-kernel - Re: [RFC] memcg/cgroup: do not fail fail on pre

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Message-ID: <507ECF28.1060602@parallels.com>
Date:	Wed, 17 Oct 2012 19:30:48 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	<linux-mm@...ck.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [RFC] memcg/cgroup: do not fail fail on pre_destroy callbacks

On 10/17/2012 05:30 PM, Michal Hocko wrote:
> Hi,
> memcg is the only controller which might fail in its pre_destroy
> callback which makes the cgroup core more complicated for no good
> reason. This is an attempt to change this unfortunate state. 
> 
> I am sending this a RFC because I would like to hear back whether the
> approach is correct. I thought that the changes would be more invasive
> but it seems that the current code was mostly prepared for this and it
> needs just some small tweaks (so I might be missing something important
> here).
> 
> The first two patches are just clean ups. They could be merged even
> without the rest.
> 
> The real change, although the code is not changed that much, is the 3rd
> patch. It changes the way how we handle mem_cgroup_move_parent failures.
> We have to realize that all those failures are *temporal*. Because we
> are either racing with the page removal or the page is temporarily off
> the LRU because of migration resp. global reclaim. As a result we do
> not fail mem_cgroup_force_empty_list if the page cannot be moved to the
> parent and rather retry until the LRU is empty.
> 
> The 4th patch is for cgroup core. I have moved cgroup_call_pre_destroy
> inside the cgroup_lock which is not very nice because the callbacks
> can take some time. Maybe we can move this call at the very end of the
> function?
> All I need for memcg is that cgroup_call_pre_destroy has been called and
> that no new cgroups can be attached to the group. The cgroup_lock is
> necessary for the later condition but if we move after CGRP_REMOVED flag
> is set then we are safe as well.
> 
> The last two patches are trivial follow ups for the cgroups core change
> because now we know that nobody will interfere with us so we can drop
> those empty && no child condition.
> 
> Comments, thoughts?
> 

I personally don't see anything fundamentally wrong with this.
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