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

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Date:	Fri, 30 Nov 2012 13:42:28 +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 01:24 PM, Michal Hocko wrote:
> On Fri 30-11-12 13:00:36, Glauber Costa wrote:
>> On 11/30/2012 07:21 AM, Kamezawa Hiroyuki wrote:
>>> (2012/11/29 6:34), Tejun Heo wrote:
>>>> Hello, guys.
>>>>
>>>> Depending on cgroup core locking - cgroup_mutex - is messy and makes
>>>> cgroup prone to locking dependency problems.  The current code already
>>>> has lock dependency loop - memcg nests get_online_cpus() inside
>>>> cgroup_mutex.  cpuset the other way around.
>>>>
>>>> Regardless of the locking details, whatever is protecting cgroup has
>>>> inherently to be something outer to most other locking constructs.
>>>> cgroup calls into a lot of major subsystems which in turn have to
>>>> perform subsystem-specific locking.  Trying to nest cgroup
>>>> synchronization inside other locks isn't something which can work
>>>> well.
>>>>
>>>> cgroup now has enough API to allow subsystems to implement their own
>>>> locking and cgroup_mutex is scheduled to be made private to cgroup
>>>> core.  This patchset makes cpuset implement its own locking instead of
>>>> relying on cgroup_mutex.
>>>>
>>>> cpuset is rather nasty in this respect.  Some of it seems to have come
>>>> from the implementation history - cgroup core grew out of cpuset - but
>>>> big part stems from cpuset's need to migrate tasks to an ancestor
>>>> cgroup when an hotunplug event makes a cpuset empty (w/o any cpu or
>>>> memory).
>>>>
>>>> This patchset decouples cpuset locking from cgroup_mutex.  After the
>>>> patchset, cpuset uses cpuset-specific cpuset_mutex instead of
>>>> cgroup_mutex.  This also removes the lockdep warning triggered during
>>>> cpu offlining (see 0009).
>>>>
>>>> Note that this leaves memcg as the only external user of cgroup_mutex.
>>>> Michal, Kame, can you guys please convert memcg to use its own locking
>>>> too?
>>>>
>>>
>>> Hmm. let me see....at quick glance cgroup_lock() is used at
>>>   hierarchy policy change
>>>   kmem_limit
>>>   migration policy change
>>>   swapiness change
>>>   oom control
>>>
>>> Because all aboves takes care of changes in hierarchy,
>>> Having a new memcg's mutex in ->create() may be a way.
>>>
>>> Ah, hm, Costa is mentioning task-attach. is the task-attach problem in memcg ?
>>>
>>
>> We disallow the kmem limit to be set if a task already exists in the
>> cgroup. So we can't allow a new task to attach if we are setting the limit.
> 
> This is racy without additional locking, isn't it?
> 
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?


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