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

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Date:	Fri, 30 Nov 2012 13:00:36 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Tejun Heo <tj@...nel.org>, <lizefan@...wei.com>,
	<paul@...lmenage.org>, <containers@...ts.linux-foundation.org>,
	<cgroups@...r.kernel.org>, <peterz@...radead.org>,
	<mhocko@...e.cz>, <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 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.


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