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SubjectRe: [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 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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