On Fri 30-11-12 12:21:32, 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 ? Yes because we do not want to leak charges if we race with one of the above hierarchy operation. Swappiness and oom control are not a big deal. Same applies to migration policy change. Those could be solved by using the same memcg lock in the attach hook. Hierarchy policy change would be a bigger issue because the task is already linked to the group when the callback is called. Same applies to kmem_limit. Sorry I didn't have time to look into this deeper so I cannot offer any solution right now. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>