On Fri 07-02-14 09:04:02, Tejun Heo wrote: > Hello, Hugh. > > On Thu, Feb 06, 2014 at 03:56:01PM -0800, Hugh Dickins wrote: > > Sometimes the cleanup after memcg hierarchy testing gets stuck in > > mem_cgroup_reparent_charges(), unable to bring non-kmem usage down to 0. > > > > There may turn out to be several causes, but a major cause is this: the > > workitem to offline parent can get run before workitem to offline child; > > parent's mem_cgroup_reparent_charges() circles around waiting for the > > child's pages to be reparented to its lrus, but it's holding cgroup_mutex > > which prevents the child from reaching its mem_cgroup_reparent_charges(). > > > > Just use an ordered workqueue for cgroup_destroy_wq. > > Hmmm... I'm not really comfortable with this. This would seal shut > any possiblity of increasing concurrency in that path, which is okay > now but I find the combination of such long term commitment and the > non-obviousness (it's not apparent from looking at memcg code why it > wouldn't deadlock) very unappealing. Besides, the only reason > offline() is currently called under cgroup_mutex is history. We can > move it out of cgroup_mutex right now. > > But even with offline being called outside cgroup_mutex, IIRC, the > described problem would still be able to deadlock as long as the tree > depth is deeper than max concurrency level of the destruction > workqueue. Sure, we can give it large enough number but it's > generally nasty. > > One thing I don't get is why memcg has such reverse dependency at all. > Why does the parent wait for its descendants to do something during > offline? Because the parent sees charges of its children but it doesn't see pages as they are on the LRU of those children. So it cannot reach 0 charges. We are are assuming that the offlining memcg doesn't have any children which sounds like a reasonable expectation to me. > Shouldn't it be able to just bail and let whatever > descendant which is stil busy propagate things upwards? That's a > usual pattern we use to tree shutdowns anyway. Would that be nasty to > implement in memcg? Hmm, this is a bit tricky. We cannot use memcg iterators to reach children because css_tryget would fail on them. We can use cgroup iterators instead, alright, and reparent pages from leafs but this all sounds like a lot of complications. Another option would be weakening css_offline reparenting and do not insist on having 0 charges. We want to get rid of as many charges as possible but do not need to have all of them gone (http://marc.info/?l=linux-kernel&m=139161412932193&w=2). The last part would be reparenting to the upmost parent which is still online. I guess this is implementable but I would prefer Hugh's fix for now and for stable. -- 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>