Commit ab3f5faa authored Feb 06, 2014 by Hugh Dickins Committed by Tejun Heo Feb 07, 2014

cgroup: use an ordered workqueue for cgroup destruction

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.

tj: Committing as the temporary fix until the reverse dependency can
be removed from memcg. Comment updated accordingly.

Fixes: e5fca243 ("cgroup: use a dedicated workqueue for cgroup destruction")
Suggested-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Tejun Heo <tj@kernel.org>

parent 0a6be655

kernel/cgroup.c

+6 −2

Original line number	Diff line number	Diff line
		@@ -4845,12 +4845,16 @@ static int __init cgroup_wq_init(void)
		/*
		* There isn't much point in executing destruction path in
		* parallel. Good chunk is serialized with cgroup_mutex anyway.
		* Use 1 for @max_active.
		*
		* XXX: Must be ordered to make sure parent is offlined after
		* children. The ordering requirement is for memcg where a
		* parent's offline may wait for a child's leading to deadlock. In
		* the long term, this should be fixed from memcg side.
		*
		* We would prefer to do this in cgroup_init() above, but that
		* is called before init_workqueues(): so leave this until after.
		*/
		cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
		cgroup_destroy_wq = alloc_ordered_workqueue("cgroup_destroy", 0);
		BUG_ON(!cgroup_destroy_wq);

		/*

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