cgroup: remove unnecessary empty check when enabling threaded mode (918a8c2c) · Commits · 戴 / test

cgroup_enable_threaded() checks that the cgroup doesn't have any tasks or children and fails the operation if so. This test is unnecessary because the first part is already checked by cgroup_can_be_thread_root() and the latter is unnecessary. The latter actually cause a behavioral oddity. Please consider the following hierarchy. All cgroups are domains. A / \ B C \ D If B is made threaded, C and D becomes invalid domains. Due to the no children restriction, threaded mode can't be enabled on C. For C and D, the only thing the user can do is removal. There is no reason for this restriction. Remove it. Acked-by:

Waiman Long <longman@redhat.com> Signed-off-by:

Tejun Heo <tj@kernel.org>

Documentation/cgroup-v2.txt

+3 −2

Original line number	Diff line number	Diff line
		@@ -274,8 +274,9 @@ thread mode, the following conditions must be met.
		- As the cgroup will join the parent's resource domain. The parent
		must either be a valid (threaded) domain or a threaded cgroup.

		- The cgroup must be empty. No enabled controllers, child cgroups or
		processes.
		- When the parent is an unthreaded domain, it must not have any domain
		controllers enabled or populated domain children. The root is
		exempt from this requirement.

		Topology-wise, a cgroup can be in an invalid state. Please consider
		the following toplogy::

kernel/cgroup/cgroup.c

+0 −7

Original line number	Diff line number	Diff line
		@@ -3146,13 +3146,6 @@ static int cgroup_enable_threaded(struct cgroup *cgrp)
		!cgroup_can_be_thread_root(dom_cgrp))
		return -EOPNOTSUPP;

		/*
		* Allow enabling thread mode only on empty cgroups to avoid
		* implicit migrations and recursive operations.
		*/
		if (cgroup_has_tasks(cgrp) \|\| css_has_online_children(&cgrp->self))
		return -EBUSY;

		/*
		* The following shouldn't cause actual migrations and should
		* always succeed.

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