[PATCH v2 03/14] memcg: update memcg_has_children() to use css_next_child()

Tejun Heo
2014-05-13T16:53:25Z

Currently, memcg_has_children() and mem_cgroup_hierarchy_write()
directly test cgroup->children for list emptiness. It's semantically
correct in traditional hierarchies as it actually wants to test for
any children dead or alive; however, cgroup->children is not a
published field and scheduled to go away.

This patch moves out .use_hierarchy test out of memcg_has_children()
and updates it to use css_next_child() to test whether there exists
any children. With .use_hierarchy test moved out, it can also be used
by mem_cgroup_hierarchy_write().

A side note: As .use_hierarchy is going away, it doesn't really matter
but I'm not sure about how it's used in __memcg_activate_kmem(). The
condition tested by memcg_has_children() is mushy when seen from
userland as its result is affected by dead csses which aren't visible
from userland. I think the rule would be a lot clearer if we have a
dedicated "freshly minted" flag which gets cleared when the first task
is migrated into it or the first child is created and then gate
activation with that.

v2: Added comment noting that testing use_hierarchy is the
responsibility of the callers of memcg_has_children() as suggested
by Michal Hocko.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
---
mm/memcontrol.c | 31 +++++++++++++++++++++----------
1 file changed, 21 insertions(+), 10 deletions(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4834,18 +4834,28 @@ static void mem_cgroup_reparent_charges(
} while (usage > 0);
}

+/*
+ * Test whether @memcg has children, dead or alive. Note that this
+ * function doesn't care whether @memcg has use_hierarchy enabled and
+ * returns %true if there are child csses according to the cgroup
+ * hierarchy. Testing use_hierarchy is the caller's responsiblity.
+ */
static inline bool memcg_has_children(struct mem_cgroup *memcg)
{
- lockdep_assert_held(&memcg_create_mutex);
+ bool ret;
+
/*
- * The lock does not prevent addition or deletion to the list
- * of children, but it prevents a new child from being
- * initialized based on this parent in css_online(), so it's
- * enough to decide whether hierarchically inherited
- * attributes can still be changed or not.
+ * The lock does not prevent addition or deletion of children, but
+ * it prevents a new child from being initialized based on this
+ * parent in css_online(), so it's enough to decide whether
+ * hierarchically inherited attributes can still be changed or not.
*/
- return memcg->use_hierarchy &&
- !list_empty(&memcg->css.cgroup->children);
+ lockdep_assert_held(&memcg_create_mutex);
+
+ rcu_read_lock();
+ ret = css_next_child(NULL, &memcg->css);
+ rcu_read_unlock();
+ return ret;
}

/*
@@ -4920,7 +4930,7 @@ static int mem_cgroup_hierarchy_write(st
*/
if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
(val == 1 || val == 0)) {
- if (list_empty(&memcg->css.cgroup->children))
+ if (!memcg_has_children(memcg))
memcg->use_hierarchy = val;
else
retval = -EBUSY;
@@ -5037,7 +5047,8 @@ static int __memcg_activate_kmem(struct
* of course permitted.
*/
mutex_lock(&memcg_create_mutex);
- if (cgroup_has_tasks(memcg->css.cgroup) || memcg_has_children(memcg))
+ if (cgroup_has_tasks(memcg->css.cgroup) ||
+ (memcg->use_hierarchy && memcg_has_children(memcg)))
err = -EBUSY;
mutex_unlock(&memcg_create_mutex);
if (err)