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Subject[PATCH 3.12 155/212] cgroup: use a dedicated workqueue for cgroup destruction
3.12-stable review patch.  If anyone has any objections, please let me know.


From: Tejun Heo <>

commit e5fca243abae1445afbfceebda5f08462ef869d3 upstream.

Since be44562613851 ("cgroup: remove synchronize_rcu() from
cgroup_diput()"), cgroup destruction path makes use of workqueue. css
freeing is performed from a work item from that point on and a later
commit, ea15f8ccdb430 ("cgroup: split cgroup destruction into two
steps"), moves css offlining to workqueue too.

As cgroup destruction isn't depended upon for memory reclaim, the
destruction work items were put on the system_wq; unfortunately, some
controller may block in the destruction path for considerable duration
while holding cgroup_mutex. As large part of destruction path is
synchronized through cgroup_mutex, when combined with high rate of
cgroup removals, this has potential to fill up system_wq's max_active
of 256.

Also, it turns out that memcg's css destruction path ends up queueing
and waiting for work items on system_wq through work_on_cpu(). If
such operation happens while system_wq is fully occupied by cgroup
destruction work items, work_on_cpu() can't make forward progress
because system_wq is full and other destruction work items on
system_wq can't make forward progress because the work item waiting
for work_on_cpu() is holding cgroup_mutex, leading to deadlock.

This can be fixed by queueing destruction work items on a separate
workqueue. This patch creates a dedicated workqueue -
cgroup_destroy_wq - for this purpose. As these work items shouldn't
have inter-dependencies and mostly serialized by cgroup_mutex anyway,
giving high concurrency level doesn't buy anything and the workqueue's
@max_active is set to 1 so that destruction work items are executed
one by one on each CPU.

Hugh Dickins: Because cgroup_init() is run before init_workqueues(),
cgroup_destroy_wq can't be allocated from cgroup_init(). Do it from a
separate core_initcall(). In the future, we probably want to reorder
so that workqueue init happens before cgroup_init().

Signed-off-by: Tejun Heo <>
Reported-by: Hugh Dickins <>
Reported-by: Shawn Bohrer <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/cgroup.c | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -90,6 +90,14 @@ static DEFINE_MUTEX(cgroup_mutex);
static DEFINE_MUTEX(cgroup_root_mutex);

+ * cgroup destruction makes heavy use of work items and there can be a lot
+ * of concurrent destructions. Use a separate workqueue so that cgroup
+ * destruction work items don't end up filling up max_active of system_wq
+ * which may lead to deadlock.
+ */
+static struct workqueue_struct *cgroup_destroy_wq;
* Generate an array of cgroup subsystem pointers. At boot time, this is
* populated with the built in subsystems, and modular subsystems are
* registered after that. The mutable section of this array is protected by
@@ -908,7 +916,7 @@ static void cgroup_free_rcu(struct rcu_h
struct cgroup *cgrp = container_of(head, struct cgroup, rcu_head);

INIT_WORK(&cgrp->destroy_work, cgroup_free_fn);
- schedule_work(&cgrp->destroy_work);
+ queue_work(cgroup_destroy_wq, &cgrp->destroy_work);

static void cgroup_diput(struct dentry *dentry, struct inode *inode)
@@ -4306,7 +4314,7 @@ static void css_free_rcu_fn(struct rcu_h
* css_put(). dput() requires process context which we don't have.
INIT_WORK(&css->destroy_work, css_free_work_fn);
- schedule_work(&css->destroy_work);
+ queue_work(cgroup_destroy_wq, &css->destroy_work);

static void css_release(struct percpu_ref *ref)
@@ -4603,7 +4611,7 @@ static void css_killed_ref_fn(struct per
container_of(ref, struct cgroup_subsys_state, refcnt);

INIT_WORK(&css->destroy_work, css_killed_work_fn);
- schedule_work(&css->destroy_work);
+ queue_work(cgroup_destroy_wq, &css->destroy_work);

@@ -5139,6 +5147,22 @@ out:
return err;

+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.
+ *
+ * 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);
+ BUG_ON(!cgroup_destroy_wq);
+ return 0;
* proc_cgroup_show()
* - Print task's cgroup paths into seq_file, one line for each hierarchy

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