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authorTejun Heo <tj@kernel.org>2015-10-15 16:41:52 -0400
committerTejun Heo <tj@kernel.org>2015-10-15 16:41:52 -0400
commited27b9f7a17ddfbc007e16d4d11f33dff4fc2de7 (patch)
tree4b9afcef40ee73ffae9999777c66c7b14febbb84 /include/linux/cgroup-defs.h
parentecb9d535df967b3ca565535e14456f612373bf5e (diff)
cgroup: don't hold css_set_rwsem across css task iteration
css_sets are synchronized through css_set_rwsem but the locking scheme is kinda bizarre. The hot paths - fork and exit - have to write lock the rwsem making the rw part pointless; furthermore, many readers already hold cgroup_mutex. One of the readers is css task iteration. It read locks the rwsem over the entire duration of iteration. This leads to silly locking behavior. When cpuset tries to migrate processes of a cgroup to a different NUMA node, css_set_rwsem is held across the entire migration attempt which can take a long time locking out forking, exiting and other cgroup operations. This patch updates css task iteration so that it locks css_set_rwsem only while the iterator is being advanced. css task iteration involves two levels - css_set and task iteration. As css_sets in use are practically immutable, simply pinning the current one is enough for resuming iteration afterwards. Task iteration is tricky as tasks may leave their css_set while iteration is in progress. This is solved by keeping track of active iterators and advancing them if their next task leaves its css_set. v2: put_task_struct() in css_task_iter_next() moved outside css_set_rwsem. A later patch will add cgroup operations to task_struct free path which may grab the same lock and this avoids deadlock possibilities. css_set_move_task() updated to use list_for_each_entry_safe() when walking task_iters and advancing them. This is necessary as advancing an iter may remove it from the list. Signed-off-by: Tejun Heo <tj@kernel.org>
Diffstat (limited to 'include/linux/cgroup-defs.h')
-rw-r--r--include/linux/cgroup-defs.h3
1 files changed, 3 insertions, 0 deletions
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 17444505c870..62413c3e2f4b 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -211,6 +211,9 @@ struct css_set {
*/
struct list_head e_cset_node[CGROUP_SUBSYS_COUNT];
+ /* all css_task_iters currently walking this cset */
+ struct list_head task_iters;
+
/* For RCU-protected deletion */
struct rcu_head rcu_head;
};