cgroup: call subsys->*attach() only for subsystems which are actually affected by migration

Currently, subsys->*attach() callbacks are called for all subsystems
which are attached to the hierarchy on which the migration is taking
place.

With cgroup_migrate_prepare_dst() filtering out identity migrations,
v1 hierarchies can avoid spurious ->*attach() callback invocations
where the source and destination csses are identical; however, this
isn't enough on v2 as only a subset of the attached controllers can be
affected on controller enable/disable.

While spurious ->*attach() invocations aren't critically broken,
they're unnecessary overhead and can lead to temporary overcharges on
certain controllers.  Fix it by tracking which subsystems are affected
by a migration and invoking ->*attach() callbacks only on those
subsystems.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 5f8c8ac5ab88..9203bfb05603 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -62,6 +62,9 @@ struct cgroup_mgctx {
 
 	/* tasks and csets to migrate */
 	struct cgroup_taskset	tset;
+
+	/* subsystems affected by migration */
+	u16			ss_mask;
 };
 
 #define CGROUP_TASKSET_INIT(tset)						\
@@ -172,7 +175,7 @@ void cgroup_migrate_add_src(struct css_set *src_cset, struct cgroup *dst_cgrp,
 			    struct cgroup_mgctx *mgctx);
 int cgroup_migrate_prepare_dst(struct cgroup_mgctx *mgctx);
 int cgroup_migrate(struct task_struct *leader, bool threadgroup,
-		   struct cgroup_mgctx *mgctx, struct cgroup_root *root);
+		   struct cgroup_mgctx *mgctx);
 
 int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 		       bool threadgroup);