tree c6819de68cbc50f5ff0a0f1324f5cd98e8928ac9
parent d1830d122e2016ffd1557d6341821182a0dd7f45
author Tejun Heo <tj@kernel.org> 1483048971 -0500
committer Tejun Heo <tj@kernel.org> 1483048971 -0500

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>