cgroup: use css_set->mg_dst_cgrp for the migration target cgroup
Migration can be multi-target on the default hierarchy when a
controller is enabled - processes belonging to each child cgroup have
to be moved to the child cgroup itself to refresh css association.
This isn't a problem for cgroup_migrate_add_src() as each source
css_set still maps to single source and target cgroups; however,
cgroup_migrate_prepare_dst() is called once after all source css_sets
are added and thus might not have a single destination cgroup. This
is currently worked around by specifying NULL for @dst_cgrp and using
the source's default cgroup as destination as the only multi-target
migration in use is self-targetting. While this works, it's subtle
and clunky.
As all taget cgroups are already specified while preparing the source
css_sets, this clunkiness can easily be removed by recording the
target cgroup in each source css_set. This patch adds
css_set->mg_dst_cgrp which is recorded on cgroup_migrate_src() and
used by cgroup_migrate_prepare_dst(). This also makes migration code
ready for arbitrary multi-target migration.
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index aae8c94..590291d 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -191,12 +191,13 @@
/*
* If this cset is acting as the source of migration the following
- * two fields are set. mg_src_cgrp is the source cgroup of the
- * on-going migration and mg_dst_cset is the destination cset the
- * target tasks on this cset should be migrated to. Protected by
- * cgroup_mutex.
+ * two fields are set. mg_src_cgrp and mg_dst_cgrp are
+ * respectively the source and destination cgroups of the on-going
+ * migration. mg_dst_cset is the destination cset the target tasks
+ * on this cset should be migrated to. Protected by cgroup_mutex.
*/
struct cgroup *mg_src_cgrp;
+ struct cgroup *mg_dst_cgrp;
struct css_set *mg_dst_cset;
/*