[PATCH 11/12] cgroup: prepare migration path for unified hierarchy

From: Tejun Heo
Date: Thu Mar 27 2014 - 22:42:38 EST

Unified hierarchy implementation would require re-migrating tasks onto
the same cgroup on the default hierarchy to reflect updated effective
csses. Update cgroup_migrate_prepare_dst() so that it accepts NULL as
the destination cgrp. When NULL is specified, the destination is
considered to be the cgroup on the default hierarchy associated with
each css_set.

After this change, the identity check in cgroup_migrate_add_src()
isn't sufficient for noop detection as the associated csses may change
without any cgroup association changing. The only way to tell whether
a migration is noop or not is testing whether the source and
destination csets are identical. The noop check in
cgroup_migrate_add_src() is removed and cset identity test is added to
cgroup_migreate_prepare_dst(). If it's detected that source and
destination csets are identical, the cset is removed removed from
@preloaded_csets and all the migration nodes are cleared which makes
cgroup_migrate() ignore the cset.

Also, make the function append the destination css_sets to
@preloaded_list so that destination css_sets always come after source

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
kernel/cgroup.c | 34 +++++++++++++++++++++++-----------
1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0c67141..5faa115 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1901,10 +1901,6 @@ static void cgroup_migrate_add_src(struct css_set *src_cset,

src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);

- /* nothing to do if this cset already belongs to the cgroup */
- if (src_cgrp == dst_cgrp)
- return;
if (!list_empty(&src_cset->mg_preload_node))

@@ -1919,13 +1915,14 @@ static void cgroup_migrate_add_src(struct css_set *src_cset,

* cgroup_migrate_prepare_dst - prepare destination css_sets for migration
- * @dst_cgrp: the destination cgroup
+ * @dst_cgrp: the destination cgroup (may be %NULL)
* @preloaded_csets: list of preloaded source css_sets
* Tasks are about to be moved to @dst_cgrp and all the source css_sets
* have been preloaded to @preloaded_csets. This function looks up and
- * pins all destination css_sets, links each to its source, and put them on
- * @preloaded_csets.
+ * pins all destination css_sets, links each to its source, and append them
+ * to @preloaded_csets. If @dst_cgrp is %NULL, the destination of each
+ * source css_set is assumed to be its cgroup on the default hierarchy.
* This function must be called after cgroup_migrate_add_src() has been
* called on each migration source css_set. After migration is performed
@@ -1936,19 +1933,34 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
struct list_head *preloaded_csets)
- struct css_set *src_cset;
+ struct css_set *src_cset, *tmp_cset;


/* look up the dst cset for each src cset and link it to src */
- list_for_each_entry(src_cset, preloaded_csets, mg_preload_node) {
+ list_for_each_entry_safe(src_cset, tmp_cset, preloaded_csets, mg_preload_node) {
struct css_set *dst_cset;

- dst_cset = find_css_set(src_cset, dst_cgrp);
+ dst_cset = find_css_set(src_cset,
+ dst_cgrp ?: src_cset->dfl_cgrp);
if (!dst_cset)
goto err;

WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);
+ /*
+ * If src cset equals dst, it's noop. Drop the src.
+ * cgroup_migrate() will skip the cset too. Note that we
+ * can't handle src == dst as some nodes are used by both.
+ */
+ if (src_cset == dst_cset) {
+ src_cset->mg_src_cgrp = NULL;
+ list_del_init(&src_cset->mg_preload_node);
+ put_css_set(src_cset, false);
+ put_css_set(dst_cset, false);
+ continue;
+ }
src_cset->mg_dst_cset = dst_cset;

if (list_empty(&dst_cset->mg_preload_node))
@@ -1957,7 +1969,7 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
put_css_set(dst_cset, false);

- list_splice(&csets, preloaded_csets);
+ list_splice_tail(&csets, preloaded_csets);
return 0;

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