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Commit e4857982 authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
parent 37ff9f8f

include/linux/cgroup-defs.h

+5 −4
Original line number Diff line number Diff line
@@ -191,12 +191,13 @@ struct css_set { 


	/*

	 * 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;



	/*

kernel/cgroup.c

+13 −13
Original line number Diff line number Diff line
@@ -2473,6 +2473,7 @@ static void cgroup_migrate_finish(struct list_head *preloaded_csets) 
	spin_lock_bh(&css_set_lock);

	list_for_each_entry_safe(cset, tmp_cset, preloaded_csets, mg_preload_node) {

		cset->mg_src_cgrp = NULL;

		cset->mg_dst_cgrp = NULL;

		cset->mg_dst_cset = NULL;

		list_del_init(&cset->mg_preload_node);

		put_css_set_locked(cset);
@@ -2511,32 +2512,31 @@ static void cgroup_migrate_add_src(struct css_set *src_cset, 
		return;



	WARN_ON(src_cset->mg_src_cgrp);

	WARN_ON(src_cset->mg_dst_cgrp);

	WARN_ON(!list_empty(&src_cset->mg_tasks));

	WARN_ON(!list_empty(&src_cset->mg_node));



	src_cset->mg_src_cgrp = src_cgrp;

	src_cset->mg_dst_cgrp = dst_cgrp;

	get_css_set(src_cset);

	list_add(&src_cset->mg_preload_node, preloaded_csets);

}



/**

 * cgroup_migrate_prepare_dst - prepare destination css_sets for migration

 * @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 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.

 * Tasks are about to be moved 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 append them to

 * @preloaded_csets.

 *

 * This function must be called after cgroup_migrate_add_src() has been

 * called on each migration source css_set.  After migration is performed

 * using cgroup_migrate(), cgroup_migrate_finish() must be called on

 * @preloaded_csets.

 */

static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,

				      struct list_head *preloaded_csets)

static int cgroup_migrate_prepare_dst(struct list_head *preloaded_csets)

{

	LIST_HEAD(csets);

	struct css_set *src_cset, *tmp_cset;
@@ -2547,8 +2547,7 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp, 
	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 ?: src_cset->dfl_cgrp);

		dst_cset = find_css_set(src_cset, src_cset->mg_dst_cgrp);

		if (!dst_cset)

			goto err;
@@ -2561,6 +2560,7 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp, 
		 */

		if (src_cset == dst_cset) {

			src_cset->mg_src_cgrp = NULL;

			src_cset->mg_dst_cgrp = NULL;

			list_del_init(&src_cset->mg_preload_node);

			put_css_set(src_cset);

			put_css_set(dst_cset);
@@ -2657,7 +2657,7 @@ static int cgroup_attach_task(struct cgroup *dst_cgrp, 
	spin_unlock_bh(&css_set_lock);



	/* prepare dst csets and commit */

	ret = cgroup_migrate_prepare_dst(dst_cgrp, &preloaded_csets);

	ret = cgroup_migrate_prepare_dst(&preloaded_csets);

	if (!ret)

		ret = cgroup_migrate(leader, threadgroup, dst_cgrp->root);
@@ -2916,7 +2916,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp) 
	spin_unlock_bh(&css_set_lock);



	/* NULL dst indicates self on default hierarchy */

	ret = cgroup_migrate_prepare_dst(NULL, &preloaded_csets);

	ret = cgroup_migrate_prepare_dst(&preloaded_csets);

	if (ret)

		goto out_finish;
@@ -4156,7 +4156,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from) 
		cgroup_migrate_add_src(link->cset, to, &preloaded_csets);

	spin_unlock_bh(&css_set_lock);



	ret = cgroup_migrate_prepare_dst(to, &preloaded_csets);

	ret = cgroup_migrate_prepare_dst(&preloaded_csets);

	if (ret)

		goto out_err;