blk-throttle: implement proper hierarchy support

Hierarchical Cgroups

====================

- Currently only CFQ supports hierarchical groups. For throttling,

  cgroup interface does allow creation of hierarchical cgroups and

  internally it treats them as flat hierarchy.

Both CFQ and throttling implement hierarchy support; however,

throttling's hierarchy support is enabled iff "sane_behavior" is

enabled from cgroup side, which currently is a development option and

not publicly available.

If somebody created a hierarchy like as follows.

			|

		     test3

  CFQ will handle the hierarchy correctly but and throttling will

  practically treat all groups at same level. For details on CFQ

  hierarchy support, refer to Documentation/block/cfq-iosched.txt.

  Throttling will treat the hierarchy as if it looks like the

CFQ by default and throttling with "sane_behavior" will handle the

hierarchy correctly.  For details on CFQ hierarchy support, refer to

Documentation/block/cfq-iosched.txt.  For throttling, all limits apply

to the whole subtree while all statistics are local to the IOs

directly generated by tasks in that cgroup.

Throttling without "sane_behavior" enabled from cgroup side will

practically treat all groups at same level as if it looks like the

following.

				pivot

			     /  /   \  \

			root  test1 test2  test3

  Nesting cgroups, while allowed, isn't officially supported and blkio

  genereates warning when cgroups nest. Once throttling implements

  hierarchy support, hierarchy will be supported and the warning will

  be removed.

Various user visible config options

===================================

CONFIG_BLK_CGROUP

	.subsys_id = blkio_subsys_id,

	.base_cftypes = blkcg_files,

	.module = THIS_MODULE,

	/*

	 * blkio subsystem is utterly broken in terms of hierarchy support.

	 * It treats all cgroups equally regardless of where they're

	 * located in the hierarchy - all cgroups are treated as if they're

	 * right below the root.  Fix it and remove the following.

	 */

	.broken_hierarchy = true,

};

EXPORT_SYMBOL_GPL(blkio_subsys);

{

	struct throtl_grp *tg = blkg_to_tg(blkg);

	struct throtl_data *td = blkg->q->td;

	struct throtl_service_queue *parent_sq;

	unsigned long flags;

	int rw;

	throtl_service_queue_init(&tg->service_queue, &td->service_queue);

	/*

	 * If sane_hierarchy is enabled, we switch to properly hierarchical

	 * behavior where limits on a given throtl_grp are applied to the

	 * whole subtree rather than just the group itself.  e.g. If 16M

	 * read_bps limit is set on the root group, the whole system can't

	 * exceed 16M for the device.

	 *

	 * If sane_hierarchy is not enabled, the broken flat hierarchy

	 * behavior is retained where all throtl_grps are treated as if

	 * they're all separate root groups right below throtl_data.

	 * Limits of a group don't interact with limits of other groups

	 * regardless of the position of the group in the hierarchy.

	 */

	parent_sq = &td->service_queue;

	if (cgroup_sane_behavior(blkg->blkcg->css.cgroup) && blkg->parent)

		parent_sq = &blkg_to_tg(blkg->parent)->service_queue;

	throtl_service_queue_init(&tg->service_queue, parent_sq);

	for (rw = READ; rw <= WRITE; rw++) {

		throtl_qnode_init(&tg->qnode_on_self[rw], tg);

		throtl_qnode_init(&tg->qnode_on_parent[rw], tg);

	 * - memcg: use_hierarchy is on by default and the cgroup file for

	 *   the flag is not created.

	 *

	 * - blkcg: blk-throttle becomes properly hierarchical.

	 *

	 * The followings are planned changes.

	 *

	 * - release_agent will be disallowed once replacement notification