On Thu, Apr 07, 2016 at 05:28:24AM -0400, Johannes Weiner wrote:

Hm? The root group can always contain tasks. It's not the only thing
the root is exempt from, it can't control any resources either:

it does in fact control resouces; the hierarchy directly affects the
proportional distribution of time.

sched_group_set_shares():

	/*
	 * We can't change the weight of the root cgroup.
	 */
	if (!tg->se[0])
		return -EINVAL;

The root has, per definition, no siblings, so setting a weight is
entirely pointless.

tg_set_cfs_bandwidth():

	if (tg == &root_task_group)
		return -EINVAL;

We have had patches to implement this, but have so far held off because
they add a bunch of cycles to some really hot paths and we'd rather not
do that. Its not impossible, or unthinkable to do this otherwise.