sched/fair: Add comments to explain select_idle_sibling()

		return i;

	/*

	 * Otherwise, iterate the domains and find an elegible idle cpu.

	 * Otherwise, iterate the domains and find an eligible idle cpu.

	 *

	 * A completely idle sched group at higher domains is more

	 * desirable than an idle group at a lower level, because lower

	 * domains have smaller groups and usually share hardware

	 * resources which causes tasks to contend on them, e.g. x86

	 * hyperthread siblings in the lowest domain (SMT) can contend

	 * on the shared cpu pipeline.

	 *

	 * However, while we prefer idle groups at higher domains

	 * finding an idle cpu at the lowest domain is still better than

	 * returning 'target', which we've already established, isn't

	 * idle.

	 */

	sd = rcu_dereference(per_cpu(sd_llc, target));

	for_each_lower_domain(sd) {

						tsk_cpus_allowed(p)))

				goto next;

			/* Ensure the entire group is idle */

			for_each_cpu(i, sched_group_cpus(sg)) {

				if (i == target || !idle_cpu(i))

					goto next;

			}

			/*

			 * It doesn't matter which cpu we pick, the

			 * whole group is idle.

			 */

			target = cpumask_first_and(sched_group_cpus(sg),

					tsk_cpus_allowed(p));

			goto done;