sched/fair: Add comments to explain select_idle_sibling()

It's not entirely obvious how the main loop in select_idle_sibling()
works on first glance. Sprinkle a few comments to explain the design
and intention behind the loop based on some conversations with Mike
and Peter.

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.com>
Cc: Mike Galbraith <mgalbraith@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1457535548-15329-1-git-send-email-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -5055,7 +5055,19 @@ static int select_idle_sibling(struct task_struct *p, int target)
 		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) {
@@ -5065,11 +5077,16 @@ static int select_idle_sibling(struct task_struct *p, int target)
 						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;