sched/fair: Explain LLC nohz kick condition

Provide a comment explaining the LLC related nohz kick in
nohz_balancer_kick().

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dietmar.Eggemann@arm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: morten.rasmussen@arm.com
Cc: vincent.guittot@linaro.org
Link: https://lkml.kernel.org/r/20190117153411.2390-3-valentin.schneider@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -9601,8 +9601,13 @@ static void nohz_balancer_kick(struct rq *rq)
 	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
 	if (sds) {
 		/*
-		 * XXX: write a coherent comment on why we do this.
-		 * See also: http://lkml.kernel.org/r/20111202010832.602203411@sbsiddha-desk.sc.intel.com
+		 * If there is an imbalance between LLC domains (IOW we could
+		 * increase the overall cache use), we need some less-loaded LLC
+		 * domain to pull some load. Likewise, we may need to spread
+		 * load within the current LLC domain (e.g. packed SMT cores but
+		 * other CPUs are idle). We can't really know from here how busy
+		 * the others are - so just get a nohz balance going if it looks
+		 * like this LLC domain has tasks we could move.
 		 */
 		nr_busy = atomic_read(&sds->nr_busy_cpus);
 		if (nr_busy > 1) {