sched/nohz: Optimize nohz_idle_balance()

Avoid calling update_blocked_averages() when it does not in fact have
any by re-using/extending update_nohz_stats().
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -7898,7 +7898,7 @@ group_type group_classify(struct sched_group *group,
 	return group_other;
 }
 
-static bool update_nohz_stats(struct rq *rq)
+static bool update_nohz_stats(struct rq *rq, bool force)
 {
 #ifdef CONFIG_NO_HZ_COMMON
 	unsigned int cpu = rq->cpu;
@@ -7909,7 +7909,7 @@ static bool update_nohz_stats(struct rq *rq)
 	if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
 		return false;
 
-	if (!time_after(jiffies, rq->last_blocked_load_update_tick))
+	if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick))
 		return true;
 
 	update_blocked_averages(cpu);
@@ -7942,7 +7942,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
 		struct rq *rq = cpu_rq(i);
 
-		if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq))
+		if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq, false))
 			env->flags |= LBF_NOHZ_AGAIN;
 
 		/* Bias balancing toward CPUs of our domain: */
@@ -9552,8 +9552,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 
 		rq = cpu_rq(balance_cpu);
 
-		update_blocked_averages(rq->cpu);
-		has_blocked_load |= rq->has_blocked_load;
+		has_blocked_load |= update_nohz_stats(rq, true);
 
 		/*
 		 * If time for next balance is due,