[RFC PATCH 2/3] sched/fair: Enable interrupts when dropping lock in newidle_balance()

[Scott Wood <swood@redhat.com>]

When combined with the next patch, which breaks out of rebalancing
when an RT task is runnable, significant latency reductions are seen
on systems with many CPUs.

Signed-off-by: Scott Wood <swood@redhat.com>
---
 kernel/sched/fair.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 74c3c5280d6b..dfde7f0ce3db 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10376,7 +10376,7 @@ static void nohz_newidle_balance(struct rq *this_rq)
 	    time_before(jiffies, READ_ONCE(nohz.next_blocked)))
 		return;
 
-	raw_spin_unlock(&this_rq->lock);
+	raw_spin_unlock_irq(&this_rq->lock);
 	/*
 	 * This CPU is going to be idle and blocked load of idle CPUs
 	 * need to be updated. Run the ilb locally as it is a good
@@ -10385,7 +10385,7 @@ static void nohz_newidle_balance(struct rq *this_rq)
 	 */
 	if (!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))
 		kick_ilb(NOHZ_STATS_KICK);
-	raw_spin_lock(&this_rq->lock);
+	raw_spin_lock_irq(&this_rq->lock);
 }
 
 #else /* !CONFIG_NO_HZ_COMMON */
@@ -10452,7 +10452,7 @@ int newidle_balance(void)
 		goto out;
 	}
 
-	raw_spin_unlock(&this_rq->lock);
+	raw_spin_unlock_irq(&this_rq->lock);
 
 	update_blocked_averages(this_cpu);
 	rcu_read_lock();
@@ -10493,7 +10493,7 @@ int newidle_balance(void)
 	}
 	rcu_read_unlock();
 
-	raw_spin_lock(&this_rq->lock);
+	raw_spin_lock_irq(&this_rq->lock);
 
 	if (curr_cost > this_rq->max_idle_balance_cost)
 		this_rq->max_idle_balance_cost = curr_cost;
-- 
2.18.2