Re: [PATCH 1/2] sched/fair: skip newidle update stats

[Peter Zijlstra]

Subject: sched/fair: Simplify newidle_balance()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri Nov 12 15:46:26 CET 2021

Move rq_{un,re}pin_lock() next to raw_spin_rq_{un,}lock().

Remove all rcu_read_{,un}lock(), since we have preempt/irqs disabled
over the whole function and those hold off RCU (again).

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c |   23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10884,15 +10884,6 @@ static int newidle_balance(struct rq *th
 	if (!cpu_active(this_cpu))
 		return 0;
 
-	/*
-	 * This is OK, because current is on_cpu, which avoids it being picked
-	 * for load-balance and preemption/IRQs are still disabled avoiding
-	 * further scheduler activity on it and we're being very careful to
-	 * re-start the picking loop.
-	 */
-	rq_unpin_lock(this_rq, rf);
-
-	rcu_read_lock();
 	sd = rcu_dereference_check_sched_domain(this_rq->sd);
 
 	if (!READ_ONCE(this_rq->rd->overload) ||
@@ -10908,18 +10899,22 @@ static int newidle_balance(struct rq *th
 			 */
 			early_stop = 1;
 		}
-		rcu_read_unlock();
 
 		goto out;
 	}
-	rcu_read_unlock();
 
+	/*
+	 * This is OK, because current is on_cpu, which avoids it being picked
+	 * for load-balance and preemption/IRQs are still disabled avoiding
+	 * further scheduler activity on it and we're being very careful to
+	 * re-start the picking loop.
+	 */
+	rq_unpin_lock(this_rq, rf);
 	raw_spin_rq_unlock(this_rq);
 
 	t0 = sched_clock_cpu(this_cpu);
 	update_blocked_averages(this_cpu);
 
-	rcu_read_lock();
 	for_each_domain(this_cpu, sd) {
 		int continue_balancing = 1;
 		u64 domain_cost;
@@ -10953,9 +10948,9 @@ static int newidle_balance(struct rq *th
 		    this_rq->ttwu_pending)
 			break;
 	}
-	rcu_read_unlock();
 
 	raw_spin_rq_lock(this_rq);
+	rq_repin_lock(this_rq, rf);
 
 	if (curr_cost > this_rq->max_idle_balance_cost)
 		this_rq->max_idle_balance_cost = curr_cost;
@@ -10982,8 +10977,6 @@ static int newidle_balance(struct rq *th
 	else if (!early_stop)
 		nohz_newidle_balance(this_rq);
 
-	rq_repin_lock(this_rq, rf);
-
 	return pulled_task;
 }