[PATCH 3/3] sched: Update ->next_balance correctly during newidle balance

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>

When newidle balancing triggers, we see that it constantly clobbers
rq->next_balance even when there is no newidle balance happening due to
the cost estimates.  Due to this, we see that periodic load balance
(rebalance_domains) may trigger way more often when the CPU is going in
and out of idle at a high rate but is no really idle. Repeatedly
triggering load balance there is a bad idea as it is a heavy operation.
It also causes increases in softirq.

Another issue is ->last_balance is not updated after newidle balance
causing mistakes in the ->next_balance calculations.

Fix by updating last_balance when a newidle load balance actually
happens and then updating next_balance. This is also how it is done in
other load balance paths.

Testing shows a significant drop in softirqs when running:
cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q

Goes from ~6k to ~800.

Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/fair.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8e276d12c3cb..b147ad09126a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12076,11 +12076,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 
 	if (!READ_ONCE(this_rq->rd->overload) ||
 	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
-
-		if (sd)
-			update_next_balance(sd, &next_balance);
 		rcu_read_unlock();
-
 		goto out;
 	}
 	rcu_read_unlock();
@@ -12095,8 +12091,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 		int continue_balancing = 1;
 		u64 domain_cost;
 
-		update_next_balance(sd, &next_balance);
-
 		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
 			break;
 
@@ -12109,6 +12103,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 			t1 = sched_clock_cpu(this_cpu);
 			domain_cost = t1 - t0;
 			update_newidle_cost(sd, domain_cost);
+			sd->last_balance = jiffies;
+			update_next_balance(sd, &next_balance);
 
 			curr_cost += domain_cost;
 			t0 = t1;
-- 
2.42.0.655.g421f12c284-goog