[PATCH 5/5] sched/fair: Skip LLC nohz logic for asymmetric systems

[Valentin Schneider <valentin.schneider@arm.com>]

The LLC nohz condition will become true as soon as >=2 CPUs in a
single LLC domain are busy. On big.LITTLE systems, this translates to
two or more CPUs of a "cluster" (big or LITTLE) being busy.

Issuing a nohz kick in these conditions isn't desired for asymmetric
systems, as if the busy CPUs can provide enough compute capacity to
the running tasks, then we can leave the nohz CPUs in peace.

Skip the LLC nohz condition for asymmetric systems, and rely on
nr_running & capacity checks to trigger nohz kicks when the system
actually needs them.

Suggested-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 291dfdb0183f..ff27bf56e8d4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9545,6 +9545,17 @@ static void nohz_balancer_kick(struct rq *rq)
 	}
 
 	rcu_read_lock();
+
+	if (static_branch_unlikely(&sched_asym_cpucapacity))
+		/*
+		 * For asymmetric systems, we do not want to nicely balance
+		 * cache use, instead we want to embrace asymmetry and only
+		 * ensure tasks have enough CPU capacity.
+		 *
+		 * Skip the LLC logic because it's not relevant in that case.
+		 */
+		goto check_capacity;
+
 	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
 	if (sds) {
 		/*
@@ -9564,6 +9575,7 @@ static void nohz_balancer_kick(struct rq *rq)
 
 	}
 
+check_capacity:
 	sd = rcu_dereference(rq->sd);
 	if (sd) {
 		if ((rq->cfs.h_nr_running >= 1) &&
-- 
2.20.1