[PATCH 4/5] sched/fair: Tune down misfit nohz kicks

[Valentin Schneider]

In

  commmit 3b1baa6496e6 ("sched/fair: Add 'group_misfit_task' load-balance type")

we set rq->misfit_task_load whenever the current running task has a
utilization greater than 80% of rq->cpu_capacity. A non-zero value in
this field enables misfit load balancing.

However, if the task being looked at is already running on a CPU of
highest capacity, there's nothing more we can do for it. We can
currently spot this in update_sd_pick_busiest(), which prevents us
from selecting a sched_group of group_type == group_misfit_task as the
busiest group, but we don't do any of that in nohz_balancer_kick().

This means that we could repeatedly kick nohz CPUs when there's no
improvements in terms of load balance to be done.

Introduce a check_misfit_status() helper that returns true iff there
is a CPU in the system that could give more CPU capacity to a rq's
misfit task - IOW, there exists a CPU of higher capacity_orig or the
rq's CPU is severely pressured by rt/IRQ.

Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
---
 kernel/sched/fair.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7658f9d76a9a..291dfdb0183f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8012,6 +8012,18 @@ check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
 				(rq->cpu_capacity_orig * 100));
 }
 
+/*
+ * Check whether a rq has a misfit task and if it looks like we can actually
+ * help that task: we can migrate the task to a CPU of higher capacity, or
+ * the task's current CPU is heavily pressured.
+ */
+static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
+{
+	return rq->misfit_task_load &&
+		(rq->cpu_capacity_orig < rq->rd->max_cpu_capacity ||
+		 check_cpu_capacity(rq, sd));
+}
+
 /*
  * Group imbalance indicates (and tries to solve) the problem where balancing
  * groups is inadequate due to ->cpus_allowed constraints.
@@ -9527,7 +9539,7 @@ static void nohz_balancer_kick(struct rq *rq)
 	if (time_before(now, nohz.next_balance))
 		goto out;
 
-	if (rq->nr_running >= 2 || rq->misfit_task_load) {
+	if (rq->nr_running >= 2) {
 		flags = NOHZ_KICK_MASK;
 		goto out;
 	}
@@ -9561,6 +9573,14 @@ static void nohz_balancer_kick(struct rq *rq)
 		}
 	}
 
+	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
+	if (sd) {
+		if (check_misfit_status(rq, sd)) {
+			flags = NOHZ_KICK_MASK;
+			goto unlock;
+		}
+	}
+
 	sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
 	if (sd) {
 		for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
-- 
2.20.1