[PATCH] sched/fair: Track target domain's avg_scan_cost in select_idle_cpu

[Yicong Yang]

We regulate the LLC domain scan in select_idle_cpu() by comparing
the average scan cost of this_sd against the average idle time of
this_rq. This is correct when the domain to scan is the LLC domain
of this cpu. But when the domain to scan is different from this
LLC domain, we'll have an inaccurate estimation of the scan cost
on the target domain as this_sd->avg_scan_cost contains contributions
of scanning other domains besides the target domain.

Track the avg_scan_cost of the target domain to make the estimation
more accurate.

Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e476f6d9435..6301740d98cb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6267,7 +6267,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 		}
 
 		avg_idle = this_rq->wake_avg_idle;
-		avg_cost = this_sd->avg_scan_cost + 1;
+		avg_cost = sd->avg_scan_cost + 1;
 
 		span_avg = sd->span_weight * avg_idle;
 		if (span_avg > 4*avg_cost)
@@ -6305,7 +6305,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 		 */
 		this_rq->wake_avg_idle -= min(this_rq->wake_avg_idle, time);
 
-		update_avg(&this_sd->avg_scan_cost, time);
+		update_avg(&sd->avg_scan_cost, time);
 	}
 
 	return idle_cpu;
-- 
2.33.0