linux-kernel - [PATCH v4 1/3] sched/fair: fix rounding issue for asym packing

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Date:   Thu, 20 Dec 2018 15:54:30 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     valentin.schneider@....com, Morten.Rasmussen@....com,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v4 1/3] sched/fair: fix rounding issue for asym packing

When check_asym_packing() is triggered, the imbalance is set to :
  busiest_stat.avg_load * busiest_stat.group_capacity / SCHED_CAPACITY_SCALE
But busiest_stat.avg_load equals :
  sgs->group_load *SCHED_CAPACITY_SCALE / sgs->group_capacity
These divisions can generate a rounding that will make imbalance slightly
lower than the weighted load of the cfs_rq.
But this is enough to skip the rq in find_busiest_queue and prevents asym
migration to happen.

Directly set imbalance to busiest's sgs->group_load to remove the rounding.

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
Reviewed-by: Valentin Schneider <valentin.schneider@....com>
---
 kernel/sched/fair.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ca46964..1e4bed4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8476,9 +8476,7 @@ static int check_asym_packing(struct lb_env *env, struct sd_lb_stats *sds)
 	if (sched_asym_prefer(busiest_cpu, env->dst_cpu))
 		return 0;

-	env->imbalance = DIV_ROUND_CLOSEST(
-		sds->busiest_stat.avg_load * sds->busiest_stat.group_capacity,
-		SCHED_CAPACITY_SCALE);
+	env->imbalance = sds->busiest_stat.group_load;

 	return 1;
 }
-- 
2.7.4