[PATCH v3 1/3] sched/fair: fix rounding issue for asym packing

From: Vincent Guittot <vincent.guittot@linaro.org>
To: peterz@infradead.org, mingo@kernel.org, linux-kernel@vger.kernel.org
Cc: valentin.schneider@arm.com, Morten.Rasmussen@arm.com,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: [PATCH v3 1/3] sched/fair: fix rounding issue for asym packing
Date: Thu, 20 Dec 2018 08:55:45 +0100	[thread overview]
Message-ID: <1545292547-18770-2-git-send-email-vincent.guittot@linaro.org> (raw)
In-Reply-To: <1545292547-18770-1-git-send-email-vincent.guittot@linaro.org>

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 sgs->group_load to remove the rounding.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 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..9b31247 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.avg_load;
 
 	return 1;
 }
-- 
2.7.4

