[PATCH v2 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 v2 1/3] sched/fair: fix rounding issue for asym packing
Date: Fri, 14 Dec 2018 17:01:55 +0100	[thread overview]
Message-ID: <1544803317-7610-2-git-send-email-vincent.guittot@linaro.org> (raw)
In-Reply-To: <1544803317-7610-1-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
busiest_stat.avg_load also comes from a division and the final rounding
can 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.

Add 1 to the avg_load to make sure that the targeted cpu will not be
skipped unexpectidly.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ca46964..c215f7a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8215,6 +8215,12 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 	/* Adjust by relative CPU capacity of the group */
 	sgs->group_capacity = group->sgc->capacity;
 	sgs->avg_load = (sgs->group_load*SCHED_CAPACITY_SCALE) / sgs->group_capacity;
+	/*
+	 * Prevent division rounding to make the computation of imbalance
+	 * slightly less than original value and to prevent the rq to be then
+	 * selected as busiest queue
+	 */
+	sgs->avg_load += 1;
 
 	if (sgs->sum_nr_running)
 		sgs->load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;
-- 
2.7.4

