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

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Date:   Tue,  7 Aug 2018 17:56:25 +0200
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 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
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@...aro.org>
---
 kernel/sched/fair.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 309c93f..c376cd0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7780,6 +7780,12 @@ 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