sched/fair: Fix rounding bug 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 from happening.

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

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: valentin.schneider@arm.com
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9693cf2ea954203b08f9579c63ff4148237fa5f7..30450901200eac65a533b8d815af8723775c009f 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8453,9 +8453,7 @@ 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;
 }