[PATCH v2] sched/fair: fix 1 task per CPU

[Vincent Guittot]

When CPUs have different capacity because of RT/DL tasks or
micro-architecture or max frequency differences, there are situation where
the imbalance is not correctly set to migrate waiting task on the idle CPU.

The UC uses the force_balance case :
	if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
	    busiest->group_no_capacity)
		goto force_balance;

But calculate_imbalance fails to set the right amount of load to migrate
a task because of the special condition:
  busiest->avg_load <= sds->avg_load || local->avg_load >= sds->avg_load)

Add in fix_small_imbalance, this special case that triggered the force
balance in order to make sure that the amount of load to migrate will be
enough.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
 kernel/sched/fair.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 309c93f..72bc5e8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8048,6 +8048,20 @@ void fix_small_imbalance(struct lb_env *env, struct sd_lb_stats *sds)
 	local = &sds->local_stat;
 	busiest = &sds->busiest_stat;
 
+	/*
+	 * There is available capacity in local group and busiest group is
+	 * overloaded but calculate_imbalance can't compute the amount of load
+	 * to migrate because load_avg became meaningless due to asymetric
+	 * capacity between groups. In such case, we only want to migrate at
+	 * least one tasks of the busiest group and rely of the average load
+	 * per task to ensure the migration.
+	 */
+	if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
+	    busiest->group_no_capacity) {
+		env->imbalance = busiest->load_per_task;
+		return;
+	}
+
 	if (!local->sum_nr_running)
 		local->load_per_task = cpu_avg_load_per_task(env->dst_cpu);
 	else if (busiest->load_per_task > local->load_per_task)
-- 
2.7.4