sched/fair: Allow calculate_imbalance() to move idle cpus

Allow calculate_imbalance() to 'create' idle cpus in the busiest group
if there are idle cpus in the local group.

Suggested-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20140729152705.GX12054@laptop.lan
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 5 insertions, 6 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 94377254254e..df1ed176c7b7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6273,12 +6273,11 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
                 return fix_small_imbalance(env, sds);
         }
 
-        if (busiest->group_type == group_overloaded) {
-                /*
-                 * Don't want to pull so many tasks that a group would go idle.
-                 * Except of course for the group_imb case, since then we might
-                 * have to drop below capacity to reach cpu-load equilibrium.
-                 */
+        /*
+         * If there aren't any idle cpus, avoid creating some.
+         */
+        if (busiest->group_type == group_overloaded &&
+            local->group_type   == group_overloaded) {
                 load_above_capacity =
                         (busiest->sum_nr_running - busiest->group_capacity_factor);