sched: Deal with low-load in wake_affine()

wake_affine() would always fail under low-load situations where
both prev and this were idle, because adding a single task will
always be a significant imbalance, even if there's nothing
around that could balance it.

Deal with this by allowing imbalance when there's nothing you
can do about it.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 11 insertions, 1 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index d7fda41ddaf0..cc97ea498f24 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1262,7 +1262,17 @@ wake_affine(struct sched_domain *this_sd, struct rq *this_rq,
         tg = task_group(p);
         weight = p->se.load.weight;
 
-        balanced = 100*(tl + effective_load(tg, this_cpu, weight, weight)) <=
+        /*
+         * In low-load situations, where prev_cpu is idle and this_cpu is idle
+         * due to the sync cause above having dropped tl to 0, we'll always have
+         * an imbalance, but there's really nothing you can do about that, so
+         * that's good too.
+         *
+         * Otherwise check if either cpus are near enough in load to allow this
+         * task to be woken on this_cpu.
+         */
+        balanced = !tl ||
+                100*(tl + effective_load(tg, this_cpu, weight, weight)) <=
                 imbalance*(load + effective_load(tg, prev_cpu, 0, weight));
 
         /*