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>

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index d7fda41..cc97ea4 100644 (file)
--- 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));
 
        /*