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@xxxxxxxxx>
Signed-off-by: Dinakar Guniguntala <dino@xxxxxxxxxx>
Index: linux-2.6.31.4-rt14-lb1/kernel/sched_fair.c
===================================================================
--- linux-2.6.31.4-rt14-lb1.orig/kernel/sched_fair.c 2009-10-21 10:49:02.000000000 -0400
+++ linux-2.6.31.4-rt14-lb1/kernel/sched_fair.c 2009-10-21 10:49:04.000000000 -0400
@@ -1264,7 +1264,17 @@
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));
/*
--
--
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