[patch -rt 16/17] sched: Deal with low-load in wake_affine()

[dino]

    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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