[tip:sched/balancing] sched: Deal with low-load in wake_affine()

tip-bot for Peter Zijlstra
2009-09-07T20:38:01Z

Commit-ID: 71a29aa7b600595d0ef373ea605ac656876d1f2f
Gitweb: http://git.kernel.org/tip/71a29aa7b600595d0ef373ea605ac656876d1f2f
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Mon, 7 Sep 2009 18:28:05 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Mon, 7 Sep 2009 20:39:06 +0200

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>

---
kernel/sched_fair.c | 12 +++++++++++-
1 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index d7fda41..cc97ea4 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));

/*