[PATCH 3/5] sched: make double-lock-balance fair

[Gregory Haskins <ghaskins@novell.com>]

double_lock balance() currently favors logically lower cpus since they
often do not have to release their own lock to acquire a second lock.
The result is that logically higher cpus can get starved when there is
a lot of pressure on the RQs.  This can result in higher latencies on
higher cpu-ids.

This patch makes the algorithm more fair by forcing all paths to have
to release both locks before acquiring them again.  Since callsites to
double_lock_balance already consider it a potential preemption/reschedule
point, they have the proper logic to recheck for atomicity violations.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
---

 kernel/sched.c |   17 +++++------------
 1 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 6e0bde6..b7326cd 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2790,23 +2790,16 @@ static int double_lock_balance(struct rq *this_rq, struct rq *busiest)
 	__acquires(busiest->lock)
 	__acquires(this_rq->lock)
 {
-	int ret = 0;