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

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Date:	Mon, 25 Aug 2008 16:15:34 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	mingo@...e.hu
Cc:	srostedt@...hat.com, peterz@...radead.org,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	npiggin@...e.de, gregory.haskins@...il.com
Subject: [PATCH 3/5] sched: make double-lock-balance fair

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@...ell.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;
-
 	if (unlikely(!irqs_disabled())) {
 		/* printk() doesn't work good under rq->lock */
 		spin_unlock(&this_rq->lock);
 		BUG_ON(1);
 	}
-	if (unlikely(!spin_trylock(&busiest->lock))) {
-		if (busiest < this_rq) {
-			spin_unlock(&this_rq->lock);
-			spin_lock(&busiest->lock);
-			spin_lock_nested(&this_rq->lock, SINGLE_DEPTH_NESTING);
-			ret = 1;
-		} else
-			spin_lock_nested(&busiest->lock, SINGLE_DEPTH_NESTING);
-	}
-	return ret;
+
+	spin_unlock(&this_rq->lock);
+	double_rq_lock(this_rq, busiest);
+
+	return 1;
 }
 
 static void double_unlock_balance(struct rq *this_rq, struct rq *busiest)

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