[tip:sched/urgent] sched: Fix load balancing performance regression in should_we_balance()

[tip-bot for Joonsoo Kim]

Commit-ID:  b0cff9d88ce2f3030f73138078c5b1019f17e1cc
Gitweb:     http://git.kernel.org/tip/b0cff9d88ce2f3030f73138078c5b1019f17e1cc
Author:     Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
AuthorDate: Tue, 10 Sep 2013 15:54:49 +0900
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 10 Sep 2013 09:20:42 +0200

sched: Fix load balancing performance regression in should_we_balance()

Commit 23f0d20 ("sched: Factor out code to should_we_balance()")
introduces the should_we_balance() function.  This function should
return 1 if this cpu is appropriate for balancing. But the newly
introduced code doesn't do so, it returns 0 instead of 1.

This introduces performance regression, reported by Dave Chinner:

                        v4 filesystem           v5 filesystem
3.11+xfsdev:            220k files/s            225k files/s
3.12-git                180k files/s            185k files/s
3.12-git-revert         245k files/s            247k files/s

You can find more detailed information at:

  https://lkml.org/lkml/2013/9/10/1

This patch corrects the return value of should_we_balance()
function as orignally intended.

With this patch, Dave Chinner reports that the regression is gone:

                        v4 filesystem           v5 filesystem
3.11+xfsdev:            220k files/s            225k files/s
3.12-git                180k files/s            185k files/s
3.12-git-revert         245k files/s            247k files/s
3.12-git-fix            249k files/s            248k files/s

Reported-by: Dave Chinner <dchinner@xxxxxxxxxx>
Tested-by: Dave Chinner <dchinner@xxxxxxxxxx>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20130910065448.GA20368@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7f0a5e6..9b3fe1c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5151,7 +5151,7 @@ static int should_we_balance(struct lb_env *env)
 	 * First idle cpu or the first cpu(busiest) in this sched group
 	 * is eligible for doing load balancing at this and above domains.
 	 */
-	return balance_cpu != env->dst_cpu;
+	return balance_cpu == env->dst_cpu;
 }
 
 /*
--
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