linux-kernel - [patch] sched: fix broken smt/mc optimizations with CFS

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Date:	Wed, 15 Aug 2007 18:01:50 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	mingo@...e.hu
Cc:	nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: [patch] sched: fix broken smt/mc optimizations with CFS

Ingo, let me know if there any side effects of this change. Thanks.
---

On a four package system with HT - HT load balancing optimizations
were broken. For example, if two tasks end up running on two logical
threads of one of the packages, scheduler is not able to pull one of
the tasks to a completely idle package.

In this scenario, for nice-0 tasks, imbalance calculated by scheduler
will be 512 and find_busiest_queue() will return 0 (as each cpu's load
is 1024 > imbalance and has only one task running).

Similarly MC scheduler optimizations also get fixed with this patch.

Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
---

diff --git a/kernel/sched.c b/kernel/sched.c
index 45e17b8..c5ac710 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2494,7 +2494,7 @@ group_next:
 	 * a think about bumping its value to force at least one task to be
 	 * moved
 	 */
-	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
+	if (*imbalance < busiest_load_per_task) {
 		unsigned long tmp, pwr_now, pwr_move;
 		unsigned int imbn;

-
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