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Date: Mon, 7 Sep 2009 20:37:28 GMT
From: tip-bot for Peter Zijlstra <a.p.zijlstra@chello.nl>
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       a.p.zijlstra@chello.nl, tglx@linutronix.de, mingo@elte.hu
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To: linux-tip-commits@vger.kernel.org
Subject: [tip:sched/balancing] sched: Deal with low-load in wake_affine()
Message-ID: <tip-71a29aa7b600595d0ef373ea605ac656876d1f2f@git.kernel.org>
Git-Commit-ID: 71a29aa7b600595d0ef373ea605ac656876d1f2f
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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));
 
 	/*