linux-kernel - [PATCH] sched/fair: remove the swap() logic in load_too

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Date:	Sat, 17 Oct 2015 11:06:32 -0400
From:	Zhihui Zhang <zzhsuny@...il.com>
To:	riel@...hat.com, peterz@...radead.org, mingo@...hat.com
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: remove the swap() logic in load_too_imbalanced()

The swap() logic was introduced before we scaled the load by the
actual CPU capacity. Now it looks funny that we swap the load but
not the CPU capacity. In fact, we probably need to check both
directions to ensure that the load of neither side increase too
high compared to the other side.

Signed-off-by: Zhihui Zhang <zzhsuny@...il.com>
---
 kernel/sched/fair.c | 36 +++++++++++++++++++-----------------
 1 file changed, 19 insertions(+), 17 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e2e348..5e8e17b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1203,7 +1203,7 @@ static void task_numa_assign(struct task_numa_env *env,
 static bool load_too_imbalanced(long src_load, long dst_load,
 				struct task_numa_env *env)
 {
-	long imb, old_imb;
+	long imb1, imb2, orig_imb1, orig_imb2;
 	long orig_src_load, orig_dst_load;
 	long src_capacity, dst_capacity;
 
@@ -1217,31 +1217,33 @@ static bool load_too_imbalanced(long src_load, long dst_load,
 	src_capacity = env->src_stats.compute_capacity;
 	dst_capacity = env->dst_stats.compute_capacity;
 
-	/* We care about the slope of the imbalance, not the direction. */
-	if (dst_load < src_load)
-		swap(dst_load, src_load);
-
-	/* Is the difference below the threshold? */
-	imb = dst_load * src_capacity * 100 -
-	      src_load * dst_capacity * env->imbalance_pct;
-	if (imb <= 0)
+	/* Does the difference in either direction exceed the threshold? */
+	imb1 = dst_load * src_capacity * 100 -
+	       src_load * dst_capacity * env->imbalance_pct;
+	imb2 = src_load * dst_capacity * 100 -
+	       dst_load * src_capacity * env->imbalance_pct;
+	if (imb1 <= 0 && imb2 <= 0)
 		return false;
 
 	/*
-	 * The imbalance is above the allowed threshold.
-	 * Compare it with the old imbalance.
+	 * At least one imbalance is above the allowed threshold.
+	 * Compare it with the original imbalance.
 	 */
 	orig_src_load = env->src_stats.load;
 	orig_dst_load = env->dst_stats.load;
 
-	if (orig_dst_load < orig_src_load)
-		swap(orig_dst_load, orig_src_load);
-
-	old_imb = orig_dst_load * src_capacity * 100 -
-		  orig_src_load * dst_capacity * env->imbalance_pct;
+	orig_imb1 = orig_dst_load * src_capacity * 100 -
+		    orig_src_load * dst_capacity * env->imbalance_pct;
+	orig_imb2 = orig_src_load * dst_capacity * 100 -
+		    orig_dst_load * src_capacity * env->imbalance_pct;
 
 	/* Would this change make things worse? */
-	return (imb > old_imb);
+	if (imb1 > 0 && imb1 > orig_imb1)
+		return true;
+	if (imb2 > 0 && imb2 > orig_imb2)
+		return true;
+
+	return false;
 }
 
 /*
-- 
1.9.1

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