linux-kernel - [095/115] sched: Set group_imb only a task can be pulled from the busiest cpu

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Date:	Tue, 15 Feb 2011 17:46:01 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Nikhil Rao <ncrao@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>
Subject: [095/115] sched: Set group_imb only a task can be pulled from the busiest cpu

2.6.32-longterm review patch.  If anyone has any objections, please let us know.

------------------

Commit: 2582f0eba54066b5e98ff2b27ef0cfa833b59f54 upstream

When cycling through sched groups to determine the busiest group, set
group_imb only if the busiest cpu has more than 1 runnable task. This patch
fixes the case where two cpus in a group have one runnable task each, but there
is a large weight differential between these two tasks. The load balancer is
unable to migrate any task from this group, and hence do not consider this
group to be imbalanced.

Signed-off-by: Nikhil Rao <ncrao@...gle.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
LKML-Reference: <1286996978-7007-3-git-send-email-ncrao@...gle.com>
[ small code readability edits ]
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Mike Galbraith <efault@....de>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
---
 kernel/sched.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3745,7 +3745,7 @@ static inline void update_sg_lb_stats(st
 			int local_group, const struct cpumask *cpus,
 			int *balance, struct sg_lb_stats *sgs)
 {
-	unsigned long load, max_cpu_load, min_cpu_load;
+	unsigned long load, max_cpu_load, min_cpu_load, max_nr_running;
 	int i;
 	unsigned int balance_cpu = -1, first_idle_cpu = 0;
 	unsigned long avg_load_per_task = 0;
@@ -3759,6 +3759,7 @@ static inline void update_sg_lb_stats(st
 	/* Tally up the load of all CPUs in the group */
 	max_cpu_load = 0;
 	min_cpu_load = ~0UL;
+	max_nr_running = 0;
 
 	for_each_cpu_and(i, sched_group_cpus(group), cpus) {
 		struct rq *rq = cpu_rq(i);
@@ -3776,8 +3777,10 @@ static inline void update_sg_lb_stats(st
 			load = target_load(i, load_idx);
 		} else {
 			load = source_load(i, load_idx);
-			if (load > max_cpu_load)
+			if (load > max_cpu_load) {
 				max_cpu_load = load;
+				max_nr_running = rq->nr_running;
+			}
 			if (min_cpu_load > load)
 				min_cpu_load = load;
 		}
@@ -3815,11 +3818,10 @@ static inline void update_sg_lb_stats(st
 	if (sgs->sum_nr_running)
 		avg_load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;
 
-	if ((max_cpu_load - min_cpu_load) > 2*avg_load_per_task)
+	if ((max_cpu_load - min_cpu_load) > 2*avg_load_per_task && max_nr_running > 1)
 		sgs->group_imb = 1;
 
-	sgs->group_capacity =
-		DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);
+	sgs->group_capacity = DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);
 }
 
 /**


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