linux-kernel - [PATCH 7/7] sched/fair: don't wake affine recently load balanced tasks

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Date:   Fri, 14 Jul 2017 13:21:04 +0000
From:   Josef Bacik <josef@...icpanda.com>
To:     mingo@...hat.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org, umgwanakikbuti@...il.com,
        tj@...nel.org, kernel-team@...com
Cc:     Josef Bacik <jbacik@...com>
Subject: [PATCH 7/7] sched/fair: don't wake affine recently load balanced tasks

From: Josef Bacik <jbacik@...com>

The wake affinity logic will move tasks between two cpu's that appear to be
loaded equally at the current time, with a slight bias towards cache locality.
However on a heavily loaded system the load balancer has a better insight into
what needs to be moved around, so instead keep track of the last time a task was
migrated by the load balancer.  If it was recent, opt to let the process stay on
it's current CPU (or an idle sibling).

Signed-off-by: Josef Bacik <jbacik@...com>
---
 include/linux/sched.h |  1 +
 kernel/sched/fair.c   | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1a0eadd..d872780 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -528,6 +528,7 @@ struct task_struct {
 	unsigned long			wakee_flip_decay_ts;
 	struct task_struct		*last_wakee;
 
+	unsigned long			last_balance_ts;
 	int				wake_cpu;
 #endif
 	int				on_rq;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 034d5df..6a98a38 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5604,6 +5604,16 @@ static int wake_wide(struct task_struct *p)
 	unsigned int slave = p->wakee_flips;
 	int factor = this_cpu_read(sd_llc_size);
 
+	/*
+	 * If we've balanced this task recently we don't want to undo all of
+	 * that hard work by the load balancer and move it to the current cpu.
+	 * Constantly overriding the load balancers decisions is going to make
+	 * it question its purpose in life and give it anxiety and self worth
+	 * issues, and nobody wants that.
+	 */
+	if (time_before(jiffies, p->last_balance_ts + HZ))
+		return 1;
+
 	if (master < slave)
 		swap(master, slave);
 	if (slave < factor || master < slave * factor)
@@ -7097,6 +7107,7 @@ static int detach_tasks(struct lb_env *env)
 			goto next;
 
 		detach_task(p, env);
+		p->last_balance_ts = jiffies;
 		list_add(&p->se.group_node, &env->tasks);
 
 		detached++;
-- 
2.9.3