linux-kernel - [PATCH 1/4 v2] sched/fair: relax constraint on task's load during load balance

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Date:   Mon, 21 Sep 2020 09:24:21 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org
Cc:     valentin.schneider@....com, pauld@...hat.com, hdanton@...a.com,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 1/4 v2] sched/fair: relax constraint on task's load during load balance

Some UCs like 9 always running tasks on 8 CPUs can't be balanced and the
load balancer currently migrates the waiting task between the CPUs in an
almost random manner. The success of a rq pulling a task depends of the
value of nr_balance_failed of its domains and its ability to be faster
than others to detach it. This behavior results in an unfair distribution
of the running time between tasks because some CPUs will run most of the
time, if not always, the same task whereas others will share their time
between several tasks.

Instead of using nr_balance_failed as a boolean to relax the condition
for detaching task, the LB will use nr_balanced_failed to relax the
threshold between the tasks'load and the imbalance. This mecanism
prevents the same rq or domain to always win the load balance fight.

Reviewed-by: Phil Auld <pauld@...hat.com>
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 33699db27ed5..d8320dc9d014 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7674,8 +7674,8 @@ static int detach_tasks(struct lb_env *env)
 			 * scheduler fails to find a good waiting task to
 			 * migrate.
 			 */
-			if (load/2 > env->imbalance &&
-			    env->sd->nr_balance_failed <= env->sd->cache_nice_tries)
+
+			if ((load >> env->sd->nr_balance_failed) > env->imbalance)
 				goto next;

 			env->imbalance -= load;
-- 
2.17.1