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

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Message-Id: <20200914100340.17608-2-vincent.guittot@linaro.org>
Date:   Mon, 14 Sep 2020 12:03:37 +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,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 1/4] 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.

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 2ba8f230feb9..765be8273292 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7669,8 +7669,8 @@ 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