linux-kernel - [PATCH 2/4 v2] sched/fair: reduce minimal imbalance threshold

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Message-Id: <20200921072424.14813-3-vincent.guittot@linaro.org>
Date:   Mon, 21 Sep 2020 09:24:22 +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 2/4 v2] sched/fair: reduce minimal imbalance threshold

The 25% default imbalance threshold for DIE and NUMA domain is large
enough to generate significant unfairness between threads. A typical
example is the case of 11 threads running on 2x4 CPUs. The imbalance of
20% between the 2 groups of 4 cores is just low enough to not trigger
the load balance between the 2 groups. We will have always the same 6
threads on one group of 4 CPUs and the other 5 threads on the other
group of CPUS. With a fair time sharing in each group, we ends up with
+20% running time for the group of 5 threads.

Consider decreasing the imbalance threshold for overloaded case where we
use the load to balance task and to ensure fair time sharing.

Acked-by: Hillf Danton <hdanton@...a.com>
Reviewed-by: Phil Auld <pauld@...hat.com>
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/topology.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 249bec7b0a4c..41df62884cea 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1349,7 +1349,7 @@ sd_init(struct sched_domain_topology_level *tl,
 		.min_interval		= sd_weight,
 		.max_interval		= 2*sd_weight,
 		.busy_factor		= 32,
-		.imbalance_pct		= 125,
+		.imbalance_pct		= 117,
 
 		.cache_nice_tries	= 0,
 
-- 
2.17.1