From c50660dcd07c1ecd7c9095e9e550c43fb309fc5e Mon Sep 17 00:00:00 2001
From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Mon, 21 Sep 2020 09:24:22 +0200
Subject: [PATCH] sched/fair: Reduce minimal imbalance threshold

References: bnc#1155798 (CPU scheduler functional and performance backports)
Git-commit: 2208cdaa56c957e20d8e16f28819aeb47851cb1e
Patch-mainline: v5.10-rc1

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.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Acked-by: Hillf Danton <hdanton@sina.com>
Link: https://lkml.kernel.org/r/20200921072424.14813-3-vincent.guittot@linaro.org
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 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 64b608d1707c..9f54e0915b4c 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1347,7 +1347,7 @@ 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,