From 9cb65154ad0867edcb7c3edc86c3408218972a37 Mon Sep 17 00:00:00 2001
From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Fri, 29 Nov 2019 15:04:47 +0100
Subject: [PATCH] sched/cfs: fix spurious active migration
References: bnc#1155798 (CPU scheduler functional and performance backports)
Patch-mainline: v5.5-rc3
Git-commit: 6cf82d559e1a1d89f06ff4d428aca479c1dd0be6
The load balance can fail to find a suitable task during the periodic check
because the imbalance is smaller than half of the load of the waiting
tasks. This results in the increase of the number of failed load balance,
which can end up to start an active migration. This active migration is
useless because the current running task is not a better choice than the
waiting ones. In fact, the current task was probably not running but
waiting for the CPU during one of the previous attempts and it had already
not been selected.
When load balance fails too many times to migrate a task, we should relax
the contraint on the maximum load of the tasks that can be migrated
similarly to what is done with cache hotness.
Before the rework, load balance used to set the imbalance to the average
load_per_task in order to mitigate such situation. This increased the
likelihood of migrating a task but also of selecting a larger task than
needed while more appropriate ones were in the list.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/1575036287-6052-1-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
kernel/sched/fair.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f48a4ecf935e..8034b2a44b49 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7344,7 +7344,14 @@ static int detach_tasks(struct lb_env *env)
load < 16 && !env->sd->nr_balance_failed)
goto next;
- if (load/2 > env->imbalance)
+ /*
+ * Make sure that we don't migrate too much load.
+ * Nevertheless, let relax the constraint if
+ * 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)
goto next;
env->imbalance -= load;