[tip: sched/urgent] sched/cfs: fix spurious active migration

[tip-bot2 for Vincent Guittot]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     6cf82d559e1a1d89f06ff4d428aca479c1dd0be6
Gitweb:        https://git.kernel.org/tip/6cf82d559e1a1d89f06ff4d428aca479c1dd0be6
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Fri, 29 Nov 2019 15:04:47 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 17 Dec 2019 13:32:48 +01:00

sched/cfs: fix spurious active migration

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/1575036287-6052-1-git-send-email-vincent.guittot@xxxxxxxxxx
---
 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 146b6c8..ba749f5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7328,7 +7328,14 @@ 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;