[tip: sched/core] sched/fair: Make sure to try to detach at least one movable task

[tip-bot2 for Vincent Guittot]

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

Commit-ID:     b0defa7ae03ecf91b8bfd10ede430cff12fcbd06
Gitweb:        https://git.kernel.org/tip/b0defa7ae03ecf91b8bfd10ede430cff12fcbd06
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Thu, 25 Aug 2022 14:27:23 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 15 Sep 2022 16:13:51 +02:00

sched/fair: Make sure to try to detach at least one movable task

During load balance, we try at most env->loop_max time to move a task.
But it can happen that the loop_max LRU tasks (ie tail of
the cfs_tasks list) can't be moved to dst_cpu because of affinity.
In this case, loop in the list until we found at least one.

The maximum of detached tasks remained the same as before.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20220825122726.20819-2-vincent.guittot@xxxxxxxxxx
---
 kernel/sched/fair.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4e5b171..dae3bfa 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8049,8 +8049,12 @@ static int detach_tasks(struct lb_env *env)
 		p = list_last_entry(tasks, struct task_struct, se.group_node);
 
 		env->loop++;
-		/* We've more or less seen every task there is, call it quits */
-		if (env->loop > env->loop_max)
+		/*
+		 * We've more or less seen every task there is, call it quits
+		 * unless we haven't found any movable task yet.
+		 */
+		if (env->loop > env->loop_max &&
+		    !(env->flags & LBF_ALL_PINNED))
 			break;
 
 		/* take a breather every nr_migrate tasks */
@@ -10179,7 +10183,9 @@ more_balance:
 
 		if (env.flags & LBF_NEED_BREAK) {
 			env.flags &= ~LBF_NEED_BREAK;
-			goto more_balance;
+			/* Stop if we tried all running tasks */
+			if (env.loop < busiest->nr_running)
+				goto more_balance;
 		}
 
 		/*