[PATCH 5.15 200/846] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

[Greg Kroah-Hartman]

From: Vincent Donnefort <vincent.donnefort@xxxxxxx>

[ Upstream commit 014ba44e8184e1acf93e0cbb7089ee847802f8f0 ]

select_idle_sibling() has a special case for tasks woken up by a per-CPU
kthread where the selected CPU is the previous one. For asymmetric CPU
capacity systems, the assumption was that the wakee couldn't have a
bigger utilization during task placement than it used to have during the
last activation. That was not considering uclamp.min which can completely
change between two task activations and as a consequence mandates the
fitness criterion asym_fits_capacity(), even for the exit path described
above.

Fixes: b4c9c9f15649 ("sched/fair: Prefer prev cpu in asymmetric wakeup path")
Signed-off-by: Vincent Donnefort <vincent.donnefort@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Link: https://lkml.kernel.org/r/20211129173115.4006346-1-vincent.donnefort@xxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c6cb8832796b5..d41f966f5866a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6431,7 +6431,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	if (is_per_cpu_kthread(current) &&
 	    in_task() &&
 	    prev == smp_processor_id() &&
-	    this_rq()->nr_running <= 1) {
+	    this_rq()->nr_running <= 1 &&
+	    asym_fits_capacity(task_util, prev)) {
 		return prev;
 	}
 
-- 
2.34.1