From 3c001412c7f4bb965332b40fa56542bc9730a784 Mon Sep 17 00:00:00 2001
From: Vincent Donnefort <vincent.donnefort@arm.com>
Date: Mon, 29 Nov 2021 17:31:15 +0000
Subject: [PATCH] sched/fair: Fix per-CPU kthread and wakee stacking for asym
 CPU capacity

References: bsc#1189999 (Scheduler functional and performance backports)
Patch-mainline: v5.17-rc1
Git-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@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lkml.kernel.org/r/20211129173115.4006346-1-vincent.donnefort@arm.com
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 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 5036864f8a2d..9b6c73ae6746 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6541,7 +6541,8 @@ 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;
 	}