linux-kernel - [PATCH 5.16 0231/1039] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

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Date:   Mon, 24 Jan 2022 19:33:40 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Vincent Donnefort <vincent.donnefort@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.16 0231/1039] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

From: Vincent Donnefort <vincent.donnefort@....com>

[ 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@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@....com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Link: https://lkml.kernel.org/r/20211129173115.4006346-1-vincent.donnefort@arm.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 97d89516fbea2..f2cf047b25e56 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6400,7 +6400,8 @@ 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