linux-kernel - [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task

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Message-Id: <20211124154239.3191366-1-vincent.donnefort@arm.com>
Date:   Wed, 24 Nov 2021 15:42:39 +0000
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org
Cc:     linux-kernel@...r.kernel.org, mgorman@...hsingularity.net,
        dietmar.eggemann@....com, valentin.schneider@....com,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task

select_idle_sibling() will return prev_cpu for the case where the task is
woken up by a per-CPU kthread. However, the idle task has been recently
modified and is now identified by is_per_cpu_kthread(), breaking the
behaviour described above. Using !is_idle_task() ensures we do not
spuriously trigger that select_idle_sibling() exit path.

Fixes: 00b89fe0197f ("sched: Make the idle task quack like a per-CPU kthread")
Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 945d987246c5..8bf95b0e368d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6399,6 +6399,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	 * pattern is IO completions.
 	 */
 	if (is_per_cpu_kthread(current) &&
+	    !is_idle_task(current) &&
 	    prev == smp_processor_id() &&
 	    this_rq()->nr_running <= 1) {
 		return prev;
-- 
2.25.1