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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Vincent Donnefort <vincent.donnefort@arm.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.15 199/846] sched/fair: Fix detection of per-CPU kthreads waking a task
Date:   Mon, 24 Jan 2022 19:35:16 +0100
Message-Id: <20220124184107.804854195@linuxfoundation.org>
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From: Vincent Donnefort <vincent.donnefort@arm.com>

[ Upstream commit 8b4e74ccb582797f6f0b0a50372ebd9fd2372a27 ]

select_idle_sibling() has a special case for tasks woken up by a per-CPU
kthread, where the selected CPU is the previous one. However, the current
condition for this exit path is incomplete. A task can wake up from an
interrupt context (e.g. hrtimer), while a per-CPU kthread is running. A
such scenario would spuriously trigger the special case described above.
Also, a recent change made the idle task like a regular per-CPU kthread,
hence making that situation more likely to happen
(is_per_cpu_kthread(swapper) being true now).

Checking for task context makes sure select_idle_sibling() will not
interpret a wake up from any other context as a wake up by a per-CPU
kthread.

Fixes: 52262ee567ad ("sched/fair: Allow a per-CPU kthread waking a task to stack on the same CPU, to fix XFS performance regression")
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lore.kernel.org/r/20211201143450.479472-1-vincent.donnefort@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/fair.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6f16dfb742462..c6cb8832796b5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6429,6 +6429,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	 * pattern is IO completions.
 	 */
 	if (is_per_cpu_kthread(current) &&
+	    in_task() &&
 	    prev == smp_processor_id() &&
 	    this_rq()->nr_running <= 1) {
 		return prev;
-- 
2.34.1