linux-kernel - [PATCH] workqueue: Always use wq_select_unbound_cpu() for WORK_CPU

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Message-ID: <20250221112003.1dSuoGyc@linutronix.de>
Date: Fri, 21 Feb 2025 12:20:03 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: open list <linux-kernel@...r.kernel.org>
Cc: Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH] workqueue: Always use wq_select_unbound_cpu() for
 WORK_CPU_UNBOUND.

If the user did not specify a CPU while enqueuing a work item then
WORK_CPU_UNBOUND is passed. In this case, for WQ_UNBOUND a CPU is
selected based on wq_unbound_cpumask while the local CPU is preferred.
For !WQ_UNBOUND the local CPU is selected.
For NOHZ_FULL system with isolated CPU wq_unbound_cpumask is set to the
not isolated (housekeeping) CPUs. This leads to different behaviour if a
work item is scheduled on an isolated CPU where
	schedule_delayed_work(, 1);

will move the timer to the housekeeping CPU and then schedule the work
there (on the housekeeping CPU) while
	schedule_delayed_work(, 0);

will schedule the work item on the isolated CPU.

The documentation says WQ_UNBOUND prefers the local CPU. It can
preferer the local CPU if it is part of wq_unbound_cpumask.

Restrict WORK_CPU_UNBOUND to wq_unbound_cpumask via
wq_select_unbound_cpu().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/workqueue.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index bfe030b443e27..134d9550538aa 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2261,12 +2261,8 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
 	rcu_read_lock();
 retry:
 	/* pwq which will be used unless @work is executing elsewhere */
-	if (req_cpu == WORK_CPU_UNBOUND) {
-		if (wq->flags & WQ_UNBOUND)
-			cpu = wq_select_unbound_cpu(raw_smp_processor_id());
-		else
-			cpu = raw_smp_processor_id();
-	}
+	if (req_cpu == WORK_CPU_UNBOUND)
+		cpu = wq_select_unbound_cpu(raw_smp_processor_id());
 
 	pwq = rcu_dereference(*per_cpu_ptr(wq->cpu_pwq, cpu));
 	pool = pwq->pool;
-- 
2.47.2