linux-kernel - [PATCH 1/2] rcu: replace use of system_wq with system_percpu

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Message-ID: <20250905090747.107232-2-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 11:07:46 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org
Cc: Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Marco Crivellari <marco.crivellari@...e.com>,
	Michal Hocko <mhocko@...e.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>
Subject: [PATCH 1/2] rcu: replace use of system_wq with system_percpu_wq

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistentcy cannot be addressed without refactoring the API.

system_wq is a per-CPU worqueue, yet nothing in its name tells about that
CPU affinity constraint, which is very often not required by users. Make
it clear by adding a system_percpu_wq.

queue_work() / queue_delayed_work() mod_delayed_work() will now use the
new per-cpu wq: whether the user still stick on the old name a warn will
be printed along a wq redirect to the new one.

This patch add the new system_percpu_wq except for mm, fs and net
subsystem, whom are handled in separated patches.

The old wq will be kept for a few release cylces.

Suggested-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
---
 kernel/rcu/tasks.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index c0cc7ae41106..5fddd7168391 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -552,13 +552,13 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
 		rtpcp_next = rtp->rtpcp_array[index];
 		if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
 			cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
-			queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
+			queue_work_on(cpuwq, system_percpu_wq, &rtpcp_next->rtp_work);
 			index++;
 			if (index < num_possible_cpus()) {
 				rtpcp_next = rtp->rtpcp_array[index];
 				if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
 					cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
-					queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
+					queue_work_on(cpuwq, system_percpu_wq, &rtpcp_next->rtp_work);
 				}
 			}
 		}
-- 
2.51.0