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

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Message-ID: <20250905090953.109082-2-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 11:09:53 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...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>,
	Ingo Molnar <mingo@...hat.com>,
	Rik van Riel <riel@...riel.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [PATCH 1/1] smp: 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/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 974f3a3962e8..c3b93476d645 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -1146,7 +1146,7 @@ int smp_call_on_cpu(unsigned int cpu, int (*func)(void *), void *par, bool phys)
 	if (cpu >= nr_cpu_ids || !cpu_online(cpu))
 		return -ENXIO;

-	queue_work_on(cpu, system_wq, &sscs.work);
+	queue_work_on(cpu, system_percpu_wq, &sscs.work);
 	wait_for_completion(&sscs.done);
 	destroy_work_on_stack(&sscs.work);

-- 
2.51.0