linux-kernel - [PATCH 2/3] mm: replace use of system_wq with system_percpu

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Message-ID: <20250905090323.103401-3-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 11:03:22 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 2/3] mm: 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 to all the mm subsystem.

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>
---
 mm/backing-dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index e9f9fdcfe052..7e672424f928 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -966,7 +966,7 @@ static int __init cgwb_init(void)
 {
 	/*
 	 * There can be many concurrent release work items overwhelming
-	 * system_wq.  Put them in a separate wq and limit concurrency.
+	 * system_percpu_wq.  Put them in a separate wq and limit concurrency.
 	 * There's no point in executing many of these in parallel.
 	 */
 	cgwb_release_wq = alloc_workqueue("cgwb_release", 0, 1);
-- 
2.51.0