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

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Message-ID: <20250905090819.107694-3-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 11:08:19 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	rust-for-linux@...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>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>
Subject: [PATCH 2/2] rust: 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>
---
 rust/kernel/workqueue.rs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
index 8ca813d68a1a..6f508c3e37e4 100644
--- a/rust/kernel/workqueue.rs
+++ b/rust/kernel/workqueue.rs
@@ -633,15 +633,15 @@ unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
     }
 }

-/// Returns the system work queue (`system_wq`).
+/// Returns the system work queue (`system_percpu_wq`).
 ///
 /// It is the one used by `schedule[_delayed]_work[_on]()`. Multi-CPU multi-threaded. There are
 /// users which expect relatively short queue flush time.
 ///
 /// Callers shouldn't queue work items which can run for too long.
 pub fn system() -> &'static Queue {
-    // SAFETY: `system_wq` is a C global, always available.
-    unsafe { Queue::from_raw(bindings::system_wq) }
+    // SAFETY: `system_percpu_wq` is a C global, always available.
+    unsafe { Queue::from_raw(bindings::system_percpu_wq) }
 }

 /// Returns the system high-priority work queue (`system_highpri_wq`).
-- 
2.51.0