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

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Message-ID: <20250905090240.102790-2-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 11:02:39 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	io-uring@...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>,
	Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH 1/2] io_uring: 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>
---
 io_uring/io_uring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index c6209fe44cb1..2a6ead3c7d36 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2986,7 +2986,7 @@ static __cold void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 	 * Use system_unbound_wq to avoid spawning tons of event kworkers
 	 * if we're exiting a ton of rings at the same time. It just adds
 	 * noise and overhead, there's no discernable change in runtime
-	 * over using system_wq.
+	 * over using system_percpu_wq.
 	 */
 	queue_work(iou_wq, &ctx->exit_work);
 }
-- 
2.51.0