[PATCH 0/2] io_uring: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[PATCH 0/2] io_uring: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Marco Crivellari]

Hi!

Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:

        "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
        https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/

=== Current situation: problems ===

Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.

This leads to different scenarios if a work item is scheduled on an isolated
CPU where "delay" value is 0 or greater then 0:
        schedule_delayed_work(, 0);

This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:

        schedule_delayed_work(, 1);

Will move the timer on an housekeeping CPU, and schedule the work there.

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.

=== Plan and future plans ===

This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.

These are the main steps:
1)  API refactoring (that this patch is introducing)
    -   Make more clear and uniform the system wq names, both per-cpu and
        unbound. This to avoid any possible confusion on what should be
        used.

    -   Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
        introduced in this patchset and used on all the callers that are not
        currently using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a future release cycle.

        Most users don't need to be per-cpu, because they don't have
        locality requirements, because of that, a next future step will be
        make "unbound" the default behavior.

2)  Check who really needs to be per-cpu
    -   Remove the WQ_PERCPU flag when is not strictly required.

3)  Add a new API (prefer local cpu)
    -   There are users that don't require a local execution, like mentioned
        above; despite that, local execution yeld to performance gain.

        This new API will prefer the local execution, without requiring it.

=== Introduced Changes by this series ===

1) [P 1-2] Replace use of system_wq and system_unbound_wq

        system_wq is a per-CPU workqueue, but his name is not clear.
        system_unbound_wq is to be used when locality is not required.

        Because of that, system_wq has been renamed in system_percpu_wq, and
        system_unbound_wq has been renamed in system_dfl_wq.

=== For Maintainers ===

There are prerequisites for this series, already merged in the master branch.
The commits are:

128ea9f6ccfb6960293ae4212f4f97165e42222d ("workqueue: Add system_percpu_wq and
system_dfl_wq")

930c2ea566aff59e962c50b2421d5fcc3b98b8be ("workqueue: Add new WQ_PERCPU flag")


Thanks!

Marco Crivellari (2):
  io_uring: replace use of system_wq with system_percpu_wq
  io_uring: replace use of system_unbound_wq with system_dfl_wq

 io_uring/io_uring.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

-- 
2.51.0

[PATCH 1/2] io_uring: replace use of system_wq with system_percpu_wq

[Marco Crivellari]

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@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.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

[PATCH 2/2] io_uring: replace use of system_unbound_wq with system_dfl_wq

[Marco Crivellari]

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_unbound_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

Adding system_dfl_wq to encourage its use when unbound work should be used.

queue_work() / queue_delayed_work() / mod_delayed_work() will now use the
new unbound wq: whether the user still use the old wq a warn will be
printed along with a wq redirect to the new one.

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.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 2a6ead3c7d36..74972ecf2045 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2983,7 +2983,7 @@ static __cold void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 
 	INIT_WORK(&ctx->exit_work, io_ring_exit_work);
 	/*
-	 * Use system_unbound_wq to avoid spawning tons of event kworkers
+	 * Use system_dfl_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_percpu_wq.
-- 
2.51.0

Re: [PATCH 0/2] io_uring: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Jens Axboe]

On Fri, 05 Sep 2025 11:02:38 +0200, Marco Crivellari wrote:
> Below is a summary of a discussion about the Workqueue API and cpu isolation
> considerations. Details and more information are available here:
> 
>         "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
>         https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
> 
> === Current situation: problems ===
> 
> [...]

Applied, thanks!

[1/2] io_uring: replace use of system_wq with system_percpu_wq
      commit: e92f5c03d32409c957864f9bc611872861f8157e
[2/2] io_uring: replace use of system_unbound_wq with system_dfl_wq
      commit: 59cfd1fa5a5b87969190c3178180d025ee9251a7

Best regards,
-- 
Jens Axboe

Re: [PATCH 0/2] io_uring: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Marco Crivellari]

On Tue, Sep 9, 2025 at 5:12 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> Applied, thanks!
>
> [1/2] io_uring: replace use of system_wq with system_percpu_wq
>       commit: e92f5c03d32409c957864f9bc611872861f8157e
> [2/2] io_uring: replace use of system_unbound_wq with system_dfl_wq
>       commit: 59cfd1fa5a5b87969190c3178180d025ee9251a7
>

Many thanks!

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product

marco.crivellari@suse.com