[PATCH 0/2] workqueue: Make WQ_UNBOUND the default behavior

[PATCH 0/2] workqueue: Make WQ_UNBOUND the default behavior

[Marco Crivellari]

Hi,

Currently alloc_workqueue() make sure one among WQ_UNBOUND or WQ_PERCPU is
present. This is enforced warning and setting WQ_PERCPU as default when
alloc_workqueue() without WQ_PERCPU or WQ_UNBOUND.

[ P1 ] change this behavior: if no flags are specified, WQ_UNBOUND is
set by default, changing the default behavior when flags are not present.

The WARN_ONCE() when neither flags are present has been removed and it has
been replaced by pr_warn_once(): from now on, every alloc_users()
without WQ_PERCPU will be considered unbound.

Q: I decided to pr_warn_once() just to make sure users are aware of the new
behavior. It makes sense or we can just drop it directly?
I don't think the WARN_ONCE() is appropriate now, for this specific path.

[ P2 ] updates the documentation accordingly.

Thanks!

Marco Crivellari (2):
  workqueue: Make alloc_workqueue() unbound by default
  doc: Update WQ_UNBOUND documentation

 Documentation/core-api/workqueue.rst |  2 ++
 kernel/workqueue.c                   | 14 +++++++-------
 2 files changed, 9 insertions(+), 7 deletions(-)

-- 
2.55.0

[PATCH 1/2] workqueue: Make alloc_workqueue() unbound by default

[Marco Crivellari]

Currently alloc_workqueue() ensure that one among WQ_PERCPU or WQ_UNBOUND
is present. Now every user has one among these two flags.

In order to achive the future goal of removing WQ_UNBOUND from the code,
start changing the default behavior making alloc_workqueue() act this way:

  - if WQ_PERCPU and WQ_UNBOUND are present: remove WQ_PERCPU (as before)
  - if neither are present: add WQ_UNBOUND

The WARN_ONCE() when neither flags are present has been removed and it has
been replaced by pr_warn_once(): from now on, every alloc_users()
without WQ_PERCPU will be considered unbound.

The pr_warn_once() will be removed along with WQ_UNBOUND in the future.

Link: https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 kernel/workqueue.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1ae3732a2c51..8681343597e8 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5928,16 +5928,16 @@ static struct workqueue_struct *__alloc_workqueue(const char *fmt,
 			     wq->name);
 
 	/*
-	 * One among WQ_PERCPU and WQ_UNBOUND must be set, but not both.
-	 * - If neither is set, default to WQ_PERCPU
-	 * - If both are set, default to WQ_UNBOUND
+	 * Every caller that doesn't explicilty specify WQ_PERCPU will be
+	 * unbound (WQ_UNBOUND) by default.
 	 *
-	 * This code can be removed after workqueue are unbound by default
+	 * If both flags are present at the same time, WQ_PERCPU will be
+	 * removed.
 	 */
 	if (unlikely(!(flags & (WQ_UNBOUND | WQ_PERCPU)))) {
-		WARN_ONCE(1, "workqueue: %s is using neither WQ_PERCPU or WQ_UNBOUND. "
-			  "Setting WQ_PERCPU.\n", wq->name);
-		flags |= WQ_PERCPU;
+		pr_warn_once("workqueue: %s is using neither WQ_PERCPU or WQ_UNBOUND. "
+			  "Setting WQ_UNBOUND.\n", wq->name);
+		flags |= WQ_UNBOUND;
 	} else if (unlikely((flags & WQ_PERCPU) && (flags & WQ_UNBOUND))) {
 		WARN_ONCE(1, "workqueue: %s uses both WQ_PERCPU and WQ_UNBOUND. "
 			  "Dropped WQ_PERCPU, keeping WQ_UNBOUND.\n", wq->name);
-- 
2.55.0

[PATCH 2/2] doc: Update WQ_UNBOUND documentation

[Marco Crivellari]

When WQ_PERCPU is not present the default behavior is now WQ_UNBOUND,
so update the documentation, reflecting this change.

Link: https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 Documentation/core-api/workqueue.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/core-api/workqueue.rst b/Documentation/core-api/workqueue.rst
index bb770f556568..3ca636e065ef 100644
--- a/Documentation/core-api/workqueue.rst
+++ b/Documentation/core-api/workqueue.rst
@@ -206,6 +206,8 @@ resources, scheduled and executed.
   * Long running CPU intensive workloads which can be better
     managed by the system scheduler.
 
+  Workqueue without ``WQ_PERCPU`` are unbound by default.
+
 ``WQ_FREEZABLE``
   A freezable wq participates in the freeze phase of the system
   suspend operations.  Work items on the wq are drained and no
-- 
2.55.0

Re: [PATCH 1/2] workqueue: Make alloc_workqueue() unbound by default

[Marco Crivellari]

Hi,

On Thu, Sep 3, 2026 at 5:22 PM Marco Crivellari
<marco.crivellari@suse.com> wrote:
> [...]
>         if (unlikely(!(flags & (WQ_UNBOUND | WQ_PERCPU)))) {
> -               WARN_ONCE(1, "workqueue: %s is using neither WQ_PERCPU or WQ_UNBOUND. "
> -                         "Setting WQ_PERCPU.\n", wq->name);
> -               flags |= WQ_PERCPU;
> +               pr_warn_once("workqueue: %s is using neither WQ_PERCPU or WQ_UNBOUND. "
> +                         "Setting WQ_UNBOUND.\n", wq->name);
> +               flags |= WQ_UNBOUND;
>         } else if (unlikely((flags & WQ_PERCPU) && (flags & WQ_UNBOUND))) {
>                 WARN_ONCE(1, "workqueue: %s uses both WQ_PERCPU and WQ_UNBOUND. "
>                           "Dropped WQ_PERCPU, keeping WQ_UNBOUND.\n", wq->name);

Please ignore the patch, I will send a v2.
This will cause issues because wq_size is calculated before:

5911     if (flags & WQ_UNBOUND)
5912         wq_size = struct_size(wq, node_nr_active, nr_node_ids + 1);
5913     else
5914         wq_size = sizeof(*wq);
5915
5916     wq = kzalloc_noprof(wq_size, GFP_KERNEL);
5917     if (!wq)
5918         return NULL;

Thanks, and sorry for the noise!

--

Marco Crivellari

SUSE Labs