Hi,

=== 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 consistency cannot be addressed without refactoring the API.

=== Recent changes to the WQ API ===

The following, address the recent changes in the Workqueue API:

- commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
- commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

The old workqueues will be removed in a future release cycle.

=== Introduced Changes by this series ===

1) [P 1-2-3]  Replace uses of system_wq and system_unbound_wq

    system_wq is a per-CPU workqueue, but his name is not clear.

    Because of that, system_wq has been replaced with system_percpu_wq.

2) [P 4] add WQ_PERCPU to alloc_workqueue()

    This change adds a new WQ_PERCPU flag to explicitly request
    alloc_workqueue() to be per-cpu when WQ_UNBOUND has not been specified.


Thanks!


Marco Crivellari (4):
  Input: gpio_keys - replace use of system_wq with system_percpu_wq
  Input: palmas-pwrbutton - replace use of system_wq with
    system_percpu_wq
  Input: mouse - replace use of system_wq with system_percpu_wq
  Input: psmouse-smbus - add WQ_PERCPU to alloc_workqueue users

 drivers/input/keyboard/gpio_keys.c    | 2 +-
 drivers/input/misc/palmas-pwrbutton.c | 2 +-
 drivers/input/mouse/psmouse-smbus.c   | 2 +-
 drivers/input/mouse/synaptics_i2c.c   | 8 ++++----
 4 files changed, 7 insertions(+), 7 deletions(-)

-- 
2.51.1

Hi Marco,

On Wed, Nov 05, 2025 at 11:35:19AM +0100, Marco Crivellari wrote:
> Hi,
> 
> === 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 consistency cannot be addressed without refactoring the API.
> 
> === Recent changes to the WQ API ===
> 
> The following, address the recent changes in the Workqueue API:
> 
> - commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
> - commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")
> 
> The old workqueues will be removed in a future release cycle.
> 
> === Introduced Changes by this series ===
> 
> 1) [P 1-2-3]  Replace uses of system_wq and system_unbound_wq
> 
>     system_wq is a per-CPU workqueue, but his name is not clear.
> 
>     Because of that, system_wq has been replaced with system_percpu_wq.
> 
> 2) [P 4] add WQ_PERCPU to alloc_workqueue()
> 
>     This change adds a new WQ_PERCPU flag to explicitly request
>     alloc_workqueue() to be per-cpu when WQ_UNBOUND has not been specified.

So neither of these 4 need percpu workqueue and instead are fine to use
"whatever is free". Maybe we can switch them to use unbound workqueues
instead?

Thanks.

-- 
Dmitry

Hi Dmitry,

On Wed, Nov 5, 2025 at 11:08 PM Dmitry Torokhov
<dmitry.torokhov@gmail.com> wrote:
> So neither of these 4 need percpu workqueue and instead are fine to use
> "whatever is free". Maybe we can switch them to use unbound workqueues
> instead?

Sure, I will send the v2 with the unbound workqueue.

Thanks!

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product