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] WQ_PERCPU added to alloc_workqueue()

    This 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 (2):
  virtio_balloon: add WQ_PERCPU to alloc_workqueue users
  vduse: add WQ_PERCPU to alloc_workqueue users

 drivers/vdpa/vdpa_user/vduse_dev.c | 3 ++-
 drivers/virtio/virtio_balloon.c    | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

-- 
2.51.1

On Fri, Nov 07, 2025 at 04:49:15PM +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] WQ_PERCPU added to alloc_workqueue()
> 
>     This 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 (2):
>   virtio_balloon: add WQ_PERCPU to alloc_workqueue users
>   vduse: add WQ_PERCPU to alloc_workqueue users

To make sure, this does not seem to introduce any
functional change - you want me to queue this now?


>  drivers/vdpa/vdpa_user/vduse_dev.c | 3 ++-
>  drivers/virtio/virtio_balloon.c    | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> -- 
> 2.51.1

On Mon, Nov 17, 2025 at 11:17 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> [...]
> To make sure, this does not seem to introduce any
> functional change - you want me to queue this now?

Hi,

Yes please, there are no functional changes as you said.
We are just marking explicitly this workqueue as per-cpu.

Thanks!

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product