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] add WQ_UNBOUND to pm_wq (alloc_workqueue() flag)

        Every alloc_workqueue() caller should use one among WQ_PERCPU or
        WQ_UNBOUND. Add WQ_UNBOUND to pm_wq to make it explicit this wq is
		unbound.

        WQ_UNBOUND will be removed in a next release cycle.


Thanks!


---
Changes in v2:
- WQ_PERCPU removed from pm_wq.

- WQ_UNBOUND has been added to pm_wq to make explicit this wq is unbound.


Marco Crivellari (1):
  PM: WQ_UNBOUND added to pm_wq workqueue

 kernel/power/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.51.0

On Fri, Sep 19, 2025 at 5:30 PM Marco Crivellari
<marco.crivellari@suse.com> wrote:
> [...]
> Changes in v2:
> - WQ_PERCPU removed from pm_wq.
>
> - WQ_UNBOUND has been added to pm_wq to make explicit this wq is unbound.
>
>
> Marco Crivellari (1):
>   PM: WQ_UNBOUND added to pm_wq workqueue
>
>  kernel/power/main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Gentle Ping. :-)

Thanks!

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product

On Fri, Oct 10, 2025 at 12:16 PM Marco Crivellari
<marco.crivellari@suse.com> wrote:
>
> On Fri, Sep 19, 2025 at 5:30 PM Marco Crivellari
> <marco.crivellari@suse.com> wrote:
> > [...]
> > Changes in v2:
> > - WQ_PERCPU removed from pm_wq.
> >
> > - WQ_UNBOUND has been added to pm_wq to make explicit this wq is unbound.
> >
> >
> > Marco Crivellari (1):
> >   PM: WQ_UNBOUND added to pm_wq workqueue
> >
> >  kernel/power/main.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Gentle Ping. :-)

Well, a merge window is in progress.  I'll get to this when it's over.

Thanks!