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] replace system_unbound_wq with system_dfl_workqueue
		system_unbound_wq is to be used when locality is not required.
		This should be the default choice. Make this clear renaming the
		workqueue in system_dfl_wq.

2)	[P 2] WQ_PERCPU added to alloc_workqueue() users
		Every alloc_workqueue() users should specify one among WQ_PERCPU
		or WQ_UNBOUND.
		
		Make this change ensuring that every alloc_workqueue() without
		WQ_UNBOUND now make use of WQ_PERCPU.


Thanks! 


Marco Crivellari (2):
  wifi: replace use of system_unbound_wq with system_dfl_wq
  wifi: WQ_PERCPU added to alloc_workqueue users

 drivers/net/wireless/ath/ath6kl/usb.c         | 2 +-
 drivers/net/wireless/quantenna/qtnfmac/core.c | 3 ++-
 drivers/net/wireless/realtek/rtlwifi/base.c   | 2 +-
 drivers/net/wireless/realtek/rtw88/usb.c      | 3 ++-
 drivers/net/wireless/silabs/wfx/main.c        | 2 +-
 drivers/net/wireless/st/cw1200/bh.c           | 4 ++--
 net/wireless/core.c                           | 4 ++--
 net/wireless/sysfs.c                          | 2 +-
 8 files changed, 12 insertions(+), 10 deletions(-)

-- 
2.51.0