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 use 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 renamed in system_percpu_wq.

2) [P 2] add WQ_PERCPU to remaining alloc_workqueue() users

        Every alloc_workqueue() caller should use one among WQ_PERCPU or
        WQ_UNBOUND. This is actually enforced warning if both or none of them
        are present at the same time.

        WQ_UNBOUND will be removed in a next release cycle.

=== For Maintainers ===

There are prerequisites for this series, already merged in the master branch.
The commits are:

128ea9f6ccfb6960293ae4212f4f97165e42222d ("workqueue: Add system_percpu_wq and
system_dfl_wq")

930c2ea566aff59e962c50b2421d5fcc3b98b8be ("workqueue: Add new WQ_PERCPU flag")


Thanks!

Marco Crivellari (2):
  power: supply: replace use of system_wq with system_percpu_wq
  power: supply: WQ_PERCPU added to alloc_workqueue users

 drivers/power/supply/ab8500_btemp.c       | 3 ++-
 drivers/power/supply/bq2415x_charger.c    | 2 +-
 drivers/power/supply/bq24190_charger.c    | 2 +-
 drivers/power/supply/bq27xxx_battery.c    | 6 +++---
 drivers/power/supply/ipaq_micro_battery.c | 3 ++-
 drivers/power/supply/rk817_charger.c      | 6 +++---
 drivers/power/supply/ucs1002_power.c      | 2 +-
 drivers/power/supply/ug3105_battery.c     | 6 +++---
 8 files changed, 16 insertions(+), 14 deletions(-)

-- 
2.51.0

On Fri, 05 Sep 2025 11:06:39 +0200, Marco Crivellari wrote:
> 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 ===
> 
> [...]

Applied, thanks!

[1/2] power: supply: replace use of system_wq with system_percpu_wq
      commit: c4a7748b551e5a06fe9a3862001192b1b5cfe195
[2/2] power: supply: WQ_PERCPU added to alloc_workqueue users
      commit: cc2ec444e461b6ca2bc73cd7cbd06aaf15bdfa1a

Best regards,
-- 
Sebastian Reichel <sebastian.reichel@collabora.com>

On Tue, Sep 9, 2025 at 2:19 AM Sebastian Reichel
<sebastian.reichel@collabora.com> wrote:
> Applied, thanks!
>
> [1/2] power: supply: replace use of system_wq with system_percpu_wq
>       commit: c4a7748b551e5a06fe9a3862001192b1b5cfe195
> [2/2] power: supply: WQ_PERCPU added to alloc_workqueue users
>       commit: cc2ec444e461b6ca2bc73cd7cbd06aaf15bdfa1a

Thank you!
-- 

Marco Crivellari

L3 Support Engineer, Technology & Product

marco.crivellari@suse.com