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
    	system_wq is a per-CPU worqueue, replaced by system_percpu_wq. Despite that,
    	system_wq in this change has been replaced by system_dfl_wq, because there
    	aren't per-cpu variables.


Thanks!


---
Changes in v2:

- Removed 1/2 from the series because already applied
- Instead of rename system_wq with system_percpu_wq, [P 1] change the workqueue
  using system_dfl_wq (the new unbound workqueue).


Marco Crivellari (1):
  ASoC: replace use of system_wq with system_dfl_wq

 sound/soc/codecs/aw88081.c          |  2 +-
 sound/soc/codecs/aw88166.c          |  2 +-
 sound/soc/codecs/aw88261.c          |  2 +-
 sound/soc/codecs/aw88395/aw88395.c  |  2 +-
 sound/soc/codecs/aw88399.c          |  2 +-
 sound/soc/codecs/cs42l43-jack.c     |  2 +-
 sound/soc/codecs/cs42l43.c          |  4 ++--
 sound/soc/codecs/es8326.c           | 12 ++++++------
 sound/soc/codecs/rt5663.c           |  6 +++---
 sound/soc/intel/boards/sof_es8336.c |  2 +-
 sound/soc/sof/intel/cnl.c           |  2 +-
 sound/soc/sof/intel/hda-ipc.c       |  2 +-
 12 files changed, 20 insertions(+), 20 deletions(-)

-- 
2.51.0

On Mon, Sep 29, 2025 at 05:50:52PM +0200, Marco Crivellari wrote:
> Hi!
> 
> Below is a summary of a discussion about the Workqueue API and cpu isolation
> considerations. Details and more information are available here:

Please don't send cover letters for single patches, if there is anything
that needs saying put it in the changelog of the patch or after the ---
if it's administrative stuff.  This reduces mail volume and ensures that 
any important information is recorded in the changelog rather than being
lost. 

On Mon, Sep 29, 2025 at 6:58 PM Mark Brown <broonie@kernel.org> wrote:
> Please don't send cover letters for single patches, if there is anything
> that needs saying put it in the changelog of the patch or after the ---
> if it's administrative stuff.  This reduces mail volume and ensures that
> any important information is recorded in the changelog rather than being
> lost.

Ok Mark, thank you. The changelog already contains all the information.

Noted for the next time.

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product