linux-kernel - Re: [PATCH 0/5] replace old wq(s), added WQ_PERCPU to alloc

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Message-ID: <CAJZ5v0gV_6+3WC6eLe3nGagx+NbmqsOFtDGFnBhyrU=H+_=+dQ@mail.gmail.com>
Date: Mon, 3 Nov 2025 18:47:18 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org, 
	Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, 
	Frederic Weisbecker <frederic@...nel.org>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Michal Hocko <mhocko@...e.com>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 0/5] replace old wq(s), added WQ_PERCPU to alloc_workqueue

On Thu, Oct 30, 2025 at 4:47 PM Marco Crivellari
<marco.crivellari@...e.com> 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]  Replace uses of system_wq and system_unbound_wq
>
>         system_wq is a per-CPU workqueue, but his name is not clear.
>         system_unbound_wq is to be used when locality is not required.
>
>         Because of that, system_wq has been replaced with system_percpu_wq, and
>         system_unbound_wq has been replaced with system_dfl_wq.
>
> 2) [P 3-4-5] WQ_PERCPU added to alloc_workqueue()
>
>         This change 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 (5):
>   ACPI: scan: replace use of system_unbound_wq with system_dfl_wq
>   ACPI: OSL: replace use of system_wq with system_percpu_wq
>   ACPI: EC: WQ_PERCPU added to alloc_workqueue users
>   ACPI: OSL: WQ_PERCPU added to alloc_workqueue users
>   ACPI: thermal: WQ_PERCPU added to alloc_workqueue users
>
>  drivers/acpi/ec.c      | 3 ++-
>  drivers/acpi/osl.c     | 6 +++---
>  drivers/acpi/scan.c    | 2 +-
>  drivers/acpi/thermal.c | 3 ++-
>  4 files changed, 8 insertions(+), 6 deletions(-)
>
> --

All applied as 6.19 material with minor tweaks in some subjects.

Thanks!