linux-kernel - Re: [PATCH v2 0/4] replace system_wq with system_dfl_wq, add WQ_UNBOUND to alloc

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Message-ID: <yyrzuyjotbvi5mmjjerbolccclx7ul5t7qc5i3un3u2wbwuwjf@ogca4giemden>
Date: Thu, 18 Dec 2025 20:54:20 -0800
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...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>
Subject: Re: [PATCH v2 0/4] replace system_wq with system_dfl_wq, add
 WQ_UNBOUND to alloc_workqueue

On Thu, Nov 06, 2025 at 03:19:51PM +0100, Marco Crivellari 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 1-2-3]  Replace uses of system_wq
> 
>     system_wq is a per-CPU workqueue, these workloads do not benefit
> 	from locality, so change the the workqueue with the unbound,
> 	system_dfl_wq.
> 
> 2) [P 4] add WQ_UNBOUND to alloc_workqueue()
> 
> 	alloc_workqueue() users must specify one among WQ_PERCPU or WQ_UNBOUND.
> 	This workload does not benefit from a per-cpu workqueue, so make the
> 	custom wq unbound with WQ_UNBOUND.
> 
> Thanks!
> 
> ---
> Changes in v2:
> - all the patches use system_dfl_wq and the custom wq is now unbound,
>   using explicitly WQ_UNBOUND.
> 
> Marco Crivellari (4):
>   Input: gpio_keys - replace use of system_wq with system_dfl_wq
>   Input: palmas-pwrbutton - replace use of system_wq with system_dfl_wq
>   Input: mouse - replace use of system_wq with system_dfl_wq
>   Input: psmouse-smbus - add WQ_UNBOUND to alloc_workqueue user
> 
>  drivers/input/keyboard/gpio_keys.c    | 2 +-
>  drivers/input/misc/palmas-pwrbutton.c | 2 +-
>  drivers/input/mouse/psmouse-smbus.c   | 2 +-
>  drivers/input/mouse/synaptics_i2c.c   | 8 ++++----
>  4 files changed, 7 insertions(+), 7 deletions(-)
> 

Applied the lot, thank you.

-- 
Dmitry