linux-kernel - [PATCH v2 0/2] replace system_unbound_wq, add WQ_PERCPU to alloc

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Message-ID: <20251114141618.172154-1-marco.crivellari@suse.com>
Date: Fri, 14 Nov 2025 15:16:16 +0100
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org
Cc: Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Marco Crivellari <marco.crivellari@...e.com>,
	Michal Hocko <mhocko@...e.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH v2 0/2] replace system_unbound_wq, add WQ_PERCPU to alloc_workqueue

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] Replace uses of system_unbound_wq

    system_wq is a per-CPU workqueue, system_unbound_wq is to be used when
	locality is not required.

	Switch to using system_dfl_wq because system_unbound_wq is going away
	as part of a workqueue restructuring.

2) [P 2] 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!

---
Changes in v2:
- improved commit logs

- rebase on 6.18-rc5

- removed "devcoredump: replace use of system_wq with system_percpu_wq":
  now is using schedule_work instead of mod_delayed_work, there are no
  wq to replace.


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

 drivers/base/core.c | 2 +-
 drivers/base/dd.c   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

-- 
2.51.1