linux-kernel - [PATCH 0/2] replace system_wq with system_percpu_wq, add WQ_PERCPU to alloc

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Message-ID: <20251105145043.231927-1-marco.crivellari@suse.com>
Date: Wed,  5 Nov 2025 15:50:41 +0100
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-bcache@...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>,
	Coly Li <colyli@...as.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>
Subject: [PATCH 0/2] replace system_wq with system_percpu_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_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 replaced with system_percpu_wq,
    keeping the same old behavior.

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!

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

 drivers/md/bcache/btree.c     |  3 ++-
 drivers/md/bcache/super.c     | 30 ++++++++++++++++--------------
 drivers/md/bcache/writeback.c |  2 +-
 3 files changed, 19 insertions(+), 16 deletions(-)

-- 
2.51.1