linux-kernel - [PATCH 0/3] replace old wq(s), added WQ_PERCPU to alloc

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Message-ID: <20251104112535.139450-1-marco.crivellari@suse.com>
Date: Tue,  4 Nov 2025 12:25:25 +0100
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-nvme@...ts.infradead.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>,
	Christoph Hellwig <hch@....de>,
	Sagi Grimberg <sagi@...mberg.me>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Justin Tee <justin.tee@...adcom.com>,
	Naresh Gottumukkala <nareshgottumukkala83@...il.com>,
	Paul Ely <paul.ely@...adcom.com>
Subject: [PATCH 0/3] replace old wq(s), added 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

    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.

2) [P 2-3] add WQ_PERCPU to all relevant alloc_workqueue() users

    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 (3):
  nvmet: replace use of system_wq with system_percpu_wq
  nvme: add WQ_PERCPU to alloc_workqueue users
  nvmet-fc: add WQ_PERCPU to alloc_workqueue users

 drivers/nvme/target/admin-cmd.c        | 2 +-
 drivers/nvme/target/core.c             | 5 +++--
 drivers/nvme/target/fabrics-cmd-auth.c | 2 +-
 drivers/nvme/target/fc.c               | 6 +++---
 drivers/nvme/target/tcp.c              | 2 +-
 5 files changed, 9 insertions(+), 8 deletions(-)

-- 
2.51.1