linux-kernel - [PATCH 0/4] replace old wq(s), added WQ_PERCPU to alloc

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Message-ID: <20251031095643.74246-1-marco.crivellari@suse.com>
Date: Fri, 31 Oct 2025 10:56:39 +0100
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-scsi@...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>,
	"James E . J . Bottomley" <James.Bottomley@...senPartnership.com>,
	"Martin K . Petersen" <martin.petersen@...cle.com>
Subject: [PATCH 0/4] 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 and system_unbound_wq

    system_unbound_wq is to be used when locality is not required.

    Because of that, system_unbound_wq has been replaced with
	system_dfl_wq, to make clear it should be used when locality
	is not required.

2) [P 2-3-4] 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 (4):
  scsi: replace use of system_unbound_wq with system_dfl_wq
  scsi: qla2xxx: WQ_PERCPU added to alloc_workqueue users
  scsi: scsi_dh_alua: WQ_PERCPU added to alloc_workqueue users
  scsi: scsi_transport_fc: WQ_PERCPU added to alloc_workqueue users

 drivers/scsi/device_handler/scsi_dh_alua.c | 2 +-
 drivers/scsi/qla2xxx/qla_os.c              | 4 ++--
 drivers/scsi/scsi_transport_fc.c           | 5 +++--
 drivers/scsi/scsi_transport_iscsi.c        | 2 +-
 4 files changed, 7 insertions(+), 6 deletions(-)

-- 
2.51.0