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

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Message-ID: <20251031105939.129200-1-marco.crivellari@suse.com>
Date: Fri, 31 Oct 2025 11:59:37 +0100
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	intel-xe@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.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>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>
Subject: [PATCH 0/2] replace system_unbound_wq, 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_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 sure this would be the default choice
	when locality is not important.

	system_dfl_wq behave like system_unbound_wq.

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.

	The behavior is the same.

Thanks!


Marco Crivellari (2):
  drm/xe: replace use of system_unbound_wq with system_dfl_wq
  drm/xe: WQ_PERCPU added to alloc_workqueue users

 drivers/gpu/drm/xe/xe_devcoredump.c     | 2 +-
 drivers/gpu/drm/xe/xe_device.c          | 4 ++--
 drivers/gpu/drm/xe/xe_execlist.c        | 2 +-
 drivers/gpu/drm/xe/xe_ggtt.c            | 2 +-
 drivers/gpu/drm/xe/xe_guc_ct.c          | 4 ++--
 drivers/gpu/drm/xe/xe_hw_engine_group.c | 3 ++-
 drivers/gpu/drm/xe/xe_oa.c              | 2 +-
 drivers/gpu/drm/xe/xe_sriov.c           | 2 +-
 drivers/gpu/drm/xe/xe_vm.c              | 4 ++--
 9 files changed, 13 insertions(+), 12 deletions(-)

-- 
2.51.0