[PATCH v2 0/2] replace system_unbound_wq, add WQ_PERCPU to alloc_workqueue

[PATCH v2 0/2] replace system_unbound_wq, add WQ_PERCPU to alloc_workqueue

[Marco Crivellari]

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

[PATCH v2 1/2] driver core: replace use of system_unbound_wq with system_dfl_wq

[Marco Crivellari]

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 consistentcy cannot be addressed without refactoring the API.

This continues the effort to refactor workqueue APIs, which began with
the introduction of new workqueues and a new alloc_workqueue flag in:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

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

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 drivers/base/dd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 13ab98e033ea..4938a132a085 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -193,7 +193,7 @@ void driver_deferred_probe_trigger(void)
 	 * Kick the re-probe thread.  It may already be scheduled, but it is
 	 * safe to kick it again.
 	 */
-	queue_work(system_unbound_wq, &deferred_probe_work);
+	queue_work(system_dfl_wq, &deferred_probe_work);
 }
 
 /**
-- 
2.51.1

[PATCH v2 2/2] driver core: WQ_PERCPU added to alloc_workqueue users

[Marco Crivellari]

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.

alloc_workqueue() treats all queues as per-CPU by default, while unbound
workqueues must opt-in via WQ_UNBOUND.

This default is suboptimal: most workloads benefit from unbound queues,
allowing the scheduler to place worker threads where they’re needed and
reducing noise when CPUs are isolated.

This continues the effort to refactor workqueue APIs, which began with
the introduction of new workqueues and a new alloc_workqueue flag in:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

This change adds a new WQ_PERCPU flag to explicitly request
alloc_workqueue() to be per-cpu when WQ_UNBOUND has not been specified.

With the introduction of the WQ_PERCPU flag (equivalent to !WQ_UNBOUND),
any alloc_workqueue() caller that doesn’t explicitly specify WQ_UNBOUND
must now use WQ_PERCPU.

Once migration is complete, WQ_UNBOUND can be removed and unbound will
become the implicit default.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 drivers/base/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index f69dc9c85954..40de2f51a1b1 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -4138,7 +4138,7 @@ int __init devices_init(void)
 	sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
 	if (!sysfs_dev_char_kobj)
 		goto char_kobj_err;
-	device_link_wq = alloc_workqueue("device_link_wq", 0, 0);
+	device_link_wq = alloc_workqueue("device_link_wq", WQ_PERCPU, 0);
 	if (!device_link_wq)
 		goto wq_err;
 
-- 
2.51.1

Re: [PATCH v2 0/2] replace system_unbound_wq, add WQ_PERCPU to alloc_workqueue

[Marco Crivellari]

On Fri, Nov 14, 2025 at 3:16 PM Marco Crivellari
<marco.crivellari@suse.com> wrote:
> 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(-)

Gentle ping.

Thanks!

--

Marco Crivellari

L3 Support Engineer