[PATCH v4 0/3] Workqueue: add WQ_PERCPU, system_dfl_wq and system_percpu_wq

[Marco Crivellari]

Hi!

Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:

    "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
    https://lore.kernel.org/all/20250221112003.1dSuoGyc@xxxxxxxxxxxxx/

=== 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 consistentcy cannot be addressed without refactoring the API.

=== Plan and future plans ===

This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.

These are the main steps:
1)  API refactoring (that this patch is introducing)
    -   Make more clear and uniform the system wq names, both per-cpu and
        unbound. This to avoid any possible confusion on what should be
        used.

    -   Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
        introduced in this patchset and used on all the callers that are not
        currently using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a future release cycle.

        Most users don't need to be per-cpu, because they don't have
        locality requirements, because of that, a next future step will be
        make "unbound" the default behavior.

2)  Check who really needs to be per-cpu
    -   Remove the WQ_PERCPU flag when is not strictly required.

3)  Add a new API (prefer local cpu)
    -   There are users that don't require a local execution, like mentioned
        above; despite that, local execution yeld to performance gain.

        This new API will prefer the local execution, without requiring it.

=== Introduced Changes by this patchset ===

1)	[P1] add system_percpu_wq and system_dfl_wq
		
	system_wq is a per-CPU workqueue, but his name is not clear.
	system_unbound_wq is to be used when locality is not required.

	Because of that, system_percpu_wq and system_dfl_wq have been
	introduced in order to replace, in future, system_wq and
	system_unbound_wq.

2)	[P2] add new WQ_PERCPU flag
		
	This patch adds the new WQ_PERCPU flag to explicitly require to be per-cpu.
	WQ_UNBOUND will be removed in a next release cycle.

3)	[P3] Doc change about WQ_PERCPU
        
	Added a short section about WQ_PERCPU and a Note under WQ_UNBOUND
	mentioning that it will be removed in the future.

---
Changes in v4:
-   Take a step back from the previous version, in order to add first the new
    wq(s) and the new flag (WQ_PERCPU), addressing later all the other changes.

Changes in v3:
-   The introduction of the new wq(s) and the WQ_PERCPU flag have been moved
    in separated patches (1 for wq(s) and 1 for WQ_PERCPU).
-   WQ_PERCPU is now added to all the alloc_workqueue callers in separated patches
    addressing few subsystems first (fs, mm, net).

Changes in v2:
-   Introduction of WQ_PERCPU change has been merged with the alloc_workqueue()
    patch that pass the WQ_PERCPU flag explicitly to every caller.
-   (2 drivers) in the code not matched by Coccinelle; WQ_PERCPU added also there.
-   WQ_PERCPU added to __WQ_BH_ALLOWS.
-   queue_work() now prints a warning (pr_warn_once()) if a user is using the
    old wq and redirect the wrong / old wq to the new one.
-   Changes to workqueue.rst about the WQ_PERCPU flag and a Note about the
    future of WQ_UNBOUND.

Marco Crivellari (3):
  Workqueue: add system_percpu_wq and system_dfl_wq
  Workqueue: add new WQ_PERCPU flag
  [Doc] Workqueue: add WQ_PERCPU

 Documentation/core-api/workqueue.rst | 10 ++++++++++
 include/linux/workqueue.h            |  9 ++++++---
 kernel/workqueue.c                   |  4 ++++
 3 files changed, 20 insertions(+), 3 deletions(-)

-- 
2.49.0