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* [PATCH v2 0/3] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users
@ 2025-09-17 15:38 Marco Crivellari
  2025-09-17 15:38 ` [PATCH v2 1/3] drivers/s390: WQ_PERCPU added to alloc_workqueue users Marco Crivellari
                   ` (3 more replies)
  0 siblings, 4 replies; 8+ messages in thread
From: Marco Crivellari @ 2025-09-17 15:38 UTC (permalink / raw)
  To: linux-kernel, linux-s390
  Cc: Tejun Heo, Lai Jiangshan, Frederic Weisbecker,
	Sebastian Andrzej Siewior, Marco Crivellari, Michal Hocko,
	Alexander Gordeev, Vasily Gorbik, Heiko Carstens

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@linutronix.de/

=== 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 series ===

1) [P 1] add WQ_PERCPU to remaining alloc_workqueue() users

        Every alloc_workqueue() caller should use one among WQ_PERCPU or
        WQ_UNBOUND. WQ_PERCPU has been added to every user that is not
		actually using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a next release cycle.

2) [P 2] Replace use of system_wq

        system_wq is a per-CPU workqueue, but his name is not clear.

        Because of that, system_wq has been renamed in system_percpu_wq.

3) [P 3] Replace use of system_wq with system_dfl_wq
			(arch/s390/kernel/hiperdispatch.c)

		 system_wq is a per-cpu wq but hiperdispatch users does not benefit
		 from it. This has been changed with system_dfl_wq (the new unbound wq).

Thanks!

---
Changes in v2:
- New in the series, 2/3: arch/s390/kernel/hiperdispatch.c does not benefit from
  system_wq: this has been converted directly to system_unbound_wq.
- the others system_wq users are converted to system_percpu_wq in patch 3/3


Marco Crivellari (3):
  drivers/s390: WQ_PERCPU added to alloc_workqueue users
  s390/diag324: replace use of system_wq with system_percpu_wq
  s390: replace use of system_wq with system_dfl_wq

 arch/s390/kernel/diag/diag324.c  | 4 ++--
 arch/s390/kernel/hiperdispatch.c | 2 +-
 drivers/s390/char/tape_3590.c    | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

-- 
2.51.0


^ permalink raw reply	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2025-09-24  9:06 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2025-09-17 15:38 [PATCH v2 0/3] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users Marco Crivellari
2025-09-17 15:38 ` [PATCH v2 1/3] drivers/s390: WQ_PERCPU added to alloc_workqueue users Marco Crivellari
2025-09-17 15:38 ` [PATCH v2 2/3] s390/diag324: replace use of system_wq with system_percpu_wq Marco Crivellari
2025-09-17 15:38 ` [PATCH v2 3/3] s390: replace use of system_wq with system_dfl_wq Marco Crivellari
2025-09-24  8:05   ` Mete Durlu
2025-09-24  8:11     ` Marco Crivellari
2025-09-24  9:03 ` [PATCH v2 0/3] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users Heiko Carstens
2025-09-24  9:05   ` Marco Crivellari

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