linux-kernel - Re: [PATCH v3 0/3] replace old wq(s), add WQ_PERCPU to alloc

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Message-ID: <34ozsv3e6ujs4rn6c2r4nrjcjifgazddy5jecwur6atfcop6vp@bunf3uyofmb4>
Date: Wed, 5 Nov 2025 08:41:52 +0000
From: Krzysztof Karas <krzysztof.karas@...el.com>
To: Marco Crivellari <marco.crivellari@...e.com>
CC: <linux-kernel@...r.kernel.org>, <intel-gfx@...ts.freedesktop.org>,
	<dri-devel@...ts.freedesktop.org>, Tejun Heo <tj@...nel.org>, Lai Jiangshan
	<jiangshanlai@...il.com>, Frederic Weisbecker <frederic@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Michal Hocko
	<mhocko@...e.com>, Jani Nikula <jani.nikula@...ux.intel.com>, Joonas Lahtinen
	<joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>, David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>
Subject: Re: [PATCH v3 0/3] replace old wq(s), add WQ_PERCPU to
 alloc_workqueue

Hi Marco,

thanks for addressing my comments!

Reviewed-by: Krzysztof Karas <krzysztof.karas@...el.com>
on the whole series.
 
Best Regards,
Krzysztof

On 2025-11-04 at 11:00:29 +0100, Marco Crivellari wrote:
> 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 1-2]  Replace uses of system_wq and system_unbound_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_wq has been replaced with system_percpu_wq, and
>     system_unbound_wq has been replaced with system_dfl_wq.
> 
> 2) [P 3] 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 3:
> - Improved commit logs
> 
> Changes in v2:
> - fix typo in patch subject (add instead of added).
> 
> - in every patch is also present the specific commit hash about the
>   workqueue API change.
> 
> - fixed commit log of P1 (removed "Adding system_dfl_wq...").
> 
> - P2: subject changed reflecting the effective change.
> 
> - rebased to v6.18-rc4.
> 
> 
> Marco Crivellari (3):
>   drm/i915: replace use of system_unbound_wq with system_dfl_wq
>   drm/i915: replace use of system_wq with system_percpu_wq in the
>     documentation
>   drm/i915: add WQ_PERCPU to alloc_workqueue users
> 
>  drivers/gpu/drm/i915/display/intel_display_driver.c | 4 ++--
>  drivers/gpu/drm/i915/display/intel_display_power.c  | 2 +-
>  drivers/gpu/drm/i915/display/intel_tc.c             | 4 ++--
>  drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c        | 2 +-
>  drivers/gpu/drm/i915/gt/uc/intel_guc.c              | 4 ++--
>  drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c           | 4 ++--
>  drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c   | 6 +++---
>  drivers/gpu/drm/i915/i915_active.c                  | 2 +-
>  drivers/gpu/drm/i915/i915_driver.c                  | 5 +++--
>  drivers/gpu/drm/i915/i915_drv.h                     | 2 +-
>  drivers/gpu/drm/i915/i915_sw_fence_work.c           | 2 +-
>  drivers/gpu/drm/i915/i915_vma_resource.c            | 2 +-
>  drivers/gpu/drm/i915/pxp/intel_pxp.c                | 2 +-
>  drivers/gpu/drm/i915/pxp/intel_pxp_irq.c            | 2 +-
>  drivers/gpu/drm/i915/selftests/i915_sw_fence.c      | 2 +-
>  drivers/gpu/drm/i915/selftests/mock_gem_device.c    | 2 +-
>  16 files changed, 24 insertions(+), 23 deletions(-)
> 
> -- 
> 2.51.1
>