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

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Message-ID: <aaac1c9b25d0fc2500e67d05948a22d77dcc72e7@intel.com>
Date: Mon, 10 Nov 2025 14:11:56 +0200
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: Krzysztof Karas <krzysztof.karas@...el.com>, 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>, 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

On Wed, 05 Nov 2025, Krzysztof Karas <krzysztof.karas@...el.com> wrote:
> Hi Marco,
>
> thanks for addressing my comments!
>
> Reviewed-by: Krzysztof Karas <krzysztof.karas@...el.com>
> on the whole series.

The series absolutely must go through both i915 and xe CI before
merging. Krzysztof, can you please make follow through with that?

BR,
Jani.

>  
> 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
>> 
>

-- 
Jani Nikula, Intel