linux-kernel - Re: [PATCH v2] drm/sched: Replace use of system_wq with system_percpu

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Message-ID: <b1f91dd8624ed65308b0420fe2d973f0bb44632d.camel@mailbox.org>
Date: Thu, 06 Nov 2025 16:37:18 +0100
From: Philipp Stanner <phasta@...lbox.org>
To: Marco Crivellari <marco.crivellari@...e.com>, 
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Cc: 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>, Matthew Brost
 <matthew.brost@...el.com>, Danilo Krummrich <dakr@...nel.org>,  Philipp
 Stanner <phasta@...nel.org>, Christian Konig
 <ckoenig.leichtzumerken@...il.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>
Subject: Re: [PATCH v2] drm/sched: Replace use of system_wq with
 system_percpu_wq

On Thu, 2025-11-06 at 16:01 +0100, Marco Crivellari wrote:
> In the general workqueue implementation, if a user enqueues 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.

Alright, just to be sure:
Behavior doesn't change and system_percpu_wq is then still a workqueue
where we can submit normal delayed_work?

The scheduler's timeout mechanism relies on scheduling and canceling
delayed work items.

> 
> This lack of consistency cannot be addressed without refactoring the API.
> For more details see the Link tag below.
> 
> This continues the effort to refactor worqueue APIs, which has begun
> with the change introducing new workqueues and a new alloc_workqueue flag:
> 
> commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
> commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")
> 
> Use the successor of system_wq, system_percpu_wq, for the scheduler's
> default timeout_wq. system_wq will be removed in a few release cycles.
> 
> Link: https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>

Thanks for the updated commit message, looks great.


P.

> ---
>  drivers/gpu/drm/scheduler/sched_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index c39f0245e3a9..13192e99637a 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1315,7 +1315,7 @@ int drm_sched_init(struct drm_gpu_scheduler *sched, const struct drm_sched_init_
>  	sched->name = args->name;
>  	sched->timeout = args->timeout;
>  	sched->hang_limit = args->hang_limit;
> -	sched->timeout_wq = args->timeout_wq ? args->timeout_wq : system_wq;
> +	sched->timeout_wq = args->timeout_wq ? args->timeout_wq : system_percpu_wq;
>  	sched->score = args->score ? args->score : &sched->_score;
>  	sched->dev = args->dev;
>