linux-kernel - Re: [PATCH 1/1] closure: replace use of system_wq with system_percpu

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Message-ID: <n7ktycej5ipkjem7hdf6bqsz3srawduxdstxe5du3gybwrm5gz@wdd3ipeyxn7a>
Date: Sat, 6 Sep 2025 14:53:45 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-bcachefs@...r.kernel.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>
Subject: Re: [PATCH 1/1] closure: replace use of system_wq with
 system_percpu_wq

On Fri, Sep 05, 2025 at 10:56:25AM +0200, Marco Crivellari wrote:
> 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.
> 
> system_wq is a per-CPU worqueue, yet nothing in its name tells about that
> CPU affinity constraint, which is very often not required by users. Make
> it clear by adding a system_percpu_wq.
> 
> queue_work() / queue_delayed_work() mod_delayed_work() will now use the
> new per-cpu wq: whether the user still stick on the old name a warn will
> be printed along a wq redirect to the new one.
> 
> This patch add the new system_percpu_wq except for mm, fs and net
> subsystem, whom are handled in separated patches.
> 
> The old wq will be kept for a few release cylces.
> 
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> ---
>  include/linux/closure.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/closure.h b/include/linux/closure.h
> index 880fe85e35e9..959b3c584254 100644
> --- a/include/linux/closure.h
> +++ b/include/linux/closure.h
> @@ -58,7 +58,7 @@
>   * bio2->bi_endio = foo_endio;
>   * bio_submit(bio2);
>   *
> - * continue_at(cl, complete_some_read, system_wq);
> + * continue_at(cl, complete_some_read, system_percpu_wq);
>   *
>   * If closure's refcount started at 0, complete_some_read() could run before the
>   * second bio was submitted - which is almost always not what you want! More
> -- 
> 2.51.0
> 

I was confused until I realized you're updating a comment.

Acked-by: Kent Overstreet <kent.overstreet@...ux.dev>