linux-kernel - Re: [PATCH v2 1/2] rust: replace use of system_unbound_wq with system_dfl

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Message-ID: <aOZgR7X5ZyXZh_jc@google.com>
Date: Wed, 8 Oct 2025 12:59:51 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...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>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>
Subject: Re: [PATCH v2 1/2] rust: replace use of system_unbound_wq with system_dfl_wq

On Mon, Sep 08, 2025 at 06:02:23PM +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_unbound_wq should be the default workqueue so as not to enforce
> locality constraints for random work whenever it's not required.
> 
> Adding system_dfl_wq to encourage its use when unbound work should be used.
> 
> The old system_unbound_wq will be kept for a few release cycles.
> 
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> ---
>  rust/kernel/workqueue.rs | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index f98bd02b838f..01e331a1f11b 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -667,11 +667,18 @@ pub fn system_long() -> &'static Queue {
>  /// Workers are not bound to any specific CPU, not concurrency managed, and all queued work items
>  /// are executed immediately as long as `max_active` limit is not reached and resources are
>  /// available.
> +///
> +/// Note: `system_unbound_wq` will be removed in a future release cycle. Use [`system_dfl_wq`] instead.
>  pub fn system_unbound() -> &'static Queue {
>      // SAFETY: `system_unbound_wq` is a C global, always available.
>      unsafe { Queue::from_raw(bindings::system_unbound_wq) }
>  }
>  
> +pub fn system_dfl() -> &'static Queue {
> +    // SAFETY: `system_dfl_wq` is a C global, always available.
> +    unsafe { Queue::from_raw(bindings::system_dfl_wq) }
> +}

Please add documentation to the new methods, similar to the
documentation that existing methods already have.

Alice