linux-kernel - Re: [PATCH v3 2/2] rust: add system_percpu() around the new system_percpu

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Message-ID: <aOaJlHEPzHcAdcUx@tardis-2.local>
Date: Wed, 8 Oct 2025 08:56:04 -0700
From: Boqun Feng <boqun.feng@...il.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>,
	Alice Ryhl <aliceryhl@...gle.com>
Subject: Re: [PATCH v3 2/2] rust: add system_percpu() around the new
 system_percpu_wq

On Wed, Oct 08, 2025 at 05:15:54PM +0200, Marco Crivellari wrote:
> The C code defines 2 new workqueues: system_percpu_wq and system_dfl_wq,
> respectively the futures replacement for system_wq and system_unbound_wq.
> 
> This change introduce system_percpu(), that use the new system_percpu_wq.
> 
> system_wq will be replaced in a future release cycle and should
> not be used.
> 
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>

If we were to expose the system_percpu_wq to Rust, then we should also
add queue_work_on() API to Rust, otherwise it's kinda pointless IMO.

PS. We can use the CpuId abstraction:

	http://rust.docs.kernel.org/kernel/cpu/struct.CpuId.html

and have an API like:

    ipml Queue {
        pub fn queue_on(&self, cpu: CpuId, w: W) -> W::EqueueOutput
    }

or maybe a different new type `PerCpuQueue`?

Regards,
Boqun

> ---
>  rust/kernel/workqueue.rs | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index 300cc2bfe012..05f213444b91 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -940,11 +940,26 @@ unsafe impl<T, const ID: u64> RawDelayedWorkItem<ID> for Pin<KBox<T>>
>  /// users which expect relatively short queue flush time.
>  ///
>  /// Callers shouldn't queue work items which can run for too long.
> +///
> +/// Note: `system_wq` will be removed in a future release cycle. Use [`system_percpu_wq`] instead.
>  pub fn system() -> &'static Queue {
>      // SAFETY: `system_wq` is a C global, always available.
>      unsafe { Queue::from_raw(bindings::system_wq) }
>  }
>  
> +/// Returns the system work queue (`system_percpu_wq`).
> +///
> +/// It is the one used by `schedule[_delayed]_work[_on]()`. Multi-CPU multi-threaded. There are
> +/// users which expect relatively short queue flush time.
> +///
> +/// Callers shouldn't queue work items which can run for too long.
> +///
> +/// Note: `system_percpu_wq` will replace ['system_wq`] in a future relase cycle.
> +pub fn system_percpu() -> &'static Queue {
> +    // SAFETY: `system_percpu_wq` is a C global, always available.
> +    unsafe { Queue::from_raw(bindings::system_percpu_wq) }
> +}
> +
>  /// Returns the system high-priority work queue (`system_highpri_wq`).
>  ///
>  /// It is similar to the one returned by [`system`] but for work items which require higher
> -- 
> 2.51.0
>