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

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Jan 09, 2026 at 05:32:23PM +0100, Marco Crivellari wrote:
> On Thu, Oct 9, 2025 at 1:17 PM Alice Ryhl <aliceryhl@google.com> wrote:
> > [...]
> > > 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
> >
> > How is it ... can we cleanly separate queues into those where you must
> > specify the cpuid, and those where you shouldn't?
> 
> Hi,
> 
> Sorry to come back to you so late. I still had many other subsystems patches,
> now there are way less, many of them are accepted.
> 
> Can I have some guidance about this? I am new to Rust.
> What's the best way in order to expose the workqueues?
> 
> If I understand correctly, the Idea is extends the functionalities of:
> 
> https://rust.docs.kernel.org/kernel/workqueue/struct.Queue.html
> 
> adding the "queue_on" function.
> 
> Creating a new type like "PerCpuQueue" I guess it means... wraps always
> the workqueue_struct structure and defines what's appropriate, like the
> new per-cpu workqueue.
> 
> Many thanks in advance!

Yes, we can provide a new `struct PerCpuQueue` with same contents as
`Queue`, where the `enqueue` and `enqueue_delayed` methods take a cpu id
parameter.

Then, all of the functions for obtaining queues at the bottom of the
file are updated to return &PerCpuQueue instead of &Queue when a cpu id
sone option here is tohould be provided to spawn on that queue.

This way, you write:

	workqueue::system().enqueue(my_work_item)

or you write:

	workqueue::system_percpu().enqueue(my_work_item, cpu_id)

This way you must supply cpu id with system_percpu_wq() but not with
system().

--

Another approach is to add a new `enqueue_cpu` to the existing `Queue`
struct. In that case, all of these four combinations become legal:

	workqueue::system().enqueue(my_work_item)
	workqueue::system().enqueue_cpu(my_work_item, cpu_id)
	workqueue::system_percpu().enqueue(my_work_item)
	workqueue::system_percpu().enqueue_cpu(my_work_item, cpu_id)

which approach is best depends on whether you want all four combinations
to be legal or not.

Alice