linux-kernel - Re: [PATCH] workqueue: rust: add creation of workqueues

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Message-ID: <59c1e09a5c47a60e26c5fb10c3305356328a98a6.camel@mailbox.org>
Date: Thu, 17 Apr 2025 09:22:40 +0200
From: Philipp Stanner <phasta@...lbox.org>
To: Tejun Heo <tj@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Philipp Stanner <phasta@...nel.org>,
  Miguel Ojeda <ojeda@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,  Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,  Benno
 Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>, Daniel Almeida
 <daniel.almeida@...labora.com>, Tamir Duberstein <tamird@...il.com>,
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: rust: add creation of workqueues

On Wed, 2025-04-16 at 09:57 -1000, Tejun Heo wrote:
> Hello, Alice.
> 
> On Wed, Apr 16, 2025 at 09:41:21PM +0200, Alice Ryhl wrote:
> ...
> > I thought about implementation approaches. The first thought that
> > sprang to mind is add a list of all delayed work items, but now I
> > think we can do better. We can have an atomic counter tracking the
> > number of delayed work items, and have destroy_workqueue() do this:
> > 
> > retry:
> > drain_workqueue(wq);
> > if (has_delayed_work_items(wq)) {
> >     wait_for_delayed_to_be_scheduled(wq);
> >     goto retry;
> > }
> > 
> > where wait_for_delayed_to_be_scheduled() either waits for the
> > counter
> > to hit zero, or waits for at least waits for one of them to be
> > scheduled. For example, maybe wait_for_delayed_to_be_scheduled()
> > could
> > add a dummy work item *without* waking up the worker threads, and
> > then
> > wait for that work item to get executed, which would effectively
> > mean
> > that it sleeps until something else wakes up a worker.
> 
> I suppose that can work too but the delays can be pretty long, so
> while
> correct, I'm not sure it'd be very pleasant to use. If we per-cpu
> lists, I
> don't think the overhead would be all that noticeable, so may as well
> do
> that?

I assume you, ultimately, mean that the list of delayed_work's would be
accessible through workqueue_struct, correct?

And then destroy_workqueue() could loop over all of them with
cancel_delayed_work_sync()?

I think that would be the cleanest possible solution.

P.

> 
> Thanks.
>