Message-ID: <CAAofZF4a6ARXOS0rmK5zY1Kd3xdODqdkj_keZmEYx8Z-JRvhng@mail.gmail.com>
Date: Fri, 5 Sep 2025 18:05:50 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: Alice Ryhl <aliceryhl@...gle.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 0/2] rust: replace wq users and add WQ_PERCPU to
 alloc_workqueue() users

Hello Alice,

Thanks for your reply!

I'm not entirely sure I understood your doubt.

system_dfl_wq and system_percpu_wq are wq(s) also present in the C code.
They are part of the prerequisite already merged in:

128ea9f6ccfb6960293ae4212f4f97165e42222d
("workqueue: Add system_percpu_wq and system_dfl_wq")

Let me know if I haven't understood correctly what you meant.

Thanks!


On Fri, Sep 5, 2025 at 1:34 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> On Fri, Sep 5, 2025 at 11:12 AM Marco Crivellari
> <marco.crivellari@...e.com> wrote:
> >
> > Below is a summary of a discussion about the Workqueue API and cpu isolation
> > considerations. Details and more information are available here:
> >
> >         "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
> >         https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
> >
> > === Current situation: problems ===
> >
> > Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
> > set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.
> >
> > This leads to different scenarios if a work item is scheduled on an isolated
> > CPU where "delay" value is 0 or greater then 0:
> >         schedule_delayed_work(, 0);
> >
> > This will be handled by __queue_work() that will queue the work item on the
> > current local (isolated) CPU, while:
> >
> >         schedule_delayed_work(, 1);
> >
> > Will move the timer on an housekeeping CPU, and schedule the work there.
> >
> > 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.
> >
> > === Plan and future plans ===
> >
> > This patchset is the first stone on a refactoring needed in order to
> > address the points aforementioned; it will have a positive impact also
> > on the cpu isolation, in the long term, moving away percpu workqueue in
> > favor to an unbound model.
> >
> > These are the main steps:
> > 1)  API refactoring (that this patch is introducing)
> >     -   Make more clear and uniform the system wq names, both per-cpu and
> >         unbound. This to avoid any possible confusion on what should be
> >         used.
> >
> >     -   Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
> >         introduced in this patchset and used on all the callers that are not
> >         currently using WQ_UNBOUND.
> >
> >         WQ_UNBOUND will be removed in a future release cycle.
> >
> >         Most users don't need to be per-cpu, because they don't have
> >         locality requirements, because of that, a next future step will be
> >         make "unbound" the default behavior.
> >
> > 2)  Check who really needs to be per-cpu
> >     -   Remove the WQ_PERCPU flag when is not strictly required.
> >
> > 3)  Add a new API (prefer local cpu)
> >     -   There are users that don't require a local execution, like mentioned
> >         above; despite that, local execution yeld to performance gain.
> >
> >         This new API will prefer the local execution, without requiring it.
> >
> > === Introduced Changes by this series ===
> >
> > 1) [P 1-2] Replace use of system_wq and system_unbound_wq
> >
> >         system_wq is a per-CPU workqueue, but his name is not clear.
> >         system_unbound_wq is to be used when locality is not required.
> >
> >         Because of that, system_wq has been renamed in system_percpu_wq, and
> >         system_unbound_wq has been renamed in system_dfl_wq.
> >
> >
> > === For Maintainers ===
> >
> > There are prerequisites for this series, already merged in the master branch.
> > The commits are:
> >
> > 128ea9f6ccfb6960293ae4212f4f97165e42222d ("workqueue: Add system_percpu_wq and
> > system_dfl_wq")
> >
> > 930c2ea566aff59e962c50b2421d5fcc3b98b8be ("workqueue: Add new WQ_PERCPU flag")
> >
> >
> > Thanks!
> >
> > Marco Crivellari (2):
> >   rust: replace use of system_unbound_wq with system_dfl_wq
> >   rust: replace use of system_wq with system_percpu_wq
> >
> >  rust/kernel/workqueue.rs | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
>
> The functions you are changing are intended to match 1-to-1 with the
> wq globals defined by the C side. Changing them so that Rust and C no
> longer agrees on what the wqs are called seems wrong. How about adding
> new functions instead?
>
> Alice



-- 

Marco Crivellari

L3 Support Engineer, Technology & Product




marco.crivellari@...e.com

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