linux-kernel - Re: [PATCH 0/3] fs: replace wq users and add WQ_PERCPU to alloc

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Message-ID: <20250915-abgearbeitet-servolenkung-d0c60406b94e@brauner>
Date: Mon, 15 Sep 2025 13:50:30 +0200
From: Christian Brauner <brauner@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...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>, Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 0/3] fs: replace wq users and add WQ_PERCPU to
 alloc_workqueue() users

On Fri, Sep 05, 2025 at 11:02:11AM +0200, Marco Crivellari wrote:
> Hi!
> 
> 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.
> 
> 2) [P 3] add WQ_PERCPU to remaining alloc_workqueue() users
> 
>         Every alloc_workqueue() caller should use one among WQ_PERCPU or
>         WQ_UNBOUND. This is actually enforced warning if both or none of them
>         are present at the same time.
> 
>         WQ_UNBOUND will be removed in a next release cycle.
> 
> === 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")

What is this based on? This doesn't apply to any v6.17-rc* tag so I
can't merge it.