linux-kernel - Re: [PATCH] i3c: master: add WQ_PERCPU to alloc

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Message-ID: <aQ5d1g95N86lXXT2@lizhi-Precision-Tower-5810>
Date: Fri, 7 Nov 2025 16:00:06 -0500
From: Frank Li <Frank.li@....com>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-i3c@...ts.infradead.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>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [PATCH] i3c: master: add WQ_PERCPU to alloc_workqueue users

On Fri, Nov 07, 2025 at 02:29:49PM +0100, Marco Crivellari wrote:
> Currently if a user enqueues 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 consistency cannot be addressed without refactoring the API.
>
> alloc_workqueue() treats all queues as per-CPU by default, while unbound
> workqueues must opt-in via WQ_UNBOUND.
>
> This default is suboptimal: most workloads benefit from unbound queues,
> allowing the scheduler to place worker threads where they’re needed and
> reducing noise when CPUs are isolated.
>
> This continues the effort to refactor workqueue APIs, which began with
> the introduction of new workqueues and a new alloc_workqueue flag in:
>
> commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
> commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")
>
> This change adds a new WQ_PERCPU flag to explicitly request
> alloc_workqueue() to be per-cpu when WQ_UNBOUND has not been specified.
>
> With the introduction of the WQ_PERCPU flag (equivalent to !WQ_UNBOUND),
> any alloc_workqueue() caller that doesn’t explicitly specify WQ_UNBOUND
> must now use WQ_PERCPU.
>
> Once migration is complete, WQ_UNBOUND can be removed and unbound will
> become the implicit default.
>
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> ---
>  drivers/i3c/master.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
> index d946db75df70..519b98c37ac7 100644
> --- a/drivers/i3c/master.c
> +++ b/drivers/i3c/master.c
> @@ -2925,7 +2925,7 @@ int i3c_master_register(struct i3c_master_controller *master,
>  	if (ret)
>  		goto err_put_dev;
>
> -	master->wq = alloc_workqueue("%s", 0, 0, dev_name(parent));
> +	master->wq = alloc_workqueue("%s", WQ_PERCPU, 0, dev_name(parent));

Maybe off topic, I think it is not neccesary to create wq for IBI at all.
it can directly use system_bh_wq, or other wq.

Frank
>  	if (!master->wq) {
>  		ret = -ENOMEM;
>  		goto err_put_dev;
> --
> 2.51.1
>