linux-kernel - Re: [PATCH] drivers/greybus: add WQ_PERCPU to alloc

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Message-ID: <aRShDB8g0hG7JcWX@hovoldconsulting.com>
Date: Wed, 12 Nov 2025 16:00:28 +0100
From: Johan Hovold <johan@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, greybus-dev@...ts.linaro.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>, Alex Elder <elder@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] drivers/greybus: add WQ_PERCPU to alloc_workqueue users

Please use just

	greybus:

as prefix.

Note that hardly any driver subsystems include "drivers/" in the commit
summary.

On Fri, Nov 07, 2025 at 02:21:49PM +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.

Apart from the naming of the WORK_CPU_UNBOUND macro I don't see the
inconsistency here. We queue on the local CPU as documented (unless the
CPU is not in the wq_unbound cpumask for unbound workqueues).

Note sure how explicitly marking percpu workqueues is going to change
this either so this paragraph doesn't seem relevant for the change at
hand.

> 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.

Fair enough, the default is about to be changed.

> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>

With an updated commit message you can add my:

Reviewed-by: Johan Hovold <johan@...nel.org>

> ---
>  drivers/greybus/operation.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/greybus/operation.c b/drivers/greybus/operation.c
> index 54ccc434a1f7..7e12ffb2dd60 100644
> --- a/drivers/greybus/operation.c
> +++ b/drivers/greybus/operation.c
> @@ -1238,7 +1238,7 @@ int __init gb_operation_init(void)
>  		goto err_destroy_message_cache;
>  
>  	gb_operation_completion_wq = alloc_workqueue("greybus_completion",
> -						     0, 0);
> +						     WQ_PERCPU, 0);
>  	if (!gb_operation_completion_wq)
>  		goto err_destroy_operation_cache;

Johan