linux-kernel - Re: [PATCH 1/2] drivers/s390: WQ_PERCPU added to alloc

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Message-ID: <1f2a16a0-9c14-4403-9c39-35f8b8138188-agordeev@linux.ibm.com>
Date: Mon, 8 Sep 2025 13:37:51 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...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>, Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH 1/2] drivers/s390: WQ_PERCPU added to alloc_workqueue
 users

On Fri, Sep 05, 2025 at 11:08:56AM +0200, Marco Crivellari wrote:
> 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.
> 
> 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 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.

A duplicate paragraph.

> This patch adds a new WQ_PERCPU flag to explicitly request the use of
> the per-CPU behavior. Both flags coexist for one release cycle to allow
> callers to transition their calls.
> 
> Once migration is complete, WQ_UNBOUND can be removed and unbound will
> become the implicit default.
> 
> 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.
> 
> All existing users have been updated accordingly.
> 
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> ---
>  drivers/s390/char/tape_3590.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/s390/char/tape_3590.c b/drivers/s390/char/tape_3590.c
> index 0d484fe43d7e..aee11fece701 100644
> --- a/drivers/s390/char/tape_3590.c
> +++ b/drivers/s390/char/tape_3590.c
> @@ -1670,7 +1670,7 @@ tape_3590_init(void)
>  
>  	DBF_EVENT(3, "3590 init\n");
>  
> -	tape_3590_wq = alloc_workqueue("tape_3590", 0, 0);
> +	tape_3590_wq = alloc_workqueue("tape_3590", WQ_PERCPU, 0);
>  	if (!tape_3590_wq)
>  		return -ENOMEM;

Acked-by: Alexander Gordeev <agordeev@...ux.ibm.com>

Thanks!