linux-kernel - Re: [PATCH] firewire: core: add WQ_PERCPU to alloc

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Message-ID: <20251108025416.GA73420@workstation.local>
Date: Sat, 8 Nov 2025 11:54:16 +0900
From: Takashi Sakamoto <o-takashi@...amocchi.jp>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux1394-devel@...ts.sourceforge.net,
	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>
Subject: Re: [PATCH] firewire: core: add WQ_PERCPU to alloc_workqueue users

Hi,

Thanks for the patch.

On Fri, Nov 07, 2025 at 12:26:19PM +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/firewire/core-transaction.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/firewire/core-transaction.c b/drivers/firewire/core-transaction.c
> index c65f491c54d0..c15dbe882cbe 100644
> --- a/drivers/firewire/core-transaction.c
> +++ b/drivers/firewire/core-transaction.c
> @@ -1437,7 +1437,8 @@ static int __init fw_core_init(void)
>  {
>  	int ret;
>  
> -	fw_workqueue = alloc_workqueue("firewire", WQ_MEM_RECLAIM, 0);
> +	fw_workqueue = alloc_workqueue("firewire", WQ_MEM_RECLAIM | WQ_PERCPU,
> +				       0);
>  	if (!fw_workqueue)
>  		return -ENOMEM;

As far as I know, there is no specific reason to use per-cpu workqueue
for this purpose in this subsystem. I believe that using unbound workqueue
would be more beneficial, since the workqueue users just operate chained
1394 OHCI DMA descriptor over system memory for asynchronous
communication.

Would it be acceptable for you to add WQ_UNBOUND flag to the workqueue?
If so, I can prepare a patch for the next merge window.


Thanks

Takashi Sakamoto