linux-kernel - Re: [PATCH v1 2/2] serial: 8250_dma: add parameter to queue work on specific cpu

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Message-ID: <aPfV6IXPk-j59AhQ@slm.duckdns.org>
Date: Tue, 21 Oct 2025 08:50:16 -1000
From: Tejun Heo <tj@...nel.org>
To: Xin Zhao <jackzxcui1989@....com>
Cc: gregkh@...uxfoundation.org, hch@...radead.org, jirislaby@...nel.org,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v1 2/2] serial: 8250_dma: add parameter to queue work on
 specific cpu

Hello,

On Wed, Oct 22, 2025 at 01:39:33AM +0800, Xin Zhao wrote:
...
> I also agree that tasks requiring continuous real-time execution should be
> handled by kthread workers. However, while the ideal situation is appealing,
> the reality is challenging. A large amount of driver code in the system uses
> the system's pwq and worker_pool management, as this API is very convenient.
> Refactoring the code carries significant change risks, and even with a team
> effort, it's hard to bear such risks.

kthread_work's API is really similar to workqueue to make it easy to switch
between the two. We probably didn't go far enough tho and it may make sense
to allow workqueue to always use dedicated fixed set of workers (e.g. by
allowing a workqueue to create a private pool of workers) when configured
from userspace so that this becomes a configuration problem which doesn't
require code changes.

> Adding flags like WQ_HIGHPRI or even introducing WQ_RT from a functional
> development perspective doesn't pose too much concern; we just need to focus

WQ_RT really doesn't make sense given that you can't even tell whether any
given work item would have a worker ready for it. What does RT priority do
when you need to go and create a new kworker?

Note that workqueue property changes don't need to be hard coded. If you
make the code use its own workqueue and set WQ_SYSFS, you can change its
properties from userspace through sysfs interface.

Thanks.

-- 
tejun