linux-kernel - Re: [RFC PATCH 0/2] RT scheduling policies for workqueues

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Date:   Mon, 28 Mar 2022 12:09:27 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Peter Hurley <peter@...leysoftware.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Esben Haabendal <esben@...nix.com>,
        Steven Walter <stevenrwalter@...il.com>,
        linux-kernel@...r.kernel.org,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        André Pribil <a.pribil@...k-ipc.com>,
        Tejun Heo <tj@...nel.org>, Jiri Slaby <jirislaby@...nel.org>,
        linux-rt-users@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] RT scheduling policies for workqueues

On 28.03.2022 12:05:57, Sebastian Andrzej Siewior wrote:
> On 2022-03-23 15:55:58 [+0100], Rasmus Villemoes wrote:
> > This RFC is motivated by an old problem in the tty layer. Ever since
> > commit a9c3f68f3cd8 (tty: Fix low_latency BUG), use of UART for
> > real-time applications has been problematic. Even if both the
> > application itself and the irq thread are set to SCHED_FIFO, the fact
> > that the flush_to_ldisc work is scheduled on the generic and global
> > system_unbound_wq (with all workers running at normal scheduling
> > priority) means that UART RX can suffer unbounded latency.
> 
> Having a kthread per "low-latency" tty instance is something I would
> prefer. The kwork corner is an anonymous worker instance and probably
> does more harm than good. Especially if it is a knob for everyone which
> is used for the wrong reasons and manages to be harmful in the end.
> With a special kthread for a particular tty, the thread can be assigned
> with the desired priority within the system and ttyS1 can be
> distinguished from ttyS0 (and so on). This turned out to be useful in a
> few setups over the years.

+1

The networking subsystem has gone the same/similar way with NAPI. NAPI
handling can be switched from the softirq to kernel thread on a per
interface basis.

regards
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
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