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Date:   Mon, 28 Mar 2022 12:09:27 +0200
From:   Marc Kleine-Budde <mkl@pengutronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Peter Hurley <peter@hurleysoftware.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Esben Haabendal <esben@geanix.com>,
        Steven Walter <stevenrwalter@gmail.com>,
        linux-kernel@vger.kernel.org,
        Oleksij Rempel <o.rempel@pengutronix.de>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        =?utf-8?B?QW5kcsOp?= Pribil <a.pribil@beck-ipc.com>,
        Tejun Heo <tj@kernel.org>, Jiri Slaby <jirislaby@kernel.org>,
        linux-rt-users@vger.kernel.org
Subject: Re: [RFC PATCH 0/2] RT scheduling policies for workqueues
Message-ID: <20220328100927.5ax34nea7sp7jdsy@pengutronix.de>
References: <20220323145600.2156689-1-linux@rasmusvillemoes.dk>
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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.
>=20
> 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

--=20
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
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