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

[Marc Kleine-Budde]

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

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