Re: tty latency and RT

[John Ogness <john.ogness@linutronix.de>]

Hi,

On 2018-07-06, André Pribil <a.pribil@beck-ipc.com> wrote:
> are there any plans to improve the latency with serial ttys and RT?
>
> I'm seeing very high latencies (up to 100 ms) when a high priority 
> RT thread accesses a serial tty port. I'm working with 4.14.52-rt34 
> on a ARM system.

As you later point out, this is a problem with the tty subsystem
(kworker) and is not RT-specific. This problem needs to be fixed in
mainline.

> The reason for this issue seems to be that a kworker thread with a 
> normal priority is involved here. When the high priority RT 
> thread needs to wait for this kworker thread, it can easily be blocked 
> by other lower priority RT threads running in the system.
>
> I have seen that Steven Walter proposed some change in 2015, where
> this kworker thread in replaced by a kthread with a RT priority if
> the low_latency mode is selected.
> See: https://www.spinics.net/lists/linux-serial/msg17782.html
>
> The follow-up discussion in this thread also proposes a solution that
> allows low_latency ports to directly execute the worker.
> As interrupts are already threaded in PREEMPT_RT, wouldn't that be a 
> solution?

The patches and discussions in that thread provide some good ground work
for a real solution. Someone needs to take more time to work with the
maintainers to get it mainline.

John Ogness