linux-kernel - Re: [PATCH v1 3/3] drivers/tty: increase priority for tty_buffer

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Date:   Thu, 10 Jan 2019 16:19:53 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 3/3] drivers/tty: increase priority for
 tty_buffer_worker

On Thu, Jan 10, 2019 at 04:54:53AM -0800, Linus Torvalds wrote:
> On Thu, Jan 10, 2019 at 2:12 AM Oleksij Rempel <o.rempel@...gutronix.de> wrote:
> >
> > sched_priority = 1 is enough to dramatically reduce latency
> > on have system load produced by tasks with default user space prio.
> 
> .. and is this perhaps a way for a user to then make the system spend
> inordinate amounts of time in the tty layer, and hurting other people?
> I'm thinking threads using pty's etc as a way to make the system
> unresponsive.
> 
> We have *never* had good results with random priority modifications.
> People used to do this for the X server, and it helped in very
> specific cases, and hurt enormously in others.
> 
> Why would anybody use a tty interface with a l;oopback adapter and
> care about latency?
> 
> I can kind of see why you want to do this from a theoretical point,
> but from a *practical* point of view it seems pointless. Why not use
> more appropriate models like networking or pipes etc. IOW, I think you
> should describe what you *really* are doing much more.
> 
> "hackbench with a loopback serial adapter" really doesn't sound like
> something that should worry a lot of people.

yes, you right.

> My gut feel is that if somebody still cares deeply about serial line
> latency, they should look at trying to see if they can do some of the
> work directly without the bounce to the workqueue. We use workqueues
> for a reason, but it's possible that some of it could be avoided at
> least in special cases... And yours sounds like a special case.

It is for industrial low latency RS-422 based application. The loopback
test is just easy way to test/reproduce it without additional hardware.

What is good, mainlineable way to implement it? 

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