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

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Date:   Mon, 28 Jan 2019 09:23:13 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 Mon, Jan 28, 2019 at 09:05:30AM +0100, Oleksij Rempel wrote:
> 
> 
> On 10.01.19 17:30, Greg Kroah-Hartman wrote:
> > On Thu, Jan 10, 2019 at 04:19:53PM +0100, Oleksij Rempel wrote:
> > > > 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?
> > 
> > What is the real problem your systems are having?  Are they serial-port
> > limited?  Is latency a big issue?  Trying to tune for a fake workload
> > isn't the best way to solve anything :)
> 
> The system in question is a high power laser cutter with live image-based inspection
> and adjustment of the cutting process. In this setup the RS422 interface is used to
> control parameters of the laser cutting unit in a tie control loop with the camera.
> This loops needs to operate at 1000 Hz.
> 
> The xy-stage moves with a speed of approx. 60m/min, i.e. within 1ms it
> moves about 1mm. For a high precision control process a jitter of ± 500 us (+/- 0.5mm)
> is unacceptable.

Are you using the rt kernel patch for this type of thing?  That should
bound your jitter at a much more deterministic level.

thanks,

greg k-h