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Message-ID: <dedfa895-ae7e-9a64-9e89-055ca85cf2e3@pengutronix.de>
Date: Mon, 28 Jan 2019 10:22:02 +0100
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 28.01.19 09:23, Greg Kroah-Hartman wrote:
> 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.
Yes, I tested it with different linux-rt version with mostly similar results:
kernel 4.8.15-rt10+
latency histogram:
0 ... < 250 usec : 1933104 transmissions
250 ... < 500 usec : 21339 transmissions
500 ... < 750 usec : 8952 transmissions
750 ... < 1000 usec : 6226 transmissions
1000 ... < 1500 usec : 7688 transmissions
1500 ... < 2000 usec : 5236 transmissions
2000 ... < 5000 usec : 11724 transmissions
5000 ... < 10000 usec : 3588 transmissions
10000 ... < 50000 usec : 2123 transmissions
50000 ... < 1000000 usec : 20 transmissions
>= 1000000 usec : 0 transmissions
kernel 4.9.0-rt1+
latency histogram:
0 ... < 250 usec : 1950222 transmissions
250 ... < 500 usec : 15041 transmissions
500 ... < 750 usec : 5968 transmissions
750 ... < 1000 usec : 4437 transmissions
1000 ... < 1500 usec : 6022 transmissions
1500 ... < 2000 usec : 4185 transmissions
2000 ... < 5000 usec : 9864 transmissions
5000 ... < 10000 usec : 2773 transmissions
10000 ... < 50000 usec : 1462 transmissions
50000 ... < 1000000 usec : 26 transmissions
>= 1000000 usec : 0 transmissions
4.19.10-rt8
latency histogram:
0 ... < 250 usec : 1906861 transmissions
250 ... < 500 usec : 35271 transmissions
500 ... < 750 usec : 13103 transmissions
750 ... < 1000 usec : 9084 transmissions
1000 ... < 1500 usec : 9434 transmissions
1500 ... < 2000 usec : 5644 transmissions
2000 ... < 5000 usec : 12737 transmissions
5000 ... < 10000 usec : 4511 transmissions
10000 ... < 50000 usec : 3201 transmissions
50000 ... < 1000000 usec : 154 transmissions
>= 1000000 usec : 0 transmissions
without extra CPU load the result on kernel 4.19.10-rt8 will be:
latency histogram:
0 ... < 250 usec : 1999992 transmissions
250 ... < 500 usec : 8 transmissions
500 ... < 750 usec : 0 transmissions
750 ... < 1000 usec : 0 transmissions
1000 ... < 1500 usec : 0 transmissions
1500 ... < 2000 usec : 0 transmissions
2000 ... < 5000 usec : 0 transmissions
5000 ... < 10000 usec : 0 transmissions
10000 ... < 50000 usec : 0 transmissions
50000 ... < 1000000 usec : 0 transmissions
>= 1000000 usec : 0 transmissions
=============================================================
test results with same load and replaced kworker with kthread and assigned an RT priority
min latency: 0 sec : 75 usec
max latency: 0 sec : 125 usec
average latency: 81 usec
latency measure cycles overall: 79000000
latency histogram:
0 ... < 250 usec : 79000000 transmissions
250 ... < 500 usec : 0 transmissions
500 ... < 750 usec : 0 transmissions
750 ... < 1000 usec : 0 transmissions
1000 ... < 1500 usec : 0 transmissions
1500 ... < 2000 usec : 0 transmissions
2000 ... < 5000 usec : 0 transmissions
5000 ... < 10000 usec : 0 transmissions
10000 ... < 50000 usec : 0 transmissions
50000 ... < 1000000 usec : 0 transmissions
>= 1000000 usec : 0 transmissions
Kind regards,
Oleksij Rempel
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