From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 53125C282CB for ; Mon, 28 Jan 2019 09:22:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 238222087F for ; Mon, 28 Jan 2019 09:22:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726846AbfA1JWH (ORCPT ); Mon, 28 Jan 2019 04:22:07 -0500 Received: from metis.ext.pengutronix.de ([85.220.165.71]:47805 "EHLO metis.ext.pengutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726672AbfA1JWF (ORCPT ); Mon, 28 Jan 2019 04:22:05 -0500 Received: from soja.hi.pengutronix.de ([2001:67c:670:100:3ad5:47ff:feaf:13da]) by metis.ext.pengutronix.de with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.89) (envelope-from ) id 1go37D-0006Sa-BP; Mon, 28 Jan 2019 10:22:03 +0100 Subject: Re: [PATCH v1 3/3] drivers/tty: increase priority for tty_buffer_worker To: Greg Kroah-Hartman Cc: Linus Torvalds , Jiri Slaby , Pengutronix Kernel Team , lkml References: <20190110101232.9398-1-o.rempel@pengutronix.de> <20190110101232.9398-4-o.rempel@pengutronix.de> <20190110151953.qpat4t7lat6plfk6@pengutronix.de> <20190110163030.GB19693@kroah.com> <7a593f3b-0019-c30f-30e8-34eae7b96cf0@pengutronix.de> <20190128082313.GA15182@kroah.com> From: Oleksij Rempel Message-ID: Date: Mon, 28 Jan 2019 10:22:02 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190128082313.GA15182@kroah.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-SA-Exim-Connect-IP: 2001:67c:670:100:3ad5:47ff:feaf:13da X-SA-Exim-Mail-From: o.rempel@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-kernel@vger.kernel.org Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |