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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 D1B50C433DF for ; Thu, 27 Aug 2020 14:30:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B8D8E22B4E for ; Thu, 27 Aug 2020 14:30:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728051AbgH0OaF convert rfc822-to-8bit (ORCPT ); Thu, 27 Aug 2020 10:30:05 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:57430 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728048AbgH0OWC (ORCPT ); Thu, 27 Aug 2020 10:22:02 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-223-AW3ZNV-SP0aGNzZHgpXBVg-1; Thu, 27 Aug 2020 15:13:22 +0100 X-MC-Unique: AW3ZNV-SP0aGNzZHgpXBVg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Thu, 27 Aug 2020 15:13:21 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Thu, 27 Aug 2020 15:13:21 +0100 From: David Laight To: "linux-kernel@vger.kernel.org" Subject: NTP adjustments to CLOCK_MONATONIC Thread-Topic: NTP adjustments to CLOCK_MONATONIC Thread-Index: AdZ8dpybr71tOIXqSjuzMi+avyML6Q== Date: Thu, 27 Aug 2020 14:13:21 +0000 Message-ID: <3df4f582304949648e104a2837555bad@AcuMS.aculab.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0.001 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I've just noticed some strange behaviour of some code that is comparing CLOCK_MONATONIC to a second (external) clock source. Normally the difference is between +/-200ns in 10ms. Which is well within the ppm errors of the crystals. However for around 90 seconds after system boot the error was massive - I have to force a difference of around 8000ns/10ms to see similar behaviour. The only explanation I can guess at is that it was subject to NTP's maximum slew rate of 0.5ms/sec (1/2000) while NTP was aligning CLOCK_REALTIME. While the NTP adjustments for clock frequency drift aren't unreasonable for CLOCK_MONATONIC, including the adjustments for the boot time offset is rather horrid. In this case they stopped after 90 seconds, but a bigger offset could take hours (or even days?) to clear. This rather breaks what timekeeping.rst says about CLOCK_MONATONIC being useful for 'reliable timestamps and measuring short time intervals accurately'. Any idea what happens to CLOCK_MONATONIC over a leap second? Its offset to CLOCK_REALTIME should really change by 1 second. I can't use CLOCK_MONATONIC_RAW because the hrtimer code doesn't support it. There are also programs that need to do things (like send RTP) every 20ms - not every 20.01ms. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)