From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754716AbbIJMGg (ORCPT ); Thu, 10 Sep 2015 08:06:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48880 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754640AbbIJMGb (ORCPT ); Thu, 10 Sep 2015 08:06:31 -0400 Date: Thu, 10 Sep 2015 14:02:20 +0200 From: Miroslav Lichvar To: John Stultz Cc: LKML , Nuno =?iso-8859-1?Q?Gon=E7alves?= , Prarit Bhargava , Richard Cochran , Ingo Molnar , Thomas Gleixner , Shuah Khan Subject: Re: [PATCH 2/2 (v2)] kselftest: timers: Add adjtick test to validate adjtimex() tick adjustments Message-ID: <20150910120220.GV24711@localhost> References: <1441840051-20244-1-git-send-email-john.stultz@linaro.org> <1441840051-20244-2-git-send-email-john.stultz@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1441840051-20244-2-git-send-email-john.stultz@linaro.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 09, 2015 at 04:07:31PM -0700, John Stultz wrote: > Recently an issue was reported that was difficult to detect except > by tweaking the adjtimex tick value, and noticing how quickly the > adjustment took to be made: > https://lkml.org/lkml/2015/9/1/488 > > Thus this patch introduces a new test which manipulates the adjtimex > tick value and validates the results are what we expect. > + if (llabs(eppm - ppm) > 10) { > + printf(" [FAILED]\n"); > + return -1; > + } > + printf(" [OK]\n"); > + return 0; This seems to work nicely with the tsc and hpet clocksources, but for some reason 10 ppm is not enough with the acpi_pm clocksource on both machines I tried this on. They both show -99988 ppm for the first test. When I modify the program to go through errors I get: Estimating tick (act: 9000 usec, -100000 ppm): 9001 usec, -99988 ppm [FAILED] Estimating tick (act: 9250 usec, -75000 ppm): 9251 usec, -74991 ppm [OK] Estimating tick (act: 9500 usec, -50000 ppm): 9501 usec, -49994 ppm [OK] Estimating tick (act: 9750 usec, -25000 ppm): 9751 usec, -24997 ppm [OK] Estimating tick (act: 10000 usec, 0 ppm): 10000 usec, 0 ppm [OK] Estimating tick (act: 10250 usec, 25000 ppm): 10249 usec, 24996 ppm [OK] Estimating tick (act: 10500 usec, 50000 ppm): 10499 usec, 49993 ppm [OK] Estimating tick (act: 10750 usec, 75000 ppm): 10749 usec, 74990 ppm [OK] The precision of the clock is better than microsecond, so that wouldn't explain a 12 ppm error over the 15 second interval. I guess it's due to a larger xtime_remainder, which basically is a hidden frequency offset added (and not multiplied) to the NTP frequency offset. Would that explain it? -- Miroslav Lichvar