From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S268484AbUHQWZf (ORCPT ); Tue, 17 Aug 2004 18:25:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S268477AbUHQWZf (ORCPT ); Tue, 17 Aug 2004 18:25:35 -0400 Received: from gateway-1237.mvista.com ([12.44.186.158]:36855 "EHLO av.mvista.com") by vger.kernel.org with ESMTP id S266814AbUHQWZF (ORCPT ); Tue, 17 Aug 2004 18:25:05 -0400 Message-ID: <412285A5.9080003@mvista.com> Date: Tue, 17 Aug 2004 15:24:37 -0700 From: George Anzinger Reply-To: george@mvista.com Organization: MontaVista Software User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.2.1) Gecko/20030225 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Tim Schmielau CC: Andrew Morton , OGAWA Hirofumi , albert@users.sourceforge.net, linux-kernel@vger.kernel.org, voland@dmz.com.pl, nicolas.george@ens.fr, kaukasoi@elektroni.ee.tut.fi, johnstul@us.ibm.com, david+powerix@blue-labs.org Subject: Re: [PATCH] Re: boot time, process start time, and NOW time References: <1087948634.9831.1154.camel@cube> <87smcf5zx7.fsf@devron.myhome.or.jp> <20040816124136.27646d14.akpm@osdl.org> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Tim Schmielau wrote: > On Mon, 16 Aug 2004, Andrew Morton wrote: > > >>OGAWA Hirofumi wrote: >> >>>Albert Cahalan writes: >>> >>> >>>>Even with the 2.6.7 kernel, I'm still getting reports of process >>>>start times wandering. Here is an example: >>>> >>>> "About 12 hours since reboot to 2.6.7 there was already a >>>> difference of about 7 seconds between the real start time >>>> and the start time reported by ps. Now, 24 hours since reboot >>>> the difference is 10 seconds." >>>> >>>>The calculation used is: >>>> >>>> now - uptime + time_from_boot_to_process_start >>> >>>Start-time and uptime is using different source. Looks like the >>>jiffies was added bogus lost counts. >>> >>>quick hack. Does this change the behavior? >> >>Where did this all end up? Complaints about wandering start times are >>persistent, and it'd be nice to get some fix in place... >> >>Thanks. >> > > > Seems my analysis of the problem wasn't perceived as such. > > The problem is that in the above calculation > > now - uptime + time_from_boot_to_process_start > > "uptime" currently is an ntp-corrected precise time, while > "time_from_boot_to_process_start" just is the free-running "jiffies" > value. I see you think you have the solution, but I guess I am just dense here. May be you could help me to see the error of my ways. Here is my thinking: "now" is from gettimeofday() and as such is ntp corrected. "uptime" is also corrected. In fact it is "now" + "wall_to_monotonic". And "wall_to_monotonic" is _only_ changed by do_settime() when the clock is set. "time_from_boot_to_process_start" is the same as "start_time" restated in seconds, i.e. it is a constant. So, either one or more of the above assumtions is wrong, or somebody is twiddling the clock. Otherwise I don't see how the start time can move at all. > > The problem is easily reproducible for me. It goes away if the change > that rebased /proc/uptime on posix monotonic time and my followup patch to > fix the resulting rounding issues in jiffies64_to_clock_t() are backed out > with the following patch. > -- George Anzinger george@mvista.com High-res-timers: http://sourceforge.net/projects/high-res-timers/ Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml