Messages in this thread | | | Date | Tue, 17 Aug 2004 15:24:37 -0700 | From | George Anzinger <> | Subject | Re: [PATCH] Re: boot time, process start time, and NOW time |
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Tim Schmielau wrote: > On Mon, 16 Aug 2004, Andrew Morton wrote: > > >>OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote: >> >>>Albert Cahalan <albert@users.sf.net> 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
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