Re: [PATCH] Re: boot time, process start time, and NOW time

[George Anzinger <george@mvista.com>]

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