Re: boot time, process start time, and NOW time

[George Anzinger]

john stultz wrote:
> On Mon, 2004-08-16 at 16:08, Tim Schmielau wrote:
>
> > On Mon, 16 Aug 2004, Andrew Morton wrote:
> >
> >
> > > OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > Albert Cahalan <albert@xxxxxxxxxxxx> 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...
> >
> >
> >
> > The trouble seems to be due to the patch below, part of a larger cleanup
> > (http://linus.bkbits.net:8080/linux-2.5/cset%403ef4851dGg0fxX58R9Zv8SIq9fzNmQ?nav=index.html|src/.|src/fs|src/fs/proc|related/fs/proc/proc_misc.c)
> > by George.
> >
> > Quoting from the changelog entry:
> >
> > "Changes the uptime code to use the posix_clock_monotonic notion of 
> > uptime instead of the jiffies.  This time will track NTP changes and so should 
> > be better than your standard wristwatch (if your using ntp)."
> >
> > George is absolutely right that it's more precise. However, it's also 
> > inconsistent with the process start times which use plain uncorrected 
> > jiffies. ps stumbles over this inconsistency.
> >
> > Simple fix: revert the patch below.
> > Complicated fix: correct process start times in fork.c (no patch provided, 
> > too complicated for me to do).
>
>
> Hmm. While that patch fixed the uptime proc entry, I thought the issue
> was with process start times. I'm looking at fixing the start_time
> assignment in proc_pid_stat(). My suspicion is that we need to use ACTHZ
> in jiffies64_to_clock_t().

I really don't see how the start_time that proc_pid_stat() is producing could be 
anything but a constant.  The complaint is that it moves, not that it is 
incorrect, right?
> Something like the patch below.
>
> thanks
> -john
>
> ===== include/linux/times.h 1.6 vs edited =====
> --- 1.6/include/linux/times.h	2004-05-10 04:25:49 -07:00
> +++ edited/include/linux/times.h	2004-08-16 16:22:13 -07:00
> @@ -48,6 +48,7 @@
>  	 * but even this doesn't overflow in hundreds of years
>  	 * in 64 bits, so..
>  	 */
> +	x = (x * ACT_HZ)>>8;  /* compensate for ACT_HZ != HZ */
>  	x *= TICK_NSEC;
>  	do_div(x, (NSEC_PER_SEC / USER_HZ));
>  #endif

--
George Anzinger   george@xxxxxxxxxx
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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