Re: /proc or ps tools bug? 2.6.3, time is off

[George Anzinger <george@mvista.com>]

Albert Cahalan wrote:
> On Wed, 2004-02-25 at 11:28, George Anzinger wrote:
> 
>>Albert Cahalan wrote:
>>
>>>On Wed, 2004-02-25 at 00:10, David Ford wrote:
> 
> 
>>>Actually, it seems that there is a -significant- time difference in this 
>>>
>>>>phantom clock now, I suspended my notebook to bring it home from the 
>>>>station, and now this time difference is greater than 9 minutes.  I 
>>>>suspect it's roughly 46 seconds plus the amount of time that my notebook 
>>>>was suspended.  Yes, I'm running ntpd.
>>>>
>>>>root     16894  0.0  0.0  1544  484 pts/3    S    Feb24   0:00 grep grep ps
>>>>Wed Feb 25 00:09:09 EST 2004
>>>
>>>OK, this is pointing right at the problem.
>>>
>>>Linux does not record process start times at all.
>>>Instead, it records the number of clock ticks
>>>from boot until the process starts.
>>>
>>>Either the boot time or current time is real.
>>>The other may be computed from the uptime, which
>>>may be measured in clock ticks.
>>
>>In 2.6.* boot time is captured at boot.  This is then adjusted when ever the 
>>clock is set.  Up time is the difference between the saved boot time and the 
>>current wall clock time.
>>
>>
>>>The clock doesn't tick when your laptop sleeps.
>>
>>I would guess that the clock adjustment made when the sleep ends is not 
>>adjusting the boot time as it should.  That code should set the clock by calling 
>>do_settimeofday() which will do the right thing.
> 
> 
> I don't think so. The problem might be fixable by advancing
> jiffies, crediting the extra ticks to idle time.
> Consider the current situation as I know it, in jiffies:
> 
> 00000 boot
> 10000 process 42 starts
> 20000 go to sleep
> 20000 wake (same jiffies, different time)
> 30000 process 51 starts
> 40000 ps examines the state of the system
> 
> Process 42 was started 10 seconds after boot. (10000 jiffies)
> Process 51 appears to be started 30 seconds after boot. (30000 jiffies + ???) 
> 
> Now we want to compute:
> 
> 1. real-world date and time for process start
> 2. length of process lifetime (real-world or not?)
> 
> What works for process 42 won't work for process 51,
> because they are on different sides of a hidden gap.
> 
> Another way to fix the problem is to move the boot time.
> It's kind of sick, but so are the alternatives.
> 
> 
>>As to small drifts of ~170 PPM, they are caused by code (ps I would guess) that 
>>assumes that jiffies is exactly 1/HZ whereas it is NOT in the 2.6.* kernel.  The 
>>size of the jiffie that the kernel uses is returned by:
>>
>>struct timespec tv;
>>:
>>:
>>clock_res(CLOCK_REALTIME, &tv);
>>
>>This will be in nanoseconds (and must be as that is what the wall clock is in).
> 
> 
> This is NOT sane. Remeber that procps doesn't get to see HZ.
> Only USER_HZ is available, as the AT_CLKTCK ELF note.
> 
> I think the way to fix this is to skip or add a tick
> every now and then, so that the long-term HZ is exact.
> 
> Another way is to simply choose between pure old-style
> tick-based timekeeping and pure new-style cycle-based
> (TSC or ACPI) timekeeping. Systems with uncooperative
> hardware have to use the old-style time keeping. This
> should simply the code greatly.

On checking the code and thinking about this, I would suggest that we change 
start_time in the task struct to be the wall time (or monotonic time if that 
seems better).  I only find two places this is used, in proc and in the 
accounting code.  Both of these could easily be changed.  Of course, even 
leaving it as it is, they could be changed to report more correct values by 
using the correct conversions to translate the system HZ to USER_HZ.

Hm, OK, I will work up a patch to do the ladder, i.e. to correctly convert the 
elapsed jiffies to USER_HZ and (for the accounting code) seconds.
> 
> 
> 
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-- 
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