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

[George Anzinger <george@mvista.com>]

john stultz wrote:
> On Thu, 2004-02-26 at 15:06, George Anzinger wrote:
> 
>>john stultz wrote:
>>
>>>On Wed, 2004-02-25 at 13:10, George Anzinger wrote:
>>>
>>>
>>>>Albert Cahalan wrote:
>>>>
>>>>
>>>>>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.
>>>
>>>
>>>Is this close to what your thinking of? 
>>>I can't reproduce the issue on my systems, so I'll need someone else to
>>>test this. 
>>
>>More or less.  I wonder if:
> 
> 
>>static inline long jiffies_to_clock_t(long x)
>>{
>>	u64 tmp = (u64)x * TICK_NSEC;
>>	div64(tmp, (NSEC_PER_SEC / USER_HZ));
>>	return (long)x;
>>}
>>might be better as it addresses the overflow issue.  Should be able to toss the 
>>#if (HZ % USER_HZ)==0 test too.  We could get carried away and do scaled math to 
>>eliminate the div64 but I don't think this path is used enough to justify the 
>>clarity ;) that would make.
> 
> 
> Sounds good to me. Would you mind sending the diff so Petri and David
> could test it?

Oops, I have been caught :)  The above was composed in the email window.  I 
don't have a 2.6.x kernel up at the moment and I don't have any free cycles...
Late next week??
> 

-- 
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