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 16:20, George Anzinger wrote:
> 
>>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??
> 
> 
> Finally got a chance to go through my work queue and yikes! This is
> seriously stale! As neither George or I have come to bat with a patch,
> I'll attempt a swing. 
> 
> Albert/David: Would you mind testing the following to see if it resolves
> the issue for you?
> 
> George: Mind skimming this to make sure its close enough to what you
> intended?

Looks rather like exactly what I intended.

-g
> 
> thanks
> -john
> 
> 
> diff -Nru a/include/linux/times.h b/include/linux/times.h
> --- a/include/linux/times.h	Tue Apr 13 15:00:25 2004
> +++ b/include/linux/times.h	Tue Apr 13 15:00:25 2004
> @@ -7,7 +7,12 @@
>  #include <asm/param.h>
>  
>  #if (HZ % USER_HZ)==0
> -# define jiffies_to_clock_t(x) ((x) / (HZ / USER_HZ))
> +static inline long jiffies_to_clock_t(long x)
> +{
> +	u64 tmp = (u64)x * TICK_NSEC;
> +	x = do_div(tmp, (NSEC_PER_SEC / USER_HZ));
> +	return (long)tmp;
> +}
>  #else
>  # define jiffies_to_clock_t(x) ((clock_t) jiffies_64_to_clock_t((u64) x))
>  #endif
> 
> 
> 
> 
> 

-- 
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