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

[George Anzinger]

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@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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