Subject: Re: /proc or ps tools bug?  2.6.3, time is off
From: john stultz <johnstul@us.ibm.com>
To: george anzinger <george@mvista.com>
Cc: Albert Cahalan <albert@users.sourceforge.net>,
       David Ford <david+powerix@blue-labs.org>,
       linux-kernel mailing list <linux-kernel@vger.kernel.org>
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Date: Tue, 13 Apr 2004 15:38:00 -0700
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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?

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


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