Re: [coLinux-users] coLinux 30945 seconds leap mystery solved

[Henry N. <hen...@ar...>]

  Hello Ron,

many thanks for your work!

A skip for negative glitches coded now. We simple ignore this time and 
wait for next call of this function.
Please see SVN r1539:
http://colinux.svn.sourceforge.net/viewvc/colinux?view=revision&revision=1539

This should fix the Bug#1780633.

Binaries for devel version 0.7.9 have here:
http://www.henrynestler.com/colinux/testing/devel-0.7.9/20101019-jiffies/

An update for released version 0.7.8 have here:
http://www.henrynestler.com/colinux/testing/stable-0.7.8/20101019-jiffies/linux-stable-svn1528-jiffies-fix.zip

Henry

Am 10.10.2010 16:50, schrieb Ron Avriel:
> Hi,
>
> After analyzing several +30945 seconds leaps, using the coLinux debug 
> version, I managed to solved the mystery.
>
> The root cause of the problem is a timer read hardware glitch.
> The glitch occurs when the 24th bit of the counter should overflow, 
> but it doesn't.
> For example:
> Instead of reading 0x16242A000004, a value of 0x162429000004 was read.
> Instead of reading 0x13510E000005, a value of 0x13510D000005 was read.
>
> This causes an error of  -4.68 seconds (for a 3579545 frequency).
> The code in callback_return_jiffies(), together with an inaccuracy in 
> co_div64() causes some interesting results:
>
> The first time the glitch occurs the time leaps forward by 24 seconds.
> When subsequent glitches occur, time will leap forward by 30945 and 
> 30956 seconds alternatively.
>
> The results can be verified with the simulation below.
>
> Problem resolution:
> It seems this clock reading problem is well known and there are 
> several workaroun ds for it.
> For example see the the thread 
> http://www.mail-archive.com/fre...@fr.../msg34826.html 
> and http://support.microsoft.com/kb/274323 .
> In any case, I think that a simple check to verify that clock did not 
> shift backward from previous read is a must.
> In addition, if the clock changed more than a threshold, then the 
> clock should be read again in loop until below threshold.