clocksource change of behavior in 2.6.22 compared to 2.6.20 causes massive system clock slowdown

["Alessandro Suardi" <alessandro.suardi@gmail.com>]

My oldish AMD K7-800's clock began falling behind after
rebooting from 2.6.20 (and 109 days uptime with a spotless
clock) into 2.6.22; time lost is about four minutes each hour.

Turns out that 2.6.22 marks my TSC as unstable and starts
using PIT instead. Rebooting 2.6.22 with clocksource=tsc
gets the original stable system time back.

Not sure whether this is supposed to be the default way of
fixing the issue on older machines, though; if anyone sees
a problem with this, I'm up for providing more information.

[root@donkey ~]# cd /sys/devices/system/clocksource/clocksource0/
[root@donkey clocksource0]# cat available_clocksource
pit jiffies tsc
[root@donkey clocksource0]# cat current_clocksource
tsc
[root@donkey clocksource0]# uname -a
Linux donkey 2.6.22 #1 PREEMPT Mon Jul 9 13:26:38 CEST 2007 i686
athlon i386 GNU/Linux
[root@donkey clocksource0]# cat /proc/cpuinfo
processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 6
model           : 4
model name      : AMD Athlon(tm) Processor
stepping        : 2
cpu MHz         : 800.068
cache size      : 256 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 1
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca
cmov pat pse36 mmx fxsr syscall mmxext 3dnowext 3dnow
bogomips        : 1603.17
clflush size    : 32


thanks, ciao,

--alessandro

"Did you get married but forgot to get divorced ?"

    (Danny and Dusty, 'The Good Old Days')
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