fluctuations in /proc/stat btime field

From: Kingsley Cheung (kingsley@aurema.com)
Date: Tue Mar 18 2003 - 23:45:54 EST

Hi,

On a dual SMP box running SuSE-2.4.19-79 (United Linux rc4), the boot
time in /proc/stat can vary 1 second when read. I haven't seen this
on a UP box. Its not a major issue, but I guess its a hindrance if
one's expecting it to never change:

gen2 12:06:52 ~: grep btime /proc/stat
btime 1046838984
gen2 12:07:06 ~: grep btime /proc/stat
btime 1046838984
gen2 12:07:07 ~: grep btime /proc/stat
btime 1046838983
gen2 12:07:07 ~: grep btime /proc/stat
btime 1046838983
gen2 12:07:08 ~: grep btime /proc/stat
btime 1046838983
gen2 12:07:09 ~: grep btime /proc/stat
btime 1046838984
gen2 12:07:10 ~: grep btime /proc/stat
btime 1046838984
gen2 12:07:11 ~: grep btime /proc/stat
btime 1046838983

I'm not familiar with the way IO-APIC timers work or the interrupt
timer itself, so can someone explain why this is the case? I'm
guessing that it might simply be a timing issue between when the
actual interrupt handling updating jiffies in do_timer and the bottom
half updating xtime.tv_sec (see kernel/timer.c).

Maybe by caching the btime value so that its only calculated once is
the way to go. Attached is the suggested fix, as well as the output
from /proc/cpuinfo & /proc/interrupts.

If someone could get back to me on this it would be much appreciated.

Thanks, 

-- 
		Kingsley
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