2.6.11 timeval_to_jiffies() wrong for ms resolution timers

["Bhavesh P. Davda" <bhavesh@avaya.com>]

setitimer for 20ms was firing at 21ms, so I wrote a simple debug module
for 2.6.11.10 kernel on i386 to do something like this:

struct timeval tv;
unsigned long jif;

tv.tv_usec = 20000;
tv.tv_sec = 0;

jif = timeval_to_jiffies(&tv);
printk("%lu usec = %lu jiffies\n", tv.tv_usec, jif);

This yields:

20000 usec = 21 jiffies

Egad!

I looked at the timeval_to_jiffies() inline function in
include/linux/jiffies.h, and after pulling my hair for a few minutes
(okay almost an hour), I decided to ask much smarter people than myself
on why it is behaving this way, and what it would take to fix it so that
"20000 usec = 20 jiffies".

I got as far as this in figuring it out for i386:

HZ=1000
SEC_CONVERSION=4194941632
USEC_CONVERSION=2199357558
USEC_ROUND=2199023255551
USEC_JIFFIE_SC=41
SEC_JIFFIE_SC=22

Thanks in advance for saving me from going bald!

- Bhavesh

-- 
Bhavesh P. Davda | Distinguished Member of Technical Staff | Avaya |
1300 West 120th Avenue | B3-B03 | Westminster, CO 80234 | U.S.A. |
Voice/Fax: 303.538.4438 | bhavesh@avaya.com
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