How is run queue length computed in linux proc filesystem

Hi, I'm trying to obtain number of runnable processes from linux kernel. sar -q gives this information readily. However I'm trying to get this value from /proc filesystem. There is no file in /proc that gives this value directly, then how is runq-sz computed. The wiki page http://en.wikipedia.org/wiki/Load_(computing) provides some insight into how run queue length is computed based on ldavg values but it is unclear. Can someone provide more pointers on this. Cheers Thanks, Kris