Issues getting IO times using Schedtimes.stp example script

[Sunny Fugate]

I'm having some difficulties in getting one of the examples scripts to 
run.  According to the documentation, the kernel.trace("sched_switch") 
probe should expose $prev, $next, and $rq.  I've played around with the 
stp script and on my system the $rq variable is never defined.  As such, 
the script never reports io scheduling times.  Other results from the same script seem ok. 

Anyone have an idea what would cause my issues with the schedtimes.stp 
example script?  

I'm running on a recent Ubuntu 11.04. 

Thanks,

Sunny