small scheduler hack/patch

I have a small "proof of concept" scheduler hack at:
http://www.freebsd.org/~julian/it.diff

what it is supposed to do is check if there is an idle CPU at the time
that a thread is made runnable (assuming that idle CPUs are halted)
and if there is, it selects an idle CPU and gives it an IPI
to wake it up. (presumably it will then pick up the work).

It's only implemented for SMP/i386 as the code to halt the cpu is only
present (from my quick view) in x86 and it doesn't make sense in UP..

any comments and suggestions? (oh, and yes it's only in the 4bsd
scheduler.. I think jeff already handles this sort of thing).