Which is the best light weight distro for learning linux kernel development. It should have lot of debugging and profiling tools available along with it :)
I've heard Linus himself uses Fedora. I'd recommend Gentoo which lets (intends) for you to hand customize your kernel, it's the perfect setting for it (and I've spent many hours squeezing out every last bit of performance for the fun of it).
Naturally Ubuntu is my preferred distro, but you may have trouble if you start hijacking and removing expected kernel features. Gentoo won't complain, and doesn't expected them around to begin with.
The distro probably doesn't make much difference since you'll be working on your own kernel and not the "kitchen sink" kernel the distros tend to provide with a bunch of patches in most cases.
If you're doing kernel development work then I suppose you want a distro that boots quickly, something like puppy might be ideal here and do your actual coding from something like Ubuntu.