What Linux kernel hacks have you made and why?

Hacks are fun if you're just playing around on one machine. If you're managing servers or need your workstation to always work after upgrades, they are a pain.

I've never modified the kernel, at most recompiled back-in-the-day before most things became dynamically loadable. There's a small number of settings that need tuning for various purposes (e.g. database server). The most challenging one was getting server motherboard BIOS/CMOS settings in sync with the kernel to get NUMA memory on multi-processor machines to behave consistently.

To me all kernel tweaking I remember is setting up tonnes of stuffs for MPTCP kernel for my pi and getting it running sith raspibian. Ofcourse, I had followed a blog with instructions but the feeling is undeniably awesome. And results are astonishing too. Really fun.