Re: RFC: A reduced Linux network stack for small systems

not quite. I'm saying: no extra optimizations, no GCC changes.
Compile kernel as-is. Most functions have a stub for mcount() already.
Use it to track whether kernel function was called or not.
Collect this data in userspace (as perf already does), add few
more functions that had 'notrace' attribute on them, and feed this into
special linker that unpacks existing vmlinux, throws away cold functions,
relocates the rest and here you have tiny vmlinux without recompilation.

That's very difficult for networking code. How would you know you 
exercised all the corner cases in the TCP stack? And you wouldn't
want a remotely exploitable system because some important error
handler is missing.

I agree it may work for some other subsystems.

-Andi