lockref scalability on x86-64 vs cpu_relax

[Luck, Tony]

> Yeah, if it was ia64-only, it's a non-issue these days. It's dead and
> in pure maintenance mode from a kernel perspective (if even that).

There's not much "simultaneous" in the SMT on ia64. One thread in a
spin loop will hog the core until the h/w switches to the other thread some
number of cycles (hundreds, thousands? I really can remember). So I
was pretty generous with dropping cpu_relax() into any kind of spin loop.

Is it time yet for:

$ git rm -r arch/ia64

-Tony