Do spin_lock and spin_unlock hurt performance of an SMP kernel on a single-cpu machine?

Question

On my Ubuntu machine, default kernel image which is running is built for smp (CONFIG_SMP=y). But this machine has only 1 cpu.

On uni-processor kernel, unlike smp kernel, spin_lock/unlock are null functions. So how does spin_lock() and spin_unlock() behave in this setup?

Is there any performance impact due to such smp specific code?

Answer 1

Current Linux kernels contain a patch that you can google for with the terms "SMP alternatives". In short, during boot, if the kernel detects that it's running on a uniprocessor machines, the spinlock functions are hot-patched with no-ops.