get_cpu() disables preemption (so you won't
suddenly get moved to another CPU) and returns the current
processor number, between 0 and NR_CPUS. Note
that the CPU numbers are not necessarily continuous. You return
it again with
put_cpu() when you are done.
If you know you cannot be preempted by another task (ie. you are in interrupt context, or have preemption disabled) you can use smp_processor_id().