num_active_cpus
static atomic_t num_active_cpus = ATOMIC_INIT(0);
atomic_add(is_active ? 1 : -1, &num_active_cpus);
active_cpus = atomic_read(&num_active_cpus);
msdata.num_threads = num_active_cpus() + 1; /* +1 for local */
cpu_stop_init_done(&done, num_active_cpus());