start_kernel()之setup_per_cpu_area()

 The routine setup_per_cpu_area() exits for the setup of a multiple processing environment .If the linux is configured without SMP support, it do nothing.

void __init setup_per_cpu_areas(void)
{
    unsigned long delta;
    unsigned int cpu;
    int rc;

    /*
     * Always reserve area for module percpu variables. That's
     * what the legacy allocator did.
     */
    rc = pcpu_embed_first_chunk(PERCPU_MODULE_RESERVE,
                 PERCPU_DYNAMIC_RESERVE, PAGE_SIZE, NULL,
                 pcpu_dfl_fc_alloc, pcpu_dfl_fc_free);
    if (rc < 0)
        panic("Failed to initialized percpu areas.");

    delta = (unsigned long)pcpu_base_addr - (unsigned long)__per_cpu_start;
    for_each_possible_cpu(cpu)
        __per_cpu_offset[cpu] = delta + pcpu_unit_offsets[cpu];
}