1. 启动1号进程

1号进程的创建

kernel的启动开始于start_kernel()函数，在这里完成各种系统初始化后最后进入rest_init函数，这整个过程都可以称之为0进程：

    asmlinkage void __init start_kernel(void)
    {
        ...
            boot_cpu_init();
            page_address_init();
            pr_notice("%s", linux_banner);
            setup_arch(&command_line);
            mm_init_owner(&init_mm, &init_task);
            mm_init_cpumask(&init_mm);
            setup_command_line(command_line);
            setup_nr_cpu_ids();
            setup_per_cpu_areas();
            smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */

            build_all_zonelists(NULL, NULL);
            page_alloc_init();

            pr_notice("Kernel command line: %s\n", boot_command_line);
            parse_early_param();
        ...
            vfs_caches_init_early();
            sort_main_extable();
            trap_init();
            mm_init();
        ...
        
            /* Do the rest non-__init'ed, we're now alive */
            rest_init();

    }

rest_init()创建了两个内核线程kernel_init和kthreadd：

    static noinline void __init_refok rest_init(void)
    {
            int pid;

            rcu_scheduler_starting();
            /*
             * We need to spawn init first so that it obtains pid 1, however
             * the init task will end up wanting to create kthreads, which, if
             * we schedule it before we create kthreadd, will OOPS.
             */
            kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
            numa_default_policy();
            pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
            rcu_read_lock();
            kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
            rcu_read_unlock();
            complete(&kthreadd_done);

            /*
             * The boot idle thread must execute schedule()
             * at least once to get things moving:
             */
            init_idle_bootup_task(current);
            schedule_preempt_disabled();
            /* Call into cpu_idle with preempt disabled */
            cpu_startup_entry(CPUHP_ONLINE);
    }

top命令可以看到kthreadd的pid为2

top.png

kernel_init后面会尝试装载init进程，并完成内核态到用户态的转换，形成用户态的祖先1号进程。

    static int __ref kernel_init(void *unused)
    {
        ...
            if (execute_command) {
                    if (!run_init_process(execute_command))
                            return 0;
                    pr_err("Failed to execute %s.  Attempting defaults...\n",
                            execute_command);
            }
            if (!run_init_process("/etc/preinit") ||
                !run_init_process("/sbin/init") ||
                !run_init_process("/etc/init") ||
                !run_init_process("/bin/init") ||
                !run_init_process("/bin/sh"))
                    return 0;

            panic("No init found.  Try passing init= option to kernel. "
                  "See Linux Documentation/init.txt for guidance.");
    }

execute_command为cmdline传递进来的init进程，不存在或运行失败后依次尝试运行：

    "/etc/preinit",     "/sbin/init",       "/etc/init",        "/bin/init",        "/bin/sh"