linux内核是在哪里创建1号进程的?

1. 请看rest_init的完整代码(不看也没关系,内核版本为5.2, init/main.c)

noinline void __ref rest_init(void)
{
        struct task_struct *tsk;
        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.
         */
        pid = kernel_thread(kernel_init, NULL, CLONE_FS);
        /*
         * Pin init on the boot CPU. Task migration is not properly working
         * until sched_init_smp() has been run. It will set the allowed
         * CPUs for init to the non isolated CPUs.
         */
        rcu_read_lock();
        tsk = find_task_by_pid_ns(pid, &init_pid_ns);
        set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
        rcu_read_unlock();

        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();

        /*
         * Enable might_sleep() and smp_processor_id() checks.
         * They cannot be enabled earlier because with CONFIG_PREEMPT=y
         * kernel_thread() would trigger might_sleep() splats. With
         * CONFIG_PREEMPT_VOLUNTARY=y the init task might have scheduled
         * already, but it's stuck on the kthreadd_done completion.
         */
        system_state = SYSTEM_SCHEDULING;

        complete(&kthreadd_done);

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

2. 从以上代码中可以看到调用了两次kernel_thread, 那么哪个是1号进程？

　　第一处pid = kernel_thread(kernel_init, NULL, CLONE_FS);即会创建1号进程init

　　第二处pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);即会创建2号进程kthreadd

3. 1号进程到底干了些什么?是个死循环吗?

　　3.1 先展示一下完整代码(init/main.c)　　　

static int __ref kernel_init(void *unused)
{
        int ret;

        kernel_init_freeable();
        /* need to finish all async __init code before freeing the memory */
        async_synchronize_full();
        ftrace_free_init_mem();
        free_initmem();
        mark_readonly();

        /*
         * Kernel mappings are now finalized - update the userspace page-table
         * to finalize PTI.
         */
        pti_finalize();

        system_state = SYSTEM_RUNNING;
        numa_default_policy();

        rcu_end_inkernel_boot();

        if (ramdisk_execute_command) {
                ret = run_init_process(ramdisk_execute_command);
                if (!ret)
                        return 0;
                pr_err("Failed to execute %s (error %d)
",
                       ramdisk_execute_command, ret);
        }

        /*
         * We try each of these until one succeeds.
         *
         * The Bourne shell can be used instead of init if we are
         * trying to recover a really broken machine.
         */
        if (execute_command) {
                ret = run_init_process(execute_command);
                if (!ret)
                        return 0;
                panic("Requested init %s failed (error %d).",
                      execute_command, ret);
        }
        if (!try_to_run_init_process("/sbin/init") ||
            !try_to_run_init_process("/etc/init") ||
            !try_to_run_init_process("/bin/init") ||
            !try_to_run_init_process("/bin/sh"))
                return 0;

        panic("No working init found.  Try passing init= option to kernel. "
              "See Linux Documentation/admin-guide/init.rst for guidance.");
}

　　3.2 简单解读一下代码

　　　　到处找那个init程序(在/,/sbin/,/etc/,/bin/等目录下找)并执行,如果没找到直接调用panic

　　3.3 不是一个死循环哦

4. 那么2号进程kthreadd又干了些什么呢? 它是个死循环吗?

　　4.1 老规矩，贴上完整代码(代码在kernel/kthread.c中)　

int kthreadd(void *unused)
{
        struct task_struct *tsk = current;

        /* Setup a clean context for our children to inherit. */
        set_task_comm(tsk, "kthreadd");
        ignore_signals(tsk);
        set_cpus_allowed_ptr(tsk, cpu_all_mask);
        set_mems_allowed(node_states[N_MEMORY]);

        current->flags |= PF_NOFREEZE;
        cgroup_init_kthreadd();

        for (;;) {
                set_current_state(TASK_INTERRUPTIBLE);
                if (list_empty(&kthread_create_list))
                        schedule();
                __set_current_state(TASK_RUNNING);

                spin_lock(&kthread_create_lock);
                while (!list_empty(&kthread_create_list)) {
                        struct kthread_create_info *create;

                        create = list_entry(kthread_create_list.next,
                                            struct kthread_create_info, list);
                        list_del_init(&create->list);
                        spin_unlock(&kthread_create_lock);

                        create_kthread(create);

                        spin_lock(&kthread_create_lock);
                }
                spin_unlock(&kthread_create_lock);
        }

        return 0;
}

　　4.2 这是个死循环

　　4.3 简单阐述2号进程干了什么?

　　　　不断从全局链表kthread_create_list中获取一个节点，然后执行节点中的函数，这样就可以做到管理调度其它内核线程的功能