kthreadd-linux下2号进程

参考：

1. linux常见进程与内核线程

2. Linux下2号进程的kthreadd--Linux进程的管理与调度（七）

本文中代码内核版本：3.2.0

kthreadd：这种内核线程只有一个，它的作用是管理调度其它的内核线程。这个线程不能关闭。它在内核初始化的时候被创建，会循环运行一个叫做kthreadd的函数，该函数的作用是运行kthread_create_list全局链表中维护的kthread。其他任务或代码想创建内核线程时需要调用kthread_create（或kthread_create_on_node）创建一个kthread，该kthread会被加入到kthread_create_list链表中，同时kthread_create会weak up kthreadd_task（即kthreadd）（增链表）。kthreadd再执行kthread时会调用老的接口——kernel_thread运行一个名叫“kthread”的内核线程去运行创建的kthread，被执行过的kthread会从kthread_create_list链表中删除（减链表），并且kthreadd会不断调用scheduler 让出CPU。kthreadd创建的kthread执行完后，会调到kthread_create()执行，之后再执行最初原任务或代码。

创建

在linux启动的C阶段start_kernel()的最后，rest_init()会开启两个进程：kernel_init，kthreadd，之后主线程变成idle线程，init/main.c。

linux下的3个特殊的进程：idle进程（PID=0），init进程（PID=1）和kthreadd（PID=2）。

* idle进程由系统自动创建, 运行在内核态   PID=0
idle进程其pid=0，其前身是系统创建的第一个进程，也是唯一一个没有通过fork或者kernel_thread产生的进程。完成加载系统后，演变为进程调度、交换。

* init进程由idle通过kernel_thread创建，在内核空间完成初始化后, 加载init程序, 并最终用户空间运行  PID=1 PPID=0
由0进程创建，完成系统的初始化. 是系统中所有其它用户进程的祖先进程 。
Linux中的所有进程都是有init进程创建并运行的。首先Linux内核启动，然后在用户空间中启动init进程，再启动其他系统进程。在系统启动完成完成后，init将变为守护进程监视系统其他进程。

* kthreadd进程由idle通过kernel_thread创建，并始终运行在内核空间, 负责所有内核线程的调度和管理 PID=2 PPID=0
它的任务就是管理和调度其他内核线程kernel_thread, 会循环执行一个kthreadd的函数，该函数的作用就是运行kthread_create_list全局链表中维护的kthread, 当我们调用kthread_create创建的内核线程会被加入到此链表中，因此所有的内核线程都是直接或者间接的以kthreadd为父进程。所有的内核线程的PPID都是2。

注：所有的内核线程在大部分时间里都处于阻塞状态(TASK_INTERRUPTIBLE)只有在系统满足进程需要的某种资源的情况下才会运行。

/*

* We need to finalize in a non-__init function, or else race conditions
* between the root thread and the init thread may cause start_kernel to
* be reaped by free_initmem before the root thread has proceeded to
* cpu_idle.
*
* gcc-3.4 accidentally inlines this function, so use noinline.
*/

static __initdata DECLARE_COMPLETION(kthreadd_done);

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);
    preempt_enable_no_resched();
    schedule();

    /* Call into cpu_idle with preempt disabled */
    preempt_disable();
    cpu_idle();
}

 kthreadd任务

函数体定义在kernel/kthread.c中。

static DEFINE_SPINLOCK(kthread_create_lock);
static LIST_HEAD(kthread_create_list);
struct task_struct *kthreadd_task;

struct kthread_create_info
{
    /* Information passed to kthread() from kthreadd. */
    int (*threadfn)(void *data);
    void *data;
    int node;

    /* Result passed back to kthread_create() from kthreadd. */
    struct task_struct *result;
    struct completion done;

    struct list_head list;
};

struct kthread {
    int should_stop;
    void *data;
    struct completion exited;
};

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_HIGH_MEMORY]);

    current->flags |= PF_NOFREEZE | PF_FREEZER_NOSIG;

    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;
}

static void create_kthread(struct kthread_create_info *create)
{
    int pid;

#ifdef CONFIG_NUMA
    current->pref_node_fork = create->node;
#endif
    /* We want our own signal handler (we take no signals by default). */
    pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
    if (pid < 0) {
        create->result = ERR_PTR(pid);
        complete(&create->done);
    }   
}

kthread任务

static int kthread(void *_create)
{
    /* Copy data: it's on kthread's stack */
    struct kthread_create_info *create = _create;
    int (*threadfn)(void *data) = create->threadfn;
    void *data = create->data;
    struct kthread self;
    int ret;

    self.should_stop = 0;
    self.data = data;
    init_completion(&self.exited);
    current->vfork_done = &self.exited;

    /* OK, tell user we're spawned, wait for stop or wakeup */
    __set_current_state(TASK_UNINTERRUPTIBLE);
    create->result = current;
    complete(&create->done);
    schedule();

    ret = -EINTR;
    if (!self.should_stop)
        ret = threadfn(data);

    /* we can't just return, we must preserve "self" on stack */
    do_exit(ret);
}

/**
 * kthread_create_on_node - create a kthread.
 * @threadfn: the function to run until signal_pending(current).
 * @data: data ptr for @threadfn.
 * @node: memory node number.
 * @namefmt: printf-style name for the thread.
 *
 * Description: This helper function creates and names a kernel
 * thread.  The thread will be stopped: use wake_up_process() to start
 * it.  See also kthread_run().
 *
 * If thread is going to be bound on a particular cpu, give its node
 * in @node, to get NUMA affinity for kthread stack, or else give -1.
 * When woken, the thread will run @threadfn() with @data as its
 * argument. @threadfn() can either call do_exit() directly if it is a
 * standalone thread for which no one will call kthread_stop(), or
 * return when 'kthread_should_stop()' is true (which means
 * kthread_stop() has been called).  The return value should be zero
 * or a negative error number; it will be passed to kthread_stop().
 *
 * Returns a task_struct or ERR_PTR(-ENOMEM).
 */
struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
                       void *data,
                       int node,
                       const char namefmt[],
                       ...)
{
    struct kthread_create_info create;

    create.threadfn = threadfn;
    create.data = data;
    create.node = node;
    init_completion(&create.done);

    spin_lock(&kthread_create_lock);
    list_add_tail(&create.list, &kthread_create_list);
    spin_unlock(&kthread_create_lock);

    wake_up_process(kthreadd_task);
    wait_for_completion(&create.done);

    if (!IS_ERR(create.result)) {
        static const struct sched_param param = { .sched_priority = 0 };
        va_list args;
        va_start(args, namefmt);
        vsnprintf(create.result->comm, sizeof(create.result->comm),
              namefmt, args);
        va_end(args);
        /*
         * root may have changed our (kthreadd's) priority or CPU mask.
         * The kernel thread should not inherit these properties.
         */
        sched_setscheduler_nocheck(create.result, SCHED_NORMAL, &param);
        set_cpus_allowed_ptr(create.result, cpu_all_mask);
    }
    return create.result;
}
EXPORT_SYMBOL(kthread_create_on_node);

kernel/kthread.c的头文件include/linux/kthread.h定义kthread_create():

#define kthread_create(threadfn, data, namefmt, arg...) \
    kthread_create_on_node(threadfn, data, -1, namefmt, ##arg)