工作队列(workqueue) create_workqueue/schedule_work/queue_work

项目需要，在驱动模块里用内核计时器timer_list实现了一个状态机。
郁闷的是，运行时总报错“Scheduling while atomic”，网上搜了一下：

"Scheduling while atomic" indicates that you've tried to sleep somewhere that you shouldn't - like within a spinlock-protected critical section or an interrupt handler.

改进程序，在计时器里使用了workqueue，搞定问题。顺便把workqueue的实现代码总结了一下


一、workqueue简介

workqueue与tasklet类似，都是允许内核代码请求某个函数在将来的时间被调用（抄《ldd3》上的）
每个workqueue就是一个内核进程。

workqueue与tasklet的区别：
   1.tasklet是通过软中断实现的，在软中断上下文中运行，tasklet代码必须是原子的
     workqueue是通过内核进程实现的，就没有上述限制的，最爽的是，工作队列函数可以休眠 
         
     PS: 我的驱动模块就是印在计时器中调用了可休眠函数，所以出现了cheduling while atomic告警
         内核计时器也是通过软中断实现的

   2.tasklet始终运行在被初始提交的同一处理器上，workqueue不一定
   3.tasklet不能确定延时时间（即使很短），workqueue可以设定延迟时间


二、workqueue的API

 

workqueue的API自2.6.20后发生了变化

#include <linux/workqueue.h>struct workqueue_struct;struct work_struct;struct workqueue_struct *create_workqueue(const char *name);void destroy_workqueue(struct workqueue_struct *queue);INIT_WORK(_work, _func);INIT_DELAYED_WORK(_work, _func);int queue_work(struct workqueue_struct *wq, struct work_struct *work);int queue_delayed_work(struct workqueue_struct *wq,struct delayed_work *dwork, unsigned long delay);int queue_delayed_work_on(int cpu, struct workqueue_struct *wq,            struct delayed_work *dwork, unsigned long delay);int cancel_work_sync(struct work_struct *work);int cancel_delayed_work_sync(struct delayed_work *dwork);void flush_workqueue(struct workqueue_struct *wq);

Workqueue编程接口

序号

接口函数

说明

1

create_workqueue

用于创建一个workqueue队列，为系统中的每个CPU都创建一个内核线程。输入参数：

@name：workqueue的名称

2

create_singlethread_workqueue

用于创建workqueue，只创建一个内核线程。输入参数：

@name：workqueue名称

3

destroy_workqueue

释放workqueue队列。输入参数：

@ workqueue_struct：需要释放的workqueue队列指针

4

schedule_work

调度执行一个具体的任务，执行的任务将会被挂入Linux系统提供的workqueue——keventd_wq输入参数：

@ work_struct：具体任务对象指针

5

schedule_delayed_work

延迟一定时间去执行一个具体的任务，功能与schedule_work类似，多了一个延迟时间，输入参数：

@work_struct：具体任务对象指针

@delay：延迟时间

6

queue_work

调度执行一个指定workqueue中的任务。输入参数：

@ workqueue_struct：指定的workqueue指针

@work_struct：具体任务对象指针

7

queue_delayed_work

延迟调度执行一个指定workqueue中的任务，功能与queue_work类似，输入参数多了一个delay。

下面实例是不指定delay时间的workqueue
(代码基于2.6.24)

struct my_work_stuct{    int test;    struct work_stuct save;};struct my_work_stuct test_work; struct workqueue_struct *test_workqueue;void do_save(struct work_struct *p_work){    struct my_work_struct *p_test_work = container_of(p_work, struct my_work_stuct, save);    printk("%d\n",p_test_work->test);}  void test_init(){    test_workqueue = create_workqueue("test_workqueue");    if (!test_workqueue)        panic("Failed to create test_workqueue\n");    INIT_WORK(&(test_work.save), do_save);    queue_work(test_workqueue, &(test_work.save));}void test_destory(void){    if(test_workqueue)        destroy_workqueue(test_workqueue);}

三、workqueue的实现


工作队列workqueue不是通过软中断实现的，它是通过内核进程实现的



首先，创建一个workqueue，实际上就是建立一个内核进程

create_workqueue("tap_workqueue")--> __create_workqueue(“tap_workqueue”, 0, 0)--> __create_workqueue_key((name), (singlethread), (freezeable), NULL, NULL){         wq = kzalloc(sizeof(*wq), GFP_KERNEL);         wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct);         wq->name = name;         wq->singlethread = singlethread;         wq->freezeable = freezeable;         INIT_LIST_HEAD(&wq->list);         for_each_possible_cpu(cpu) {             cwq = init_cpu_workqueue(wq, cpu);             err = create_workqueue_thread(cwq, cpu);             start_workqueue_thread(cwq, cpu);         }    }

create_workqueue_thread 建立了一个内核进程 worker_thread（linux_2_6_24/kernel/workqueue.c）

create_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu){    struct workqueue_struct *wq = cwq->wq;    const char *fmt = is_single_threaded(wq) ? "%s" : "%s/%d";    struct task_struct *p;    p = kthread_create(worker_thread, cwq, fmt, wq->name, cpu);    if (IS_ERR(p))        return PTR_ERR(p);    cwq->thread = p;    return 0;}

内核进程worker_thread做的事情很简单，死循环而已，不停的执行workqueue上的work_list
（linux_2_6_24/kernel/workqueue.c）

int worker_thread (void *__cwq){    struct cpu_workqueue_struct *cwq = __cwq;    /*下面定义等待队列项*/    DEFINE_WAIT(wait);    /*下面freezeable一般为0*/    if (cwq->wq->freezeable)        set_freezable();    /*提高优先级别*/    set_user_nice(current, -5);    for (;;) {        /*在cwq->more_work上等待, 若有人调用queue_work,该函数将调用wake_up(&cwq->more_work) 激活本进程*/        prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE);        /*work队列空则切换出去*/        if (!freezing(current) && !kthread_should_stop() && list_empty(&cwq->worklist))            schedule();        /*切换回来则结束等待 说明有人唤醒cwq->more_work上的等待 有work需要处理*/        finish_wait(&cwq->more_work, &wait);        /*下面空，因为没有定义电源管理*/        try_to_freeze();        if (kthread_should_stop())            break;        /*run_workqueue依次处理工作队列上所有的work*/        run_workqueue(cwq);    }    return 0;}/*run_workqueue依次处理工作队列上所有的work*/static void run_workqueue(struct cpu_workqueue_struct *cwq){    spin_lock_irq(&cwq->lock);    cwq->run_depth++;    if (cwq->run_depth > 3) {        /* morton gets to eat his hat */        printk("%s: recursion depth exceeded: %d\n",            __FUNCTION__, cwq->run_depth);        dump_stack();    }    while (!list_empty(&cwq->worklist)) {        struct work_struct *work = list_entry(cwq->worklist.next,                        struct work_struct, entry);        work_func_t f = work->func;#ifdef CONFIG_LOCKDEP        /*         * It is permissible to free the struct work_struct         * from inside the function that is called from it,         * this we need to take into account for lockdep too.         * To avoid bogus "held lock freed" warnings as well         * as problems when looking into work->lockdep_map,         * make a copy and use that here.         */        struct lockdep_map lockdep_map = work->lockdep_map;#endif        cwq->current_work = work;        list_del_init(cwq->worklist.next);        spin_unlock_irq(&cwq->lock);        BUG_ON(get_wq_data(work) != cwq);        work_clear_pending(work);        lock_acquire(&cwq->wq->lockdep_map, 0, 0, 0, 2, _THIS_IP_);        lock_acquire(&lockdep_map, 0, 0, 0, 2, _THIS_IP_);        f(work); /*执行work项中的func*/               lock_release(&lockdep_map, 1, _THIS_IP_);        lock_release(&cwq->wq->lockdep_map, 1, _THIS_IP_);        if (unlikely(in_atomic() || lockdep_depth(current) > 0)) {            printk(KERN_ERR "BUG: workqueue leaked lock or atomic: "                    "%s/0x%08x/%d\n",                    current->comm, preempt_count(),                        task_pid_nr(current));            printk(KERN_ERR " last function: ");            print_symbol("%s\n", (unsigned long)f);            debug_show_held_locks(current);            dump_stack();        }        spin_lock_irq(&cwq->lock);        cwq->current_work = NULL;    }    cwq->run_depth--;    spin_unlock_irq(&cwq->lock);}

将一个work加入到指定workqueue的work_list中（文件linux_2_6_24/kernel/workqueue.c）

int fastcall queue_work(struct workqueue_struct *wq, struct work_struct *work){    int ret = 0;    if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) {        BUG_ON(!list_empty(&work->entry));        __queue_work(wq_per_cpu(wq, get_cpu()), work);        put_cpu();        ret = 1;    }    return ret;} /* Preempt must be disabled. */static void __queue_work(struct cpu_workqueue_struct *cwq, struct work_struct *work){    unsigned long flags;    spin_lock_irqsave(&cwq->lock, flags);    insert_work(cwq, work, 1);    spin_unlock_irqrestore(&cwq->lock, flags);}static void insert_work(struct cpu_workqueue_struct *cwq,                struct work_struct *work, int tail){    set_wq_data(work, cwq);    /*     * Ensure that we get the right work->data if we see the     * result of list_add() below, see try_to_grab_pending().     */    smp_wmb();    if (tail)        list_add_tail(&work->entry, &cwq->worklist);    else        list_add(&work->entry, &cwq->worklist);    wake_up(&cwq->more_work);}

四、共享队列

其实内核有自己的一个workqueue，叫keventd_wq，这个工作队列也叫做“共享队列”。
do_basic_setup --> init_workqueues --> create_workqueue("events"); 

若驱动模块使用的workqueue功能很简单的话，可以使用“共享队列”，不用自己再建一个队列
使用共享队列，有这样一套API

int schedule_work(struct work_struct *work){    queue_work(keventd_wq, work);}int schedule_delayed_work(struct delayed_work *dwork,unsigned long delay){    timer_stats_timer_set_start_info(&dwork->timer);    return queue_delayed_work(keventd_wq, dwork, delay);}void flush_scheduled_work(void){    flush_workqueue(keventd_wq);}