转自：http://blog.chinaunix.net/uid-24148050-id-296982.html
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：
- 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. 而workqueue是通过内核进程实现的，就没有上述限制的，而且工作队列函数可以休眠。
- tasklet始终运行在被初始提交的同一处理器上，workqueue不一定
- tasklet不能确定延时时间（即使很短），workqueue可以设定延迟时间
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
二、workqueue的API
workqueue的API自2.6.20后发生了变化
1 #include <linux/workqueue.h> 2 struct workqueue_struct; 3 struct work_struct; 4 struct workqueue_struct *create_workqueue(const char *name); 5 void destroy_workqueue(struct workqueue_struct *queue); 6 INIT_WORK(_work, _func); 7 INIT_DELAYED_WORK(_work, _func); 8 int queue_work(struct workqueue_struct *wq, struct work_struct *work); 9 int queue_delayed_work(struct workqueue_struct *wq,struct delayed_work *dwork, unsigned long delay); 10 int queue_delayed_work_on(int cpu, struct workqueue_struct *wq, 11 struct delayed_work *dwork, unsigned long delay); 12 int cancel_work_sync(struct work_struct *work); 13 int cancel_delayed_work_sync(struct delayed_work *dwork); 14 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)
1 struct my_work_stuct{ 2 int test; 3 struct work_stuct save; 4 }; 5 struct my_work_stuct test_work; 6 struct workqueue_struct *test_workqueue; 7 void do_save(struct work_struct *p_work) 8 { 9 struct my_work_struct *p_test_work = container_of(p_work, struct my_work_stuct, save); 10 printk("%d ",p_test_work->test); 11 } 12 13 void test_init() 14 { 15 test_workqueue = create_workqueue("test_workqueue"); 16 if (!test_workqueue) 17 panic("Failed to create test_workqueue "); 18 INIT_WORK(&(test_work.save), do_save); 19 queue_work(test_workqueue, &(test_work.save)); 20 } 21 void test_destory(void) 22 { 23 if(test_workqueue) 24 destroy_workqueue(test_workqueue); 25 }
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
1 create_workqueue("tap_workqueue") 2 --> __create_workqueue(“tap_workqueue”, 0, 0) 3 --> __create_workqueue_key((name), (singlethread), (freezeable), NULL, NULL){ 4 wq = kzalloc(sizeof(*wq), GFP_KERNEL); 5 wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct); 6 wq->name = name; 7 wq->singlethread = singlethread; 8 wq->freezeable = freezeable; 9 INIT_LIST_HEAD(&wq->list); 10 for_each_possible_cpu(cpu) { 11 cwq = init_cpu_workqueue(wq, cpu); 12 err = create_workqueue_thread(cwq, cpu); 13 start_workqueue_thread(cwq, cpu); 14 } 15 }
create_workqueue_thread 建立了一个内核进程 worker_thread（linux_2_6_24/kernel/workqueue.c）
1 create_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu) 2 { 3 struct workqueue_struct *wq = cwq->wq; 4 const char *fmt = is_single_threaded(wq) ? "%s" : "%s/%d"; 5 struct task_struct *p; 6 p = kthread_create(worker_thread, cwq, fmt, wq->name, cpu); 7 if (IS_ERR(p)) 8 return PTR_ERR(p); 9 cwq->thread = p; 10 return 0; 11 }
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）
1 int worker_thread (void *__cwq) 2 { 3 struct cpu_workqueue_struct *cwq = __cwq; 4 /*下面定义等待队列项*/ 5 DEFINE_WAIT(wait); 6 /*下面freezeable一般为0*/ 7 if (cwq->wq->freezeable) 8 set_freezable(); 9 /*提高优先级别*/ 10 set_user_nice(current, -5); 11 for (;;) { 12 /*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*/ 13 prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE); 14 /*work队列空则切换出去*/ 15 if (!freezing(current) && !kthread_should_stop() && list_empty(&cwq->worklist)) 16 schedule(); 17 /*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*/ 18 finish_wait(&cwq->more_work, &wait); 19 /*下面空，因为没有定义电源管理*/ 20 try_to_freeze(); 21 if (kthread_should_stop()) 22 break; 23 /*run_workqueue依次处理工作队列上所有的work*/ 24 run_workqueue(cwq); 25 } 26 return 0; 27 } 28 /*run_workqueue依次处理工作队列上所有的work*/ 29 static void run_workqueue(struct cpu_workqueue_struct *cwq) 30 { 31 spin_lock_irq(&cwq->lock); 32 cwq->run_depth++; 33 if (cwq->run_depth > 3) { 34 /* morton gets to eat his hat */ 35 printk("%s: recursion depth exceeded: %d ", 36 __FUNCTION__, cwq->run_depth); 37 dump_stack(); 38 } 39 while (!list_empty(&cwq->worklist)) { 40 struct work_struct *work = list_entry(cwq->worklist.next, 41 struct work_struct, entry); 42 work_func_t f = work->func; 43 #ifdef CONFIG_LOCKDEP 44 /* 45 * It is permissible to free the struct work_struct 46 * from inside the function that is called from it, 47 * this we need to take into account for lockdep too. 48 * To avoid bogus "held lock freed" warnings as well 49 * as problems when looking into work->lockdep_map, 50 * make a copy and use that here. 51 */ 52 struct lockdep_map lockdep_map = work->lockdep_map; 53 #endif 54 cwq->current_work = work; 55 list_del_init(cwq->worklist.next); 56 spin_unlock_irq(&cwq->lock); 57 BUG_ON(get_wq_data(work) != cwq); 58 work_clear_pending(work); 59 lock_acquire(&cwq->wq->lockdep_map, 0, 0, 0, 2, _THIS_IP_); 60 lock_acquire(&lockdep_map, 0, 0, 0, 2, _THIS_IP_); 61 f(work); /*执行work项中的func*/ 62 63 lock_release(&lockdep_map, 1, _THIS_IP_); 64 lock_release(&cwq->wq->lockdep_map, 1, _THIS_IP_); 65 if (unlikely(in_atomic() || lockdep_depth(current) > 0)) { 66 printk(KERN_ERR "BUG: workqueue leaked lock or atomic: " 67 "%s/0x%08x/%d ", 68 current->comm, preempt_count(), 69 task_pid_nr(current)); 70 printk(KERN_ERR " last function: "); 71 print_symbol("%s ", (unsigned long)f); 72 debug_show_held_locks(current); 73 dump_stack(); 74 } 75 spin_lock_irq(&cwq->lock); 76 cwq->current_work = NULL; 77 } 78 cwq->run_depth--; 79 spin_unlock_irq(&cwq->lock); 80 }
将一个work加入到指定workqueue的work_list中（文件linux_2_6_24/kernel/workqueue.c）
1 int fastcall queue_work(struct workqueue_struct *wq, struct work_struct *work) 2 { 3 int ret = 0; 4 if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) { 5 BUG_ON(!list_empty(&work->entry)); 6 __queue_work(wq_per_cpu(wq, get_cpu()), work); 7 put_cpu(); 8 ret = 1; 9 } 10 return ret; 11 } 12 /* Preempt must be disabled. */ 13 static void __queue_work(struct cpu_workqueue_struct *cwq, struct work_struct *work) 14 { 15 unsigned long flags; 16 spin_lock_irqsave(&cwq->lock, flags); 17 insert_work(cwq, work, 1); 18 spin_unlock_irqrestore(&cwq->lock, flags); 19 } 20 static void insert_work(struct cpu_workqueue_struct *cwq, 21 struct work_struct *work, int tail) 22 { 23 set_wq_data(work, cwq); 24 /* 25 * Ensure that we get the right work->data if we see the 26 * result of list_add() below, see try_to_grab_pending(). 27 */ 28 smp_wmb(); 29 if (tail) 30 list_add_tail(&work->entry, &cwq->worklist); 31 else 32 list_add(&work->entry, &cwq->worklist); 33 wake_up(&cwq->more_work); 34 }
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
1 int schedule_work(struct work_struct *work) 2 { 3 queue_work(keventd_wq, work); 4 } 5 int schedule_delayed_work(struct delayed_work *dwork,unsigned long delay) 6 { 7 timer_stats_timer_set_start_info(&dwork->timer); 8 return queue_delayed_work(keventd_wq, dwork, delay); 9 } 10 void flush_scheduled_work(void) 11 { 12 flush_workqueue(keventd_wq); 13 }