工作队列(workqueue)

工作队列(workqueue) 2011-05-07 19:53:31

分类: LINUX

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

  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\n",p_test_work->test);
  11. }

  12. void test_init()
  13. {
  14. test_workqueue = create_workqueue("test_workqueue");
  15. if (!test_workqueue)
  16. panic("Failed to create test_workqueue\n");

  17. INIT_WORK(&(test_work.save), do_save);

  18. queue_work(test_workqueue, &(test_work.save));
  19. }

  20. void test_destory(void)
  21. {
  22. if(test_workqueue)
  23. destroy_workqueue(test_workqueue);
  24. }


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
工作队列(workqueue)_第1张图片


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\n",
  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. lock_release(&lockdep_map, 1, _THIS_IP_);
  63. lock_release(&cwq->wq->lockdep_map, 1, _THIS_IP_);

  64. if (unlikely(in_atomic() || lockdep_depth(current) > 0)) {
  65. printk(KERN_ERR "BUG: workqueue leaked lock or atomic: "
  66. "%s/0x%08x/%d\n",
  67. current->comm, preempt_count(),
  68. task_pid_nr(current));
  69. printk(KERN_ERR " last function: ");
  70. print_symbol("%s\n", (unsigned long)f);
  71. debug_show_held_locks(current);
  72. dump_stack();
  73. }

  74. spin_lock_irq(&cwq->lock);
  75. cwq->current_work = NULL;
  76. }
  77. cwq->run_depth--;
  78. spin_unlock_irq(&cwq->lock);
  79. }


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)
  1. {
  2. int ret = 0;

  3. if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) {
  4. BUG_ON(!list_empty(&work->entry));
  5. __queue_work(wq_per_cpu(wq, get_cpu()), work);
  6. put_cpu();
  7. ret = 1;
  8. }
  9. return ret;
  10. }


  11. /* Preempt must be disabled. */
  12. static void __queue_work(struct cpu_workqueue_struct *cwq, struct work_struct *work)
  13. {
  14. unsigned long flags;

  15. spin_lock_irqsave(&cwq->lock, flags);
  16. insert_work(cwq, work, 1);
  17. spin_unlock_irqrestore(&cwq->lock, flags);
  18. }

  19. static void insert_work(struct cpu_workqueue_struct *cwq,
  20. struct work_struct *work, int tail)
  21. {
  22. set_wq_data(work, cwq);
  23. /*
  24. * Ensure that we get the right work->data if we see the
  25. * result of list_add() below, see try_to_grab_pending().
  26. */
  27. smp_wmb();
  28. if (tail)
  29. list_add_tail(&work->entry, &cwq->worklist);
  30. else
  31. list_add(&work->entry, &cwq->worklist);
  32. wake_up(&cwq->more_work);
  33. }



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. }

你可能感兴趣的:(工作队列(workqueue))