[内核]Linux workqueue

时间:2021-07-23 20:36:06

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

 #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

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”, , )
--> __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 ;
}

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, -);
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 ;
}
/*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 > ) {
/* 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, , , , , _THIS_IP_);
lock_acquire(&lockdep_map, , , , , _THIS_IP_);
f(work); /*执行work项中的func*/ lock_release(&lockdep_map, , _THIS_IP_);
lock_release(&cwq->wq->lockdep_map, , _THIS_IP_);
if (unlikely(in_atomic() || lockdep_depth(current) > )) {
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 = ;
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 = ;
}
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, );
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);
}