一、用法
struct cpu_workqueue_struct {

spinlock_t lock;

long remove_sequence; /* Least-recently added (next to run) */
long insert_sequence; /* Next to add */

struct list_head worklist;
wait_queue_head_t more_work;
wait_queue_head_t work_done;

struct workqueue_struct *wq;
struct task_struct *thread;

int run_depth; /* Detect run_workqueue() recursion depth */
} ____cacheline_aligned;

/*
* The externally visible workqueue abstraction is an array of
* per-CPU workqueues:
*/
struct workqueue_struct {
struct cpu_workqueue_struct *cpu_wq;
const char *name;
struct list_head list; /* Empty if single thread */
};

工作队列的使用很简单。
1.首先是建立一个工作队列：
struct workqueue_struct *keventd_wq;
keventd_wq = create_workqueue("events");
2.然后就是在这个队列中insert你所要做的“工作”：
DECLARE_WORK(work, func, data)
queue_work(keventd_wq, work);

struct work_struct {
unsigned long pending;
struct list_head entry;
void (*func)(void *);
void *data;
void *wq_data;
struct timer_list timer;
};

初始化有两种方法。
一种为静态方法：
#define __WORK_INITIALIZER(n, f, d) { \
.entry = { &(n).entry, &(n).entry }, \
.func = (f), \
.data = (d), \
.timer = TIMER_INITIALIZER(NULL, 0, 0), \
}

#define DECLARE_WORK(n, f, d) \
struct work_struct n = __WORK_INITIALIZER(n, f, d)

另一种为动态方法：
/*
* initialize all of a work-struct:
*/
#define INIT_WORK(_work, _func, _data) \
do { \
INIT_LIST_HEAD(&(_work)->entry); \
(_work)->pending = 0; \
PREPARE_WORK((_work), (_func), (_data)); \
init_timer(&(_work)->timer); \
} while (0)


二、执行过程
create_workqueue() -> __create_workqueue()

struct workqueue_struct *__create_workqueue(const char *name,
int singlethread)
{
int cpu, destroy = 0;
struct workqueue_struct *wq;
struct task_struct *p;

wq = kzalloc(sizeof(*wq), GFP_KERNEL);
//为每个CPU建立一个结构
wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct);
...
wq->name = name;
mutex_lock(&workqueue_mutex);
if (singlethread) {
...
} else {
list_add(&wq->list, &workqueues);
for_each_online_cpu(cpu) {
//为每个CPU创建一个线程
p = create_workqueue_thread(wq, cpu);
if (p) {
kthread_bind(p, cpu);
//唤醒这个线程执行工作
wake_up_process(p);
} else
destroy = 1;
}
}
mutex_unlock(&workqueue_mutex);
...
return wq;
}

create_workqueue() -> __create_workqueue() -> create_workqueue_thread()

static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,
int cpu)
{
struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
struct task_struct *p;

spin_lock_init(&cwq->lock);
cwq->wq = wq;
cwq->thread = NULL;
cwq->insert_sequence = 0;
cwq->remove_sequence = 0;
INIT_LIST_HEAD(&cwq->worklist);
init_waitqueue_head(&cwq->more_work);
init_waitqueue_head(&cwq->work_done);

if (is_single_threaded(wq))
p = kthread_create(worker_thread, cwq, "%s", wq->name);
else
//创建一个线程，这个线程以cwq为数据执行worker_thread这个函数
p = kthread_create(worker_thread, cwq, "%s/%d", wq->name, cpu);
if (IS_ERR(p))
return NULL;
cwq->thread = p; //
return p;
}


create_workqueue() -> __create_workqueue() -> create_workqueue_thread() -> worker_thread()

//本函数在一个死循环等待工作的到来，这一般在睡眠状态中，等待被唤醒执行工作
//当有工作到来时queue_work()会将这个线程唤醒
static int worker_thread(void *__cwq)
{
struct cpu_workqueue_struct *cwq = __cwq;
DECLARE_WAITQUEUE(wait, current);
...

current->flags |= PF_NOFREEZE;

//设置优先级
set_user_nice(current, -5);
...
set_current_state(TASK_INTERRUPTIBLE);
while (!kthread_should_stop()) {
//将本线程加入睡眠队列，用于睡眠后可以被唤醒
add_wait_queue(&cwq->more_work, &wait);

//如果没用被执行的“工作”，则将自己切换出去，进入睡眠状态
if (list_empty(&cwq->worklist))
schedule();
else //否则或是被唤醒
__set_current_state(TASK_RUNNING);
remove_wait_queue(&cwq->more_work, &wait);

//工作队列非空，执行工作
if (!list_empty(&cwq->worklist))
run_workqueue(cwq);
set_current_state(TASK_INTERRUPTIBLE);
}
__set_current_state(TASK_RUNNING);
return 0;
}


create_workqueue() -> __create_workqueue() -> create_workqueue_thread()
-> worker_thread() -> run_workqueue()
//该函数执行真正的工作
static void run_workqueue(struct cpu_workqueue_struct *cwq)
{
unsigned long flags;

spin_lock_irqsave(&cwq->lock, flags);
...
//顺次执行队列中的所有工作
while (!list_empty(&cwq->worklist)) {
struct work_struct *work = list_entry(cwq->worklist.next,
struct work_struct, entry);
void (*f) (void *) = work->func;
void *data = work->data;

//从队列中删除待执行的任务
list_del_init(cwq->worklist.next);
spin_unlock_irqrestore(&cwq->lock, flags);

BUG_ON(work->wq_data != cwq);
clear_bit(0, &work->pending);

//执行“工作”
f(data);

spin_lock_irqsave(&cwq->lock, flags);
cwq->remove_sequence++;
wake_up(&cwq->work_done); //
}
cwq->run_depth--;
spin_unlock_irqrestore(&cwq->lock, flags);
}

三、工作线程创建的详细过程
create_workqueue() -> __create_workqueue() -> create_workqueue_thread()
-> kthread_create()

struct task_struct *kthread_create(int (*threadfn)(void *data),
void *data,
const char namefmt[],
...)
{
//初始化用于创建线程的辅助结构
struct kthread_create_info create;
DECLARE_WORK(work, keventd_create_kthread, &create);
create.threadfn = threadfn;
create.data = data;
init_completion(&create.started);
init_completion(&create.done);

if (!helper_wq) //首先创建辅助工作队列
work.func(work.data);
else {
//注意，“创建一个工作队列”这个工作本身又是属于helper_wq工作队列
//的一项工作，所以，将这个工作加入的辅助工作队列中等待执行。
queue_work(helper_wq, &work);
wait_for_completion(&create.done);
}
...
return create.result;
}


create_workqueue() -> __create_workqueue() -> create_workqueue_thread()
-> kthread_create()-> keventd_create_kthread()
//终会调用kernel_thread为每个工作队列创建一个线程
//这样，被创建的线程会以create为数据执行kthread(如下),而kthread中则执行create中的threadfn(data)，
//即为create_workqueue_thread中的worker_thread(cwq)，即为我们工作队列要执行的函数了。

static void keventd_create_kthread(void *_create)
{
struct kthread_create_info *create = _create;
int pid;

/* 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);
} else {
wait_for_completion(&create->started);
read_lock(&tasklist_lock);
create->result = find_task_by_pid(pid);
read_unlock(&tasklist_lock);
}
complete(&create->done);
}

static int kthread(void *_create)
{
struct kthread_create_info *create = _create;
int (*threadfn)(void *data);
void *data;
...
threadfn = create->threadfn;
data = create->data;
...
if (!kthread_should_stop())
ret = threadfn(data);
...
return 0;
}

四、插入“工作”
/* 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);
work->wq_data = cwq;
//将当前工作插入到工作队列中待待执行
list_add_tail(&work->entry, &cwq->worklist);
cwq->insert_sequence++;
wake_up(&cwq->more_work); //唤醒相应线程
spin_unlock_irqrestore(&cwq->lock, flags);
}


int fastcall queue_work(struct workqueue_struct *wq, struct work_struct *work)
{
int ret = 0, cpu = get_cpu();

//如里当前工作未在队列中才插入
if (!test_and_set_bit(0, &work->pending)) {
...
__queue_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
ret = 1;
}
put_cpu();
return ret;
}

文章来源CU社区：工作队列分析