Linux内核中工作队列的操作

作者：yfydz

1. 前言

工作队列(workqueue)的Linux内核中的定义的用来处理不是很紧急事件的回调方式处理方法.


以下代码的linux内核版本为2.6.19.2, 源代码文件主要为kernel/workqueue.c.


2. 数据结构

/* include/linux/workqueue.h */

// 工作节点结构

struct work_struct {

// 等待时间

unsigned long pending;

// 链表节点

struct list_head entry;

// workqueue回调函数

void (*func)(void *);

// 回调函数func的数据

void *data;

// 指向CPU相关数据, 一般指向struct cpu_workqueue_struct结构

void *wq_data;

// 定时器

struct timer_list timer;

};


struct execute_work {

struct work_struct work;

};


/* kernel/workqueue.c */

/*

* The per-CPU workqueue (if single thread, we always use the first

* possible cpu).

*

* The sequence counters are for flush_scheduled_work(). It wants to wait

* until all currently-scheduled works are completed, but it doesn't

* want to be livelocked by new, incoming ones. So it waits until

* remove_sequence is >= the insert_sequence which pertained when

* flush_scheduled_work() was called.

*/

// 这个结构是针对每个CPU的

struct cpu_workqueue_struct {

// 结构锁

spinlock_t lock;

// 下一个要执行的节点序号

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

// 下一个要插入节点的序号

long insert_sequence; /* Next to add */

// 工作机构链表节点

struct list_head worklist;

// 要进行处理的等待队列

wait_queue_head_t more_work;

// 处理完的等待队列

wait_queue_head_t work_done;

// 工作队列节点

struct workqueue_struct *wq;

// 进程指针

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:

*/

// 工作队列结构

struct workqueue_struct {

struct cpu_workqueue_struct *cpu_wq;

const char *name;

struct list_head list; /* Empty if single thread */

};


kernel/workqueue.c中定义了一个工作队列链表, 所有工作队列可以挂接到这个链表中:

static LIST_HEAD(workqueues);


3. 一些宏定义

/* include/linux/workqueue.h */

// 初始化工作队列

#define __WORK_INITIALIZER(n, f, d) { \

// 初始化list

.entry = { &(n).entry, &(n).entry }, \

// 回调函数

.func = (f), \

// 回调函数参数

.data = (d), \

// 初始化定时器

.timer = TIMER_INITIALIZER(NULL, 0, 0), \

}


// 声明工作队列并初始化

#define DECLARE_WORK(n, f, d) \

struct work_struct n = __WORK_INITIALIZER(n, f, d)

/*

* initialize a work-struct's func and data pointers:

*/

// 重新定义工作结构参数

#define PREPARE_WORK(_work, _func, _data) \

do { \

(_work)->func = _func; \

(_work)->data = _data; \

} while (0)

/*

* initialize all of a work-struct:

*/

// 初始化工作结构, 和__WORK_INITIALIZER功能相同,不过__WORK_INITIALIZER用在

// 参数初始化定义, 而该宏用在程序之中对工作结构赋值

#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)


4. 操作函数


4.1 创建工作队列


一般的创建函数是create_workqueue, 但这其实只是一个宏:

/* include/linux/workqueue.h */

#define create_workqueue(name) __create_workqueue((name), 0)

在workqueue的初始化函数中, 定义了一个针对内核中所有线程可用的事件工作队列, 其他内核线程建立的事件工作结构就都挂接到该队列:

void init_workqueues(void)

{

...

keventd_wq = create_workqueue("events");

...

}


核心创建函数是__create_workqueue:


struct workqueue_struct *__create_workqueue(const char *name,

int singlethread)

{

int cpu, destroy = 0;

struct workqueue_struct *wq;

struct task_struct *p;

// 分配工作队列结构空间

wq = kzalloc(sizeof(*wq), GFP_KERNEL);

if (!wq)

return NULL;

// 为每个CPU分配单独的工作队列空间

wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct);

if (!wq->cpu_wq) {

kfree(wq);

return NULL;

}

wq->name = name;

mutex_lock(&workqueue_mutex);

if (singlethread) {

// 使用create_workqueue宏时该参数始终为0

// 如果是单一线程模式, 在单线程中调用各个工作队列

// 建立一个的工作队列内核线程

INIT_LIST_HEAD(&wq->list);

// 建立工作队列的线程

p = create_workqueue_thread(wq, singlethread_cpu);

if (!p)

destroy = 1;

else

// 唤醒该线程

wake_up_process(p);

} else {

// 链表模式, 将工作队列添加到工作队列链表

list_add(&wq->list, &workqueues);

// 为每个CPU建立一个工作队列线程

for_each_online_cpu(cpu) {

p = create_workqueue_thread(wq, cpu);

if (p) {

// 绑定CPU

kthread_bind(p, cpu);

// 唤醒线程

wake_up_process(p);

} else

destroy = 1;

}

}

mutex_unlock(&workqueue_mutex);

/*

* Was there any error during startup? If yes then clean up:

*/

if (destroy) {

// 建立线程失败, 释放工作队列

destroy_workqueue(wq);

wq = NULL;

}

return wq;

}

EXPORT_SYMBOL_GPL(__create_workqueue);


// 创建工作队列线程

static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,

int cpu)

{

// 每个CPU的工作队列

struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);

struct task_struct *p;

spin_lock_init(&cwq->lock);

// 初始化

cwq->wq = wq;

cwq->thread = NULL;

cwq->insert_sequence = 0;

cwq->remove_sequence = 0;

INIT_LIST_HEAD(&cwq->worklist);

// 初始化等待队列more_work, 该队列处理要执行的工作结构

init_waitqueue_head(&cwq->more_work);

// 初始化等待队列work_done, 该队列处理执行完的工作结构

init_waitqueue_head(&cwq->work_done);

// 建立内核线程work_thread

if (is_single_threaded(wq))

p = kthread_create(worker_thread, cwq, "%s", wq->name);

else

p = kthread_create(worker_thread, cwq, "%s/%d", wq->name, cpu);

if (IS_ERR(p))

return NULL;

// 保存线程指针

cwq->thread = p;

return p;

}

static int worker_thread(void *__cwq)

{

struct cpu_workqueue_struct *cwq = __cwq;

// 声明一个等待队列

DECLARE_WAITQUEUE(wait, current);

// 信号

struct k_sigaction sa;

sigset_t blocked;

current->flags |= PF_NOFREEZE;

// 降低进程优先级, 工作进程不是个很紧急的进程,不和其他进程抢占CPU,通常在系统空闲时运行

set_user_nice(current, -5);

/* Block and flush all signals */

// 阻塞所有信号

sigfillset(&blocked);

sigprocmask(SIG_BLOCK, &blocked, NULL);

flush_signals(current);

/*

* We inherited MPOL_INTERLEAVE from the booting kernel.

* Set MPOL_DEFAULT to insure node local allocations.

*/

numa_default_policy();

/* SIG_IGN makes children autoreap: see do_notify_parent(). */

// 信号处理都是忽略

sa.sa.sa_handler = SIG_IGN;

sa.sa.sa_flags = 0;

siginitset(&sa.sa.sa_mask, sigmask(SIGCHLD));

do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);

// 进程可中断

set_current_state(TASK_INTERRUPTIBLE);

// 进入循环, 没明确停止该进程就一直运行

while (!kthread_should_stop()) {

// 设置more_work等待队列, 当有新work结构链入队列中时会激发此等待队列

add_wait_queue(&cwq->more_work, &wait);

if (list_empty(&cwq->worklist))

// 工作队列为空, 睡眠

schedule();

else

// 进行运行状态

__set_current_state(TASK_RUNNING);

// 删除等待队列

remove_wait_queue(&cwq->more_work, &wait);

// 按链表遍历执行工作任务

if (!list_empty(&cwq->worklist))

run_workqueue(cwq);

// 执行完工作, 设置进程是可中断的, 重新循环等待工作

set_current_state(TASK_INTERRUPTIBLE);

}

__set_current_state(TASK_RUNNING);

return 0;

}


// 运行工作结构

static void run_workqueue(struct cpu_workqueue_struct *cwq)

{

unsigned long flags;

/*

* Keep taking off work from the queue until

* done.

*/

// 加锁

spin_lock_irqsave(&cwq->lock, flags);

// 统计已经递归调用了多少次了

cwq->run_depth++;

if (cwq->run_depth > 3) {

// 递归调用此时太多

/* morton gets to eat his hat */

printk("%s: recursion depth exceeded: %d\n",

__FUNCTION__, cwq->run_depth);

dump_stack();

}

// 遍历工作链表

while (!list_empty(&cwq->worklist)) {

// 获取的是next节点的

struct work_struct *work = list_entry(cwq->worklist.next,

struct work_struct, entry);

void (*f) (void *) = work->func;

void *data = work->data;

// 删除节点, 同时节点中的list参数清空

list_del_init(cwq->worklist.next);

// 解锁

// 现在在执行以下代码时可以中断,run_workqueue本身可能会重新被调用, 所以要判断递归深度

spin_unlock_irqrestore(&cwq->lock, flags);

BUG_ON(work->wq_data != cwq);

// 工作结构已经不在链表中

clear_bit(0, &work->pending);

// 执行工作函数

f(data);

// 重新加锁

spin_lock_irqsave(&cwq->lock, flags);

// 执行完的工作序列号递增

cwq->remove_sequence++;

// 唤醒工作完成等待队列, 供释放工作队列

wake_up(&cwq->work_done);

}

// 减少递归深度

cwq->run_depth--;

// 解锁

spin_unlock_irqrestore(&cwq->lock, flags);

}


4.2 释放工作队列

/**

* destroy_workqueue - safely terminate a workqueue

* @wq: target workqueue

*

* Safely destroy a workqueue. All work currently pending will be done first.

*/

void destroy_workqueue(struct workqueue_struct *wq)

{

int cpu;

// 清除当前工作队列中的所有工作

flush_workqueue(wq);

/* We don't need the distraction of CPUs appearing and vanishing. */

mutex_lock(&workqueue_mutex);

// 结束该工作队列的线程

if (is_single_threaded(wq))

cleanup_workqueue_thread(wq, singlethread_cpu);

else {

for_each_online_cpu(cpu)

cleanup_workqueue_thread(wq, cpu);

list_del(&wq->list);

}

mutex_unlock(&workqueue_mutex);

// 释放工作队列中对应每个CPU的工作队列数据

free_percpu(wq->cpu_wq);

kfree(wq);

}

EXPORT_SYMBOL_GPL(destroy_workqueue);


/**

* flush_workqueue - ensure that any scheduled work has run to completion.

* @wq: workqueue to flush

*

* Forces execution of the workqueue and blocks until its completion.

* This is typically used in driver shutdown handlers.

*

* This function will sample each workqueue's current insert_sequence number and

* will sleep until the head sequence is greater than or equal to that. This

* means that we sleep until all works which were queued on entry have been

* handled, but we are not livelocked by new incoming ones.

*

* This function used to run the workqueues itself. Now we just wait for the

* helper threads to do it.

*/

void fastcall flush_workqueue(struct workqueue_struct *wq)

{

// 该进程可以睡眠

might_sleep();

// 清空每个CPU上的工作队列

if (is_single_threaded(wq)) {

/* Always use first cpu's area. */

flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));

} else {

int cpu;

mutex_lock(&workqueue_mutex);

for_each_online_cpu(cpu)

flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));

mutex_unlock(&workqueue_mutex);

}

}

EXPORT_SYMBOL_GPL(flush_workqueue);


flush_workqueue的核心处理函数为flush_cpu_workqueue:

static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)

{

if (cwq->thread == current) {

// 如果是工作队列进程正在被调度

/*

* Probably keventd trying to flush its own queue. So simply run

* it by hand rather than deadlocking.

*/

// 执行完该工作队列

run_workqueue(cwq);

} else {

// 定义等待

DEFINE_WAIT(wait);

long sequence_needed;

// 加锁

spin_lock_irq(&cwq->lock);

// 最新工作结构序号

sequence_needed = cwq->insert_sequence;

// 该条件是判断队列中是否还有没有执行的工作结构

while (sequence_needed - cwq->remove_sequence > 0) {

// 有为执行的工作结构

// 通过work_done等待队列等待

prepare_to_wait(&cwq->work_done, &wait,

TASK_UNINTERRUPTIBLE);

// 解锁

spin_unlock_irq(&cwq->lock);

// 睡眠, 由wake_up(&cwq->work_done)来唤醒

schedule();

// 重新加锁

spin_lock_irq(&cwq->lock);

}

// 等待清除

finish_wait(&cwq->work_done, &wait);

spin_unlock_irq(&cwq->lock);

}

}


4.3 调度工作


在大多数情况下, 并不需要自己建立工作队列,而是只定义工作, 将工作结构挂接到内核预定义的事件工作队列中调度, 在kernel/workqueue.c中定义了一个静态全局量的工作队列keventd_wq:

static struct workqueue_struct *keventd_wq;


4.3.1 立即调度

// 在其他函数中使用以下函数来调度工作结构, 是把工作结构挂接到工作队列中进行调度

/**

* schedule_work - put work task in global workqueue

* @work: job to be done

*

* This puts a job in the kernel-global workqueue.

*/

// 调度工作结构, 将工作结构添加到事件工作队列keventd_wq

int fastcall schedule_work(struct work_struct *work)

{

return queue_work(keventd_wq, work);

}

EXPORT_SYMBOL(schedule_work);


/**

* queue_work - queue work on a workqueue

* @wq: workqueue to use

* @work: work to queue

*

* Returns 0 if @work was already on a queue, non-zero otherwise.

*

* We queue the work to the CPU it was submitted, but there is no

* guarantee that it will be processed by that CPU.

*/

int fastcall queue_work(struct workqueue_struct *wq, struct work_struct *work)

{

int ret = 0, cpu = get_cpu();

if (!test_and_set_bit(0, &work->pending)) {

// 工作结构还没在队列, 设置pending标志表示把工作结构挂接到队列中

if (unlikely(is_single_threaded(wq)))

cpu = singlethread_cpu;

BUG_ON(!list_empty(&work->entry));

// 进行具体的排队

__queue_work(per_cpu_ptr(wq->cpu_wq, cpu), work);

ret = 1;

}

put_cpu();

return ret;

}

EXPORT_SYMBOL_GPL(queue_work);

/* Preempt must be disabled. */

// 不能被抢占

static void __queue_work(struct cpu_workqueue_struct *cwq,

struct work_struct *work)

{

unsigned long flags;

// 加锁

spin_lock_irqsave(&cwq->lock, flags);

// 指向CPU工作队列

work->wq_data = cwq;

// 挂接到工作链表

list_add_tail(&work->entry, &cwq->worklist);

// 递增插入的序列号

cwq->insert_sequence++;

// 唤醒等待队列准备处理工作结构

wake_up(&cwq->more_work);

spin_unlock_irqrestore(&cwq->lock, flags);

}


4.3.2 延迟调度


4.3.2.1 schedule_delayed_work

/**

* schedule_delayed_work - put work task in global workqueue after delay

* @work: job to be done

* @delay: number of jiffies to wait

*

* After waiting for a given time this puts a job in the kernel-global

* workqueue.

*/

// 延迟调度工作, 延迟一定时间后再将工作结构挂接到工作队列

int fastcall schedule_delayed_work(struct work_struct *work, unsigned long delay)

{

return queue_delayed_work(keventd_wq, work, delay);

}

EXPORT_SYMBOL(schedule_delayed_work);


/**

* queue_delayed_work - queue work on a workqueue after delay

* @wq: workqueue to use

* @work: work to queue

* @delay: number of jiffies to wait before queueing

*

* Returns 0 if @work was already on a queue, non-zero otherwise.

*/

int fastcall queue_delayed_work(struct workqueue_struct *wq,

struct work_struct *work, unsigned long delay)

{

int ret = 0;

// 定时器, 此时的定时器应该是不起效的, 延迟将通过该定时器来实现

struct timer_list *timer = &work->timer;

if (!test_and_set_bit(0, &work->pending)) {

// 工作结构还没在队列, 设置pending标志表示把工作结构挂接到队列中

// 如果现在定时器已经起效, 出错

BUG_ON(timer_pending(timer));

// 工作结构已经挂接到链表, 出错

BUG_ON(!list_empty(&work->entry));

/* This stores wq for the moment, for the timer_fn */

// 保存工作队列的指针

work->wq_data = wq;

// 定时器初始化

timer->expires = jiffies + delay;

timer->data = (unsigned long)work;

// 定时函数

timer->function = delayed_work_timer_fn;

// 定时器生效, 定时到期后再添加到工作队列

add_timer(timer);

ret = 1;

}

return ret;

}

EXPORT_SYMBOL_GPL(queue_delayed_work);



// 定时中断函数

static void delayed_work_timer_fn(unsigned long __data)

{

struct work_struct *work = (struct work_struct *)__data;

struct workqueue_struct *wq = work->wq_data;

// 获取CPU

int cpu = smp_processor_id();

if (unlikely(is_single_threaded(wq)))

cpu = singlethread_cpu;

// 将工作结构添加到工作队列,注意这是在时间中断调用

__queue_work(per_cpu_ptr(wq->cpu_wq, cpu), work);

}


4.3.2.2 schedule_delayed_work_on


指定CPU的延迟调度工作结构, 和schedule_delayed_work相比增加了一个CPU参数, 其他都相同

/**

* schedule_delayed_work_on - queue work in global workqueue on CPU after delay

* @cpu: cpu to use

* @work: job to be done

* @delay: number of jiffies to wait

*

* After waiting for a given time this puts a job in the kernel-global

* workqueue on the specified CPU.

*/

int schedule_delayed_work_on(int cpu,

struct work_struct *work, unsigned long delay)

{

return queue_delayed_work_on(cpu, keventd_wq, work, delay);

}


/**

* queue_delayed_work_on - queue work on specific CPU after delay

* @cpu: CPU number to execute work on

* @wq: workqueue to use

* @work: work to queue

* @delay: number of jiffies to wait before queueing

*

* Returns 0 if @work was already on a queue, non-zero otherwise.

*/

int queue_delayed_work_on(int cpu, struct workqueue_struct *wq,

struct work_struct *work, unsigned long delay)

{

int ret = 0;

struct timer_list *timer = &work->timer;

if (!test_and_set_bit(0, &work->pending)) {

BUG_ON(timer_pending(timer));

BUG_ON(!list_empty(&work->entry));

/* This stores wq for the moment, for the timer_fn */

work->wq_data = wq;

timer->expires = jiffies + delay;

timer->data = (unsigned long)work;

timer->function = delayed_work_timer_fn;

add_timer_on(timer, cpu);

ret = 1;

}

return ret;

}

EXPORT_SYMBOL_GPL(queue_delayed_work_on);


5. 结论


工作队列和定时器函数处理有点类似, 都是执行一定的回调函数, 但和定时器处理函数不同的是定时器回调函数只执行一次, 而且执行定时器回调函数的时候是在时钟中断中, 限制比较多, 因此回调程序不能太复杂; 而工作队列是通过内核线程实现, 一直有效, 可重复执行, 由于执行时降低了线程的优先级, 执行时可能休眠, 因此工作队列处理的应该是那些不是很紧急的任务, 如垃圾回收处理等, 通常在系统空闲时执行，在xfrm库中就广泛使用了workqueue，使用时，只需要定义work结构，然后调用schedule_(delayed_)work即可。