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kernel, linux, 操作系统

Linux workqueue简单笔记

最近再看《深入理解LINUX内核》,简单的笔记下workqueue的实现吧。

workqueue.h

/*
 * workqueue.h --- work queue handling for Linux.
 */

#ifndef _LINUX_WORKQUEUE_H
#define _LINUX_WORKQUEUE_H

#include <linux/timer.h>
#include <linux/linkage.h>
#include <linux/bitops.h>

struct workqueue_struct;


/*
插入到工作队列中的任务结构体
*/
struct work_struct {
	unsigned long pending; /* 该任务是否已经要执行了 */
	struct list_head entry; /* 任务在队列中是链表存储 */
	void (*func)(void *); /* 回调函数 */
	void *data;             /* 传入参数 */
	void *wq_data;      /*  保存该任务所属的工作队列 */
	struct timer_list timer; /* 任务可能要延时执行 */
};


/* 初始化一个任务结构体*/
#define __WORK_INITIALIZER(n, f, d) {				\
        .entry	= { &(n).entry, &(n).entry },			\
	.func = (f),						\
	.data = (d),						\
	.timer = TIMER_INITIALIZER(NULL, 0, 0),			\
	}


/* 初始化一个任务结构体*/
#define DECLARE_WORK(n, f, d)					\
	struct work_struct n = __WORK_INITIALIZER(n, f, d)

/*
 * initialize a work-struct's func and data pointers:
 */
 /* 初始化一个任务结构体指针 */
#define PREPARE_WORK(_work, _func, _data)			\
	do {							\
		(_work)->func = _func;				\
		(_work)->data = _data;				\
	} while (0)

/*
 * initialize all of a work-struct:
 */
 /* 初始化一个任务结构体指针 */
#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)

extern struct workqueue_struct *__create_workqueue(const char *name,
						   int singlethread);
#define create_workqueue(name) __create_workqueue((name), 0)
#define create_singlethread_workqueue(name) __create_workqueue((name), 1)

extern void destroy_workqueue(struct workqueue_struct *wq);

extern int FASTCALL(queue_work(struct workqueue_struct *wq, struct work_struct *work));
extern int FASTCALL(queue_delayed_work(struct workqueue_struct *wq, struct work_struct *work, unsigned long delay));
extern void FASTCALL(flush_workqueue(struct workqueue_struct *wq));

extern int FASTCALL(schedule_work(struct work_struct *work));
extern int FASTCALL(schedule_delayed_work(struct work_struct *work, unsigned long delay));

extern int schedule_delayed_work_on(int cpu, struct work_struct *work, unsigned long delay);
extern void flush_scheduled_work(void);
extern int current_is_keventd(void);
extern int keventd_up(void);

extern void init_workqueues(void);

/*
 * Kill off a pending schedule_delayed_work().  Note that the work callback
 * function may still be running on return from cancel_delayed_work().  Run
 * flush_scheduled_work() to wait on it.
 */
/*
取消一个任务。
*/
static inline int cancel_delayed_work(struct work_struct *work)
{
	int ret;

	ret = del_timer_sync(&work->timer);
	if (ret)
		clear_bit(0, &work->pending);
	return ret;
}

#endif

workqueue.c

/*
 * linux/kernel/workqueue.c
 *
 * Generic mechanism for defining kernel helper threads for running
 * arbitrary tasks in process context.
 *
 * Started by Ingo Molnar, Copyright (C) 2002
 *
 * Derived from the taskqueue/keventd code by:
 *
 *   David Woodhouse <dwmw2@infradead.org>
 *   Andrew Morton <andrewm@uow.edu.au>
 *   Kai Petzke <wpp@marie.physik.tu-berlin.de>
 *   Theodore Ts'o <tytso@mit.edu>
 */

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/signal.h>
#include <linux/completion.h>
#include <linux/workqueue.h>
#include <linux/slab.h>
#include <linux/cpu.h>
#include <linux/notifier.h>
#include <linux/kthread.h>

/*
 * The per-CPU workqueue (if single thread, we always use cpu 0's).
 *
 * The sequence counters are for flush_scheduled_work().  It wants to wait
 * until 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.
 */
 /*
多CPU环境下,每个CPU上都维护一个工作队列
*/
struct cpu_workqueue_struct {

	spinlock_t lock;  /* 锁 */

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

	struct list_head worklist;/* 工作队列上挂着的任务链表 */
	wait_queue_head_t more_work;
	wait_queue_head_t work_done;

	struct workqueue_struct *wq; /* 该CPU工作队列所属的总工作队列 */
	task_t *thread;/* 该工作队列的工作线程 */

/* 如果有递归情况发生,计算下深度 */
	int run_depth;		/* Detect run_workqueue() recursion depth */
} ____cacheline_aligned;

/*
 * The externally visible workqueue abstraction is an array of
 * per-CPU workqueues:
 */

/*
总工作队列结构体
*/
struct workqueue_struct {
	struct cpu_workqueue_struct cpu_wq[NR_CPUS];/* 在每个CPU上都有对应的工作队列 */
	const char *name;/* 该结构体的名称 */
	struct list_head list; 	/* Empty if single thread *//* 内核中把所有申请的工作队列全部串起来,便于管理,
	如果是单CPU队列,就置为NULL 	*/

};

/* All the per-cpu workqueues on the system, for hotplug cpu to add/remove
   threads to each one as cpus come/go. */

/*
内核中把所有申请的工作队列全部串起来,便于管理。
这里定义了一个锁和一个链表头。
*/
/*
注意这里串起来的队列全部是多CPU队列,原因为当CPU发生变更时可以
进行遍历更改。由于单CPU不会出现这个问题,因此单CPU队列不会放在这个
队列中
*/

static DEFINE_SPINLOCK(workqueue_lock);
static LIST_HEAD(workqueues);


/*
判断一个工作队列是否是单CPU型的。
*/
/* If it's single threaded, it isn't in the list of workqueues. */
static inline int is_single_threaded(struct workqueue_struct *wq)
{
	return list_empty(&wq->list);
}

/*
将一个工作任务插入到一个工作队列中
*/
/* 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);/* 先禁掉中断,并且锁上该工作队列*/
	work->wq_data = cwq;/* 更新工作任务对应的工作队列 */
	list_add_tail(&work->entry, &cwq->worklist);/* 讲工作任务添加到工作队列内部的链表上 */
	cwq->insert_sequence++;/* 累加插入计数 */
	wake_up(&cwq->more_work);/* 唤醒工作队列上的等待队列 */
	spin_unlock_irqrestore(&cwq->lock, flags);
}

/*
 * Queue work on a workqueue. Return non-zero if it was successfully
 * added.
 *
 * We queue the work to the CPU it was submitted, but there is no
 * guarantee that it will be processed by that CPU.
 */
 /*
将一个工作任务插入到一个工作队列中
*/
int fastcall queue_work(struct workqueue_struct *wq, struct work_struct *work)
{
/*
获取当前CPU的锁定
*/
	int ret = 0, cpu = get_cpu();

/* 如果pending为1,说明已经加到工作队列上了,
不能重复加入了
*/
	if (!test_and_set_bit(0, &work->pending)) {

/* 如果当前的工作队列是单CPU类型的,就不用指定当前CPU 了*/
		if (unlikely(is_single_threaded(wq)))
			cpu = 0;
		BUG_ON(!list_empty(&work->entry));
		__queue_work(wq->cpu_wq + cpu, work);
		ret = 1;
	}
	put_cpu();
	return ret;
}

/*
内部工具函数,如果一个任务被延迟执行,那么它不会被立即
加入到工作队列中,而是会先放到一个定时器中,当定时器到期
时,会进入该函数,在该函数中,才会放到工作队列中。
*/
static void delayed_work_timer_fn(unsigned long __data)
{
/*从该任务中找出要加入的队列,加进去*/
	struct work_struct *work = (struct work_struct *)__data;
	struct workqueue_struct *wq = work->wq_data;
	int cpu = smp_processor_id();

	if (unlikely(is_single_threaded(wq)))
		cpu = 0;

	__queue_work(wq->cpu_wq + cpu, work);
}

/*
添加一个需要延迟的任务,其实是先加入一个定时
*/
int fastcall queue_delayed_work(struct workqueue_struct *wq,
			struct work_struct *work, unsigned long delay)
{
	int ret = 0;
	struct timer_list *timer = &work->timer;

/*
同样,先看下该任务是否已经被加入了
*/
	if (!test_and_set_bit(0, &work->pending)) {
		BUG_ON(timer_pending(timer));
		BUG_ON(!list_empty(&work->entry));

/* 先不加人工作队列,而是先定时 */
		/* 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(timer);
		ret = 1;
	}
	return ret;
}

/* 内部工具函数,工作队列的线程函数,用来不停的遍历任务并执行 */
static inline void run_workqueue(struct cpu_workqueue_struct *cwq)
{
	unsigned long flags;

	/*
	* Keep taking off work from the queue until
	* done.
	*/
	spin_lock_irqsave(&cwq->lock, flags);/* 禁掉中断,并且锁上该工作队列*/
	cwq->run_depth++;/* 记下递归值,防止太深了 */
	if (cwq->run_depth > 3) {
		/* morton gets to eat his hat */
		printk("%s: recursion depth exceeded: %d\n",
			__FUNCTION__, cwq->run_depth);
		dump_stack();
	}
	/* 如果工作队列上的任务链表不空,就一个一个执行 */
	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;

		/* 将这个任务从工作队列链表中删除 */
		list_del_init(cwq->worklist.next);
		spin_unlock_irqrestore(&cwq->lock, flags);

		/* 注意这里先解锁,然后再加锁。原因是因为
		工作任务中的回调函数有可能会修改该工作队列,比如最简单的,把自己
		重新加入到该队列中,如果不解锁,很可能造成死锁。
		*/
		BUG_ON(work->wq_data != cwq);
		clear_bit(0, &work->pending);
		f(data);/* 执行该工作任务中的回调函数 */

		spin_lock_irqsave(&cwq->lock, flags);
		cwq->remove_sequence++;/* 累加删除计数 */
		wake_up(&cwq->work_done);/* 唤醒工作队列上的完成队列 */
	}
	cwq->run_depth--;
	spin_unlock_irqrestore(&cwq->lock, flags);
}

/* 工作队列的线程函数 */
static int worker_thread(void *__cwq)
{
	struct cpu_workqueue_struct *cwq = __cwq;
	DECLARE_WAITQUEUE(wait, current);
	struct k_sigaction sa;
	sigset_t blocked;

	current->flags |= PF_NOFREEZE;

	set_user_nice(current, -5);

	/* Block and flush all signals */
	sigfillset(&blocked);
	sigprocmask(SIG_BLOCK, &blocked, NULL);
	flush_signals(current);

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

	set_current_state(TASK_INTERRUPTIBLE);
	while (!kthread_should_stop()) {
		add_wait_queue(&cwq->more_work, &wait);
		if (list_empty(&cwq->worklist))
			schedule();
		else
			__set_current_state(TASK_RUNNING);
		remove_wait_queue(&cwq->more_work, &wait);

		if (!list_empty(&cwq->worklist))
			run_workqueue(cwq);
		set_current_state(TASK_INTERRUPTIBLE);
	}
	__set_current_state(TASK_RUNNING);
	return 0;
}


/*
刷新工作队列,就是让工作队列中的工作任务尽可能快的执行完
*/
static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
{
	if (cwq->thread == current) {
		/*
		* Probably keventd trying to flush its own queue. So simply run
		* it by hand rather than deadlocking.
		*/
		/*可以自己刷新自己*/
		run_workqueue(cwq);
	} else {
		DEFINE_WAIT(wait);
		long sequence_needed;

		spin_lock_irq(&cwq->lock);
		sequence_needed = cwq->insert_sequence;

		while (sequence_needed - cwq->remove_sequence > 0) {
			prepare_to_wait(&cwq->work_done, &wait,
					TASK_UNINTERRUPTIBLE);
			spin_unlock_irq(&cwq->lock);
			schedule();/* 不停的让出CPU,好让其他线程可以工作 */
			spin_lock_irq(&cwq->lock);
		}
		finish_wait(&cwq->work_done, &wait);
		spin_unlock_irq(&cwq->lock);
	}
}

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

 /*
刷新工作队列,就是让工作队列中的工作任务尽可能快的执行完
*/
void fastcall flush_workqueue(struct workqueue_struct *wq)
{
	might_sleep();

	if (is_single_threaded(wq)) {
		/* Always use cpu 0's area. */
		flush_cpu_workqueue(wq->cpu_wq + 0);
	} else {
		int cpu;

		lock_cpu_hotplug();
		for_each_online_cpu(cpu)
			flush_cpu_workqueue(wq->cpu_wq + cpu);
		unlock_cpu_hotplug();
	}
}

/* 创建工作队列的线程 */
static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,
						  int cpu)
{
	struct cpu_workqueue_struct *cwq = 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);

	/* 设置下线程的名字 */
	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;
	cwq->thread = p;
	return p;
}

/* 创建一个工作队列 */
struct workqueue_struct *__create_workqueue(const char *name,
					   int singlethread)
{
	int cpu, destroy = 0;
	struct workqueue_struct *wq;
	struct task_struct *p;

	BUG_ON(strlen(name) > 10);

	wq = kmalloc(sizeof(*wq), GFP_KERNEL);
	if (!wq)
		return NULL;
	memset(wq, 0, sizeof(*wq));

	wq->name = name;
	/* We don't need the distraction of CPUs appearing and vanishing. */
	lock_cpu_hotplug();
	if (singlethread) {
		INIT_LIST_HEAD(&wq->list);
		/* 如果是单CPU类型队列,就使用0 */
		p = create_workqueue_thread(wq, 0);
		if (!p)
			destroy = 1;
		else
			wake_up_process(p);
	} else {
		spin_lock(&workqueue_lock);
		list_add(&wq->list, &workqueues);
		spin_unlock(&workqueue_lock);
		for_each_online_cpu(cpu) {
			p = create_workqueue_thread(wq, cpu);
			if (p) {
				kthread_bind(p, cpu);
				wake_up_process(p);
			} else
				destroy = 1;
		}
	}
	unlock_cpu_hotplug();

	/*
	* Was there any error during startup? If yes then clean up:
	*/
	if (destroy) {
		destroy_workqueue(wq);
		wq = NULL;
	}
	return wq;
}

static void cleanup_workqueue_thread(struct workqueue_struct *wq, int cpu)
{
	struct cpu_workqueue_struct *cwq;
	unsigned long flags;
	struct task_struct *p;

	cwq = wq->cpu_wq + cpu;
	spin_lock_irqsave(&cwq->lock, flags);
	p = cwq->thread;
	cwq->thread = NULL;
	spin_unlock_irqrestore(&cwq->lock, flags);
	if (p)
		kthread_stop(p);
}

/* 释放一个工作队列 */
void destroy_workqueue(struct workqueue_struct *wq)
{
	int cpu;

	flush_workqueue(wq);

	/* We don't need the distraction of CPUs appearing and vanishing. */
	lock_cpu_hotplug();
	if (is_single_threaded(wq))
		cleanup_workqueue_thread(wq, 0);
	else {
		for_each_online_cpu(cpu)
			cleanup_workqueue_thread(wq, cpu);
		spin_lock(&workqueue_lock);
		list_del(&wq->list);
		spin_unlock(&workqueue_lock);
	}
	unlock_cpu_hotplug();
	kfree(wq);
}

/*
如果只是简单的一个任务,为了这个任务搞一个新的工作队列太麻烦了,
可以使用内部预配置的一个工作队列
*/
static struct workqueue_struct *keventd_wq;

int fastcall schedule_work(struct work_struct *work)
{
	return queue_work(keventd_wq, work);
}

int fastcall schedule_delayed_work(struct work_struct *work, unsigned long delay)
{
	return queue_delayed_work(keventd_wq, work, delay);
}

int schedule_delayed_work_on(int cpu,
			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 keventd_wq for the moment, for the timer_fn */
		work->wq_data = keventd_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;
}

void flush_scheduled_work(void)
{
	flush_workqueue(keventd_wq);
}

int keventd_up(void)
{
	return keventd_wq != NULL;
}

/*
判断当前上下文是不是在keventd_wq中
*/
int current_is_keventd(void)
{
	struct cpu_workqueue_struct *cwq;
	int cpu = smp_processor_id();	/* preempt-safe: keventd is per-cpu */
	int ret = 0;

	BUG_ON(!keventd_wq);

	cwq = keventd_wq->cpu_wq + cpu;
	if (current == cwq->thread)
		ret = 1;

	return ret;

}


/*
如果CPU支持热插拔,会使用下面的回调函数
*/
#ifdef CONFIG_HOTPLUG_CPU
/* Take the work from this (downed) CPU. */
static void take_over_work(struct workqueue_struct *wq, unsigned int cpu)
{
	struct cpu_workqueue_struct *cwq = wq->cpu_wq + cpu;
	LIST_HEAD(list);
	struct work_struct *work;

	spin_lock_irq(&cwq->lock);
	list_splice_init(&cwq->worklist, &list);

	while (!list_empty(&list)) {
		printk("Taking work for %s\n", wq->name);
		work = list_entry(list.next,struct work_struct,entry);
		list_del(&work->entry);
		__queue_work(wq->cpu_wq + smp_processor_id(), work);
	}
	spin_unlock_irq(&cwq->lock);
}

/* We're holding the cpucontrol mutex here */
static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
				 unsigned long action,
				 void *hcpu)
{
	unsigned int hotcpu = (unsigned long)hcpu;
	struct workqueue_struct *wq;

	switch (action) {
	case CPU_UP_PREPARE:
		/* Create a new workqueue thread for it. */
		list_for_each_entry(wq, &workqueues, list) {
			if (create_workqueue_thread(wq, hotcpu) < 0) {
				printk("workqueue for %i failed\n", hotcpu);
				return NOTIFY_BAD;
			}
		}
		break;

	case CPU_ONLINE:
		/* Kick off worker threads. */
		list_for_each_entry(wq, &workqueues, list) {
			kthread_bind(wq->cpu_wq[hotcpu].thread, hotcpu);
			wake_up_process(wq->cpu_wq[hotcpu].thread);
		}
		break;

	case CPU_UP_CANCELED:
		list_for_each_entry(wq, &workqueues, list) {
			/* Unbind so it can run. */
			kthread_bind(wq->cpu_wq[hotcpu].thread,
				    smp_processor_id());
			cleanup_workqueue_thread(wq, hotcpu);
		}
		break;

	case CPU_DEAD:
		list_for_each_entry(wq, &workqueues, list)
			cleanup_workqueue_thread(wq, hotcpu);
		list_for_each_entry(wq, &workqueues, list)
			take_over_work(wq, hotcpu);
		break;
	}

	return NOTIFY_OK;
}
#endif

void init_workqueues(void)
{
	hotcpu_notifier(workqueue_cpu_callback, 0);
	keventd_wq = create_workqueue("events");
	BUG_ON(!keventd_wq);
}

EXPORT_SYMBOL_GPL(__create_workqueue);
EXPORT_SYMBOL_GPL(queue_work);
EXPORT_SYMBOL_GPL(queue_delayed_work);
EXPORT_SYMBOL_GPL(flush_workqueue);
EXPORT_SYMBOL_GPL(destroy_workqueue);

EXPORT_SYMBOL(schedule_work);
EXPORT_SYMBOL(schedule_delayed_work);
EXPORT_SYMBOL(schedule_delayed_work_on);
EXPORT_SYMBOL(flush_scheduled_work);

 

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