魅族内核团队： Linux Workqueue

由于内核的workqueue变迁一直在发生，而一般的内核书又比较老，跟不上时代。特转发魅族内核团队的这篇文章：
http://kernel.meizu.com/linux-workqueue.html

Workqueue 是内核里面很重要的一个机制，特别是内核驱动，一般的小型任务 (work) 都不会自己起一个线程来处理，而是扔到 Workqueue 中处理。Workqueue 的主要工作就是用进程上下文来处理内核中大量的小任务。

Workqueue 是内核里面很重要的一个机制，特别是内核驱动，一般的小型任务 (work) 都不会自己起一个线程来处理，而是扔到 Workqueue 中处理。Workqueue 的主要工作就是用进程上下文来处理内核中大量的小任务。

本文的代码分析基于 Linux kernel 3.18.22，最好的学习方法还是 “read the fucking source code”

1.CMWQ 的几个基本概念

关于 workqueue 中几个概念都是 work 相关的数据结构非常容易混淆，大概可以这样来理解：

• work ：工作。

• workqueue ：工作的集合。workqueue 和 work 是一对多的关系。

• worker ：工人。在代码中 worker 对应一个 work_thread() 内核线程。

• worker_pool：工人的集合。worker_pool 和 worker 是一对多的关系。

• pwq(pool_workqueue)：中间人 / 中介，负责建立起 workqueue 和 worker_pool 之间的关系。workqueue 和 pwq 是一对多的关系，pwq 和 worker_pool 是一对一的关系。

最终的目的还是把 work( 工作 ) 传递给 worker( 工人 ) 去执行，中间的数据结构和各种关系目的是把这件事组织的更加清晰高效。

1.1 worker_pool

每个执行 work 的线程叫做 worker，一组 worker 的集合叫做 worker_pool。CMWQ 的精髓就在 worker_pool 里面 worker 的动态增减管理上 manage_workers()。

CMWQ 对 worker_pool 分成两类：

• normal worker_pool，给通用的 workqueue 使用；

• unbound worker_pool，给 WQ_UNBOUND 类型的的 workqueue 使用；

1.1.1 normal worker_pool

默认 work 是在 normal worker_pool 中处理的。系统的规划是每个 CPU 创建两个 normal worker_pool：一个 normal 优先级 (nice=0)、一个高优先级 (nice=HIGHPRI_NICE_LEVEL)，对应创建出来的 worker 的进程 nice 不一样。

每个 worker 对应一个 worker_thread() 内核线程，一个 worker_pool 包含一个或者多个 worker，worker_pool 中 worker 的数量是根据 worker_pool 中 work 的负载来动态增减的。

我们可以通过 ps | grep kworker 命令来查看所有 worker 对应的内核线程，normal worker_pool 对应内核线程 (worker_thread()) 的命名规则是这样的：

snprintf(id_buf, sizeof(id_buf), "%d:%d%s", pool->cpu, id,
pool->attrs->nice < 0  ? "H" : "");

worker->task = kthread_create_on_node(worker_thread, worker, pool->node,
     "kworker/%s", id_buf);

so 类似名字是 normal worker_pool：

shell@PRO5:/ $ ps | grep "kworker"
root      14    2     0      0     worker_thr 0000000000 S kworker/1:0H// cpu1 高优先级 worker_pool 的第 0 个 worker 进程
root      17    2     0      0     worker_thr 0000000000 S kworker/2:0// cpu2 低优先级 worker_pool 的第 0 个 worker 进程
root      18    2     0      0     worker_thr 0000000000 S kworker/2:0H// cpu2 高优先级 worker_pool 的第 0 个 worker 进程
root      23699 2     0      0     worker_thr 0000000000 S kworker/0:1// cpu0 低优先级 worker_pool 的第 1 个 worker 进程

对应的拓扑图如下：

以下是 normal worker_pool 详细的创建过程代码分析：

• kernel/workqueue.c:

• init_workqueues() -> init_worker_pool()/create_worker()

static int __init init_workqueues(void)

{

int std_nice[NR_STD_WORKER_POOLS] = { 0, HIGHPRI_NICE_LEVEL };

int i, cpu;

// (1) 给每个 cpu 创建对应的 worker_pool

/* initialize CPU pools */

for_each_possible_cpu(cpu) {

struct worker_pool *pool;

i = 0;

for_each_cpu_worker_pool(pool, cpu) {

BUG_ON(init_worker_pool(pool));

// 指定 cpu

pool->cpu = cpu;

cpumask_copy(pool->attrs->cpumask, cpumask_of(cpu));

// 指定进程优先级 nice

pool->attrs->nice = std_nice[i++];

pool->node = cpu_to_node(cpu);

/* alloc pool ID */

mutex_lock(&wq_pool_mutex);

BUG_ON(worker_pool_assign_id(pool));

mutex_unlock(&wq_pool_mutex);

}

}

// (2) 给每个 worker_pool 创建第一个 worker

/* create the initial worker */

for_each_online_cpu(cpu) {

struct worker_pool *pool;

for_each_cpu_worker_pool(pool, cpu) {

pool->flags &= ~POOL_DISASSOCIATED;

BUG_ON(!create_worker(pool));

}

}

}

| →

static int init_worker_pool(struct worker_pool *pool)

{

spin_lock_init(&pool->lock);

pool->id = -1;

pool->cpu = -1;

pool->node = NUMA_NO_NODE;

pool->flags |= POOL_DISASSOCIATED;

// (1.1) worker_pool 的 work list，各个 workqueue 把 work 挂载到这个链表上，

// 让 worker_pool 对应的多个 worker 来执行

INIT_LIST_HEAD(&pool->worklist);

// (1.2) worker_pool 的 idle worker list，

// worker 没有活干时，不会马上销毁，先进入 idle 状态备选

INIT_LIST_HEAD(&pool->idle_list);

// (1.3) worker_pool 的 busy worker list，

// worker 正在干活，在执行 work

hash_init(pool->busy_hash);

// (1.4) 检查 idle 状态 worker 是否需要 destroy 的 timer

init_timer_deferrable(&pool->idle_timer);

pool->idle_timer.function = idle_worker_timeout;

pool->idle_timer.data = (unsigned long)pool;

// (1.5) 在 worker_pool 创建新的 worker 时，检查是否超时的 timer

setup_timer(&pool->mayday_timer, pool_mayday_timeout,

    (unsigned long)pool);

mutex_init(&pool->manager_arb);

mutex_init(&pool->attach_mutex);

INIT_LIST_HEAD(&pool->workers);

ida_init(&pool->worker_ida);

INIT_HLIST_NODE(&pool->hash_node);

pool->refcnt = 1;

/* shouldn't fail above this point */

pool->attrs = alloc_workqueue_attrs(GFP_KERNEL);

if (!pool->attrs)

return -ENOMEM;

return 0;

}

| →

static struct worker *create_worker(struct worker_pool *pool)

{

struct worker *worker = NULL;

int id = -1;

char id_buf[16];

/* ID is needed to determine kthread name */

id = ida_simple_get(&pool->worker_ida, 0, 0, GFP_KERNEL);

if (id < 0)

goto fail;

worker = alloc_worker(pool->node);

if (!worker)

goto fail;

worker->pool = pool;

worker->id = id;

if (pool->cpu >= 0)

// (2.1) 给 normal worker_pool 的 worker 构造进程名

snprintf(id_buf, sizeof(id_buf), "%d:%d%s", pool->cpu, id,

pool->attrs->nice < 0  ? "H" : "");

else

// (2.2) 给 unbound worker_pool 的 worker 构造进程名

snprintf(id_buf, sizeof(id_buf), "u%d:%d", pool->id, id);

// (2.3) 创建 worker 对应的内核进程

worker->task = kthread_create_on_node(worker_thread, worker, pool->node,

      "kworker/%s", id_buf);

if (IS_ERR(worker->task))

goto fail;

// (2.4) 设置内核进程对应的优先级 nice

set_user_nice(worker->task, pool->attrs->nice);

/* prevent userland from meddling with cpumask of workqueue workers */

worker->task->flags |= PF_NO_SETAFFINITY;

// (2.5) 将 worker 和 worker_pool 绑定

/* successful, attach the worker to the pool */

worker_attach_to_pool(worker, pool);

// (2.6) 将 worker 初始状态设置成 idle，

// wake_up_process 以后，worker 自动 leave idle 状态

/* start the newly created worker */

spin_lock_irq(&pool->lock);

worker->pool->nr_workers++;

worker_enter_idle(worker);

wake_up_process(worker->task);

spin_unlock_irq(&pool->lock);

return worker;

fail:

if (id >= 0)

ida_simple_remove(&pool->worker_ida, id);

kfree(worker);

return NULL;

}

|| →

static void worker_attach_to_pool(struct worker *worker,

   struct worker_pool *pool)

{

mutex_lock(&pool->attach_mutex);

// (2.5.1) 将 worker 线程和 cpu 绑定

/*

* set_cpus_allowed_ptr() will fail if the cpumask doesn't have any

* online CPUs.  It'll be re-applied when any of the CPUs come up.

*/

set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);

/*

* The pool->attach_mutex ensures %POOL_DISASSOCIATED remains

* stable across this function.  See the comments above the

* flag definition for details.

*/

if (pool->flags & POOL_DISASSOCIATED)

worker->flags |= WORKER_UNBOUND;

// (2.5.2) 将 worker 加入 worker_pool 链表

list_add_tail(&worker->node, &pool->workers);

mutex_unlock(&pool->attach_mutex);

}

1.1.2 unbound worker_pool

大部分的 work 都是通过 normal worker_pool 来执行的 ( 例如通过 schedule_work()、schedule_work_on() 压入到系统 workqueue(system_wq) 中的 work)，最后都是通过 normal worker_pool 中的 worker 来执行的。这些 worker 是和某个 CPU 绑定的，work 一旦被 worker 开始执行，都是一直运行到某个 CPU 上的不会切换 CPU。

unbound worker_pool 相对应的意思，就是 worker 可以在多个 CPU 上调度的。但是他其实也是绑定的，只不过它绑定的单位不是 CPU 而是 node。所谓的 node 是对 NUMA(Non Uniform Memory Access Architecture) 系统来说的，NUMA 可能存在多个 node，每个 node 可能包含一个或者多个 CPU。

unbound worker_pool 对应内核线程 (worker_thread()) 的命名规则是这样的：

so 类似名字是 unbound worker_pool：

shell@PRO5:/ $ ps | grep "kworker"

root      23906 2     0      0     worker_thr 0000000000 S kworker/u20:2// unbound pool 20 的第 2 个 worker 进程

root      24564 2     0      0     worker_thr 0000000000 S kworker/u20:0// unbound pool 20 的第 0 个 worker 进程

root      24622 2     0      0     worker_thr 0000000000 S kworker/u21:1// unbound pool 21 的第 1 个 worker 进程

unbound worker_pool 也分成两类：

• unbound_std_wq。每个 node 对应一个 worker_pool，多个 node 就对应多个 worker_pool;

对应的拓扑图如下：

unbound_std_wq topology

• ordered_wq。所有 node 对应一个 default worker_pool；

对应的拓扑图如下：

以下是 unbound worker_pool 详细的创建过程代码分析：

• kernel/workqueue.c:

• init_workqueues() -> unbound_std_wq_attrs/ordered_wq_attrs

• kernel/workqueue.c:

• __alloc_workqueue_key() -> alloc_and_link_pwqs() -> apply_workqueue_attrs() -> alloc_unbound_pwq()/numa_pwq_tbl_install()

struct workqueue_struct *__alloc_workqueue_key(const char *fmt,

       unsigned int flags,

       int max_active,

       struct lock_class_key *key,

       const char *lock_name, ...)

{

size_t tbl_size = 0;

va_list args;

struct workqueue_struct *wq;

struct pool_workqueue *pwq;

/* see the comment above the definition of WQ_POWER_EFFICIENT */

if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient)

flags |= WQ_UNBOUND;

/* allocate wq and format name */

if (flags & WQ_UNBOUND)

tbl_size = nr_node_ids * sizeof(wq->numa_pwq_tbl[0]);

// (1) 分配 workqueue_struct 数据结构

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

if (!wq)

return NULL;

if (flags & WQ_UNBOUND) {

wq->unbound_attrs = alloc_workqueue_attrs(GFP_KERNEL);

if (!wq->unbound_attrs)

goto err_free_wq;

}

va_start(args, lock_name);

vsnprintf(wq->name, sizeof(wq->name), fmt, args);

va_end(args);

// (2) pwq 最多放到 worker_pool 中的 work 数

max_active = max_active ?: WQ_DFL_ACTIVE;

max_active = wq_clamp_max_active(max_active, flags, wq->name);

/* init wq */

wq->flags = flags;

wq->saved_max_active = max_active;

mutex_init(&wq->mutex);

atomic_set(&wq->nr_pwqs_to_flush, 0);

INIT_LIST_HEAD(&wq->pwqs);

INIT_LIST_HEAD(&wq->flusher_queue);

INIT_LIST_HEAD(&wq->flusher_overflow);

INIT_LIST_HEAD(&wq->maydays);

lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);

INIT_LIST_HEAD(&wq->list);

// (3) 给 workqueue 分配对应的 pool_workqueue

// pool_workqueue 将 workqueue 和 worker_pool 链接起来

if (alloc_and_link_pwqs(wq) < 0)

goto err_free_wq;

// (4) 如果是 WQ_MEM_RECLAIM 类型的 workqueue

// 创建对应的 rescuer_thread() 内核进程

/*

* Workqueues which may be used during memory reclaim should

* have a rescuer to guarantee forward progress.

*/

if (flags & WQ_MEM_RECLAIM) {

struct worker *rescuer;

rescuer = alloc_worker(NUMA_NO_NODE);

if (!rescuer)

goto err_destroy;

rescuer->rescue_wq = wq;

rescuer->task = kthread_create(rescuer_thread, rescuer, "%s",

       wq->name);

if (IS_ERR(rescuer->task)) {

kfree(rescuer);

goto err_destroy;

}

wq->rescuer = rescuer;

rescuer->task->flags |= PF_NO_SETAFFINITY;

wake_up_process(rescuer->task);

}

// (5) 如果是需要，创建 workqueue 对应的 sysfs 文件

if ((wq->flags & WQ_SYSFS) && workqueue_sysfs_register(wq))

goto err_destroy;

/*

* wq_pool_mutex protects global freeze state and workqueues list.

* Grab it, adjust max_active and add the new @wq to workqueues

* list.

*/

mutex_lock(&wq_pool_mutex);

mutex_lock(&wq->mutex);

for_each_pwq(pwq, wq)

pwq_adjust_max_active(pwq);

mutex_unlock(&wq->mutex);

// (6) 将新的 workqueue 加入到全局链表 workqueues 中

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

mutex_unlock(&wq_pool_mutex);

return wq;

err_free_wq:

free_workqueue_attrs(wq->unbound_attrs);

kfree(wq);

return NULL;

err_destroy:

destroy_workqueue(wq);

return NULL;

}

| →

static int alloc_and_link_pwqs(struct workqueue_struct *wq)

{

bool highpri = wq->flags & WQ_HIGHPRI;

int cpu, ret;

// (3.1) normal workqueue

// pool_workqueue 链接 workqueue 和 worker_pool 的过程

if (!(wq->flags & WQ_UNBOUND)) {

// 给 workqueue 的每个 cpu 分配对应的 pool_workqueue，赋值给 wq->cpu_pwqs

wq->cpu_pwqs = alloc_percpu(struct pool_workqueue);

if (!wq->cpu_pwqs)

return -ENOMEM;

for_each_possible_cpu(cpu) {

struct pool_workqueue *pwq =

per_cpu_ptr(wq->cpu_pwqs, cpu);

struct worker_pool *cpu_pools =

per_cpu(cpu_worker_pools, cpu);

// 将初始化时已经创建好的 normal worker_pool，赋值给 pool_workqueue

init_pwq(pwq, wq, &cpu_pools[highpri]);

mutex_lock(&wq->mutex);

// 将 pool_workqueue 和 workqueue 链接起来

link_pwq(pwq);

mutex_unlock(&wq->mutex);

}

return 0;

} else if (wq->flags & __WQ_ORDERED) {

// (3.2) unbound ordered_wq workqueue

// pool_workqueue 链接 workqueue 和 worker_pool 的过程

ret = apply_workqueue_attrs(wq, ordered_wq_attrs[highpri]);

/* there should only be single pwq for ordering guarantee */

WARN(!ret && (wq->pwqs.next != &wq->dfl_pwq->pwqs_node ||

      wq->pwqs.prev != &wq->dfl_pwq->pwqs_node),

     "ordering guarantee broken for workqueue %s\n", wq->name);

return ret;

} else {

// (3.3) unbound unbound_std_wq workqueue

// pool_workqueue 链接 workqueue 和 worker_pool 的过程

return apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);

}

}

|| →

int apply_workqueue_attrs(struct workqueue_struct *wq,

  const struct workqueue_attrs *attrs)

{

// (3.2.1) 根据的 ubound 的 ordered_wq_attrs/unbound_std_wq_attrs

// 创建对应的 pool_workqueue 和 worker_pool

// 其中 worker_pool 不是默认创建好的，是需要动态创建的，对应的 worker 内核进程也要重新创建

// 创建好的 pool_workqueue 赋值给 pwq_tbl[node]

/*

* If something goes wrong during CPU up/down, we'll fall back to

* the default pwq covering whole @attrs->cpumask.  Always create

* it even if we don't use it immediately.

*/

dfl_pwq = alloc_unbound_pwq(wq, new_attrs);

if (!dfl_pwq)

goto enomem_pwq;

for_each_node(node) {

if (wq_calc_node_cpumask(attrs, node, -1, tmp_attrs->cpumask)) {

pwq_tbl[node] = alloc_unbound_pwq(wq, tmp_attrs);

if (!pwq_tbl[node])

goto enomem_pwq;

} else {

dfl_pwq->refcnt++;

pwq_tbl[node] = dfl_pwq;

}

}

/* save the previous pwq and install the new one */

// (3.2.2) 将临时 pwq_tbl[node] 赋值给 wq->numa_pwq_tbl[node]

for_each_node(node)

pwq_tbl[node] = numa_pwq_tbl_install(wq, node, pwq_tbl[node]);

}

||| →

static struct pool_workqueue *alloc_unbound_pwq(struct workqueue_struct *wq,

const struct workqueue_attrs *attrs)

{

struct worker_pool *pool;

struct pool_workqueue *pwq;

lockdep_assert_held(&wq_pool_mutex);

// (3.2.1.1) 如果对应 attrs 已经创建多对应的 unbound_pool，则使用已有的 unbound_pool

// 否则根据 attrs 创建新的 unbound_pool

pool = get_unbound_pool(attrs);

if (!pool)

return NULL;

pwq = kmem_cache_alloc_node(pwq_cache, GFP_KERNEL, pool->node);

if (!pwq) {

put_unbound_pool(pool);

return NULL;

}

init_pwq(pwq, wq, pool);

return pwq;

}

1.2 worker

每个 worker 对应一个 worker_thread() 内核线程，一个 worker_pool 对应一个或者多个 worker。多个 worker 从同一个链表中 worker_pool->worklist 获取 work 进行处理。

所以这其中有几个重点：

• worker 怎么处理 work；

• worker_pool 怎么动态管理 worker 的数量；

1.2.1 worker 处理 work

处理 work 的过程主要在 worker_thread() -> process_one_work() 中处理，我们具体看看代码的实现过程。

• kernel/workqueue.c:

• worker_thread() -> process_one_work()

static int worker_thread(void *__worker){
struct worker *worker = __worker;
struct worker_pool *pool = worker->pool;

/* tell the scheduler that this is a workqueue worker */
worker->task->flags |= PF_WQ_WORKER;woke_up:
spin_lock_irq(&pool->lock);

// (1) 是否 die/* am I supposed to die? */
if (unlikely(worker->flags & WORKER_DIE)) {
spin_unlock_irq(&pool->lock);
WARN_ON_ONCE(!list_empty(&worker->entry));
worker->task->flags &= ~PF_WQ_WORKER;

set_task_comm(worker->task, "kworker/dying");
ida_simple_remove(&pool->worker_ida, worker->id);
worker_detach_from_pool(worker, pool);
kfree(worker);
return 0;
}

// (2) 脱离 idle 状态// 被唤醒之前 worker 都是 idle 状态worker_leave_idle(worker);recheck:

// (3) 如果需要本 worker 继续执行则继续，否则进入 idle 状态// need more worker 的条件： (pool->worklist != 0) && (pool->nr_running == 0)// worklist 上有 work 需要执行，并且现在没有处于 running 的 work/* no more worker necessary? */
if (!need_more_worker(pool))
goto sleep;

// (4) 如果 (pool->nr_idle == 0)，则启动创建更多的 worker// 说明 idle 队列中已经没有备用 worker 了，先创建 一些 worker 备用/* do we need to manage? */
if (unlikely(!may_start_working(pool)) && manage_workers(worker))
goto recheck;

/*
* ->scheduled list can only be filled while a worker is
* preparing to process a work or actually processing it.
* Make sure nobody diddled with it while I was sleeping.
*/
WARN_ON_ONCE(!list_empty(&worker->scheduled));

/*
* Finish PREP stage.  We're guaranteed to have at least one idle
* worker or that someone else has already assumed the manager
* role.  This is where @worker starts participating in concurrency
* management if applicable and concurrency management is restored
* after being rebound.  See rebind_workers() for details.
*/
worker_clr_flags(worker, WORKER_PREP | WORKER_REBOUND);

do {
// (5) 如果 pool->worklist 不为空，从其中取出一个 work 进行处理struct work_struct *work =
list_first_entry(&pool->worklist,
struct work_struct, entry);

if (likely(!(*work_data_bits(work) & WORK_STRUCT_LINKED))) {
/* optimization path, not strictly necessary */
// (6) 执行正常的 workprocess_one_work(worker, work);
if (unlikely(!list_empty(&worker->scheduled)))
process_scheduled_works(worker);
} else {
// (7) 执行系统特意 scheduled 给某个 worker 的 work// 普通的 work 是放在池子的公共 list 中的 pool->worklist// 只有一些特殊的 work 被特意派送给某个 worker 的 worker->scheduled// 包括：1、执行 flush_work 时插入的 barrier work；// 2、collision 时从其他 worker 推送到本 worker 的 workmove_linked_works(work, &worker->scheduled, NULL);
process_scheduled_works(worker);
}
// (8) worker keep_working 的条件：// pool->worklist 不为空 && (pool->nr_running <= 1)} while (keep_working(pool));

worker_set_flags(worker, WORKER_PREP);supposedsleep:
// (9) worker 进入 idle 状态/*
* pool->lock is held and there's no work to process and no need to
* manage, sleep.  Workers are woken up only while holding
* pool->lock or from local cpu, so setting the current state
* before releasing pool->lock is enough to prevent losing any
* event.
*/
worker_enter_idle(worker);
__set_current_state(TASK_INTERRUPTIBLE);
spin_unlock_irq(&pool->lock);
schedule();
goto woke_up;}| →static void process_one_work(struct worker *worker, struct work_struct *work)__releases(&pool->lock)__acquires(&pool->lock){
struct pool_workqueue *pwq = get_work_pwq(work);
struct worker_pool *pool = worker->pool;
bool cpu_intensive = pwq->wq->flags & WQ_CPU_INTENSIVE;
int work_color;
struct worker *collision;#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;

lockdep_copy_map(&lockdep_map, &work->lockdep_map);#endif/* ensure we're on the correct CPU */
WARN_ON_ONCE(!(pool->flags & POOL_DISASSOCIATED) &&
    raw_smp_processor_id() != pool->cpu);

// (8.1) 如果 work 已经在 worker_pool 的其他 worker 上执行，// 将 work 放入对应 worker 的 scheduled 队列中延后执行/*
* A single work shouldn't be executed concurrently by
* multiple workers on a single cpu.  Check whether anyone is
* already processing the work.  If so, defer the work to the
* currently executing one.
*/
collision = find_worker_executing_work(pool, work);
if (unlikely(collision)) {
move_linked_works(work, &collision->scheduled, NULL);
return;
}

// (8.2) 将 worker 加入 busy 队列 pool->busy_hash/* claim and dequeue */
debug_work_deactivate(work);
hash_add(pool->busy_hash, &worker->hentry, (unsigned long)work);
worker->current_work = work;
worker->current_func = work->func;
worker->current_pwq = pwq;
work_color = get_work_color(work);

list_del_init(&work->entry);

// (8.3) 如果 work 所在的 wq 是 cpu 密集型的 WQ_CPU_INTENSIVE// 则当前 work 的执行脱离 worker_pool 的动态调度，成为一个独立的线程/*
* CPU intensive works don't participate in concurrency management.
* They're the scheduler's responsibility.  This takes @worker out
* of concurrency management and the next code block will chain
* execution of the pending work items.
*/
if (unlikely(cpu_intensive))
worker_set_flags(worker, WORKER_CPU_INTENSIVE);

// (8.4) 在 UNBOUND 或者 CPU_INTENSIVE work 中判断是否需要唤醒 idle worker// 普通 work 不会执行这个操作/*
* Wake up another worker if necessary.  The condition is always
* false for normal per-cpu workers since nr_running would always
* be >= 1 at this point.  This is used to chain execution of the
* pending work items for WORKER_NOT_RUNNING workers such as the
* UNBOUND and CPU_INTENSIVE ones.
*/
if (need_more_worker(pool))
wake_up_worker(pool);

/*
* Record the last pool and clear PENDING which should be the last
* update to @work.  Also, do this inside @pool->lock so that
* PENDING and queued state changes happen together while IRQ is
* disabled.
*/
set_work_pool_and_clear_pending(work, pool->id);

spin_unlock_irq(&pool->lock);

lock_map_acquire_read(&pwq->wq->lockdep_map);
lock_map_acquire(&lockdep_map);
trace_workqueue_execute_start(work);
// (8.5) 执行 work 函数worker->current_func(work);
/*
* While we must be careful to not use "work" after this, the trace
* point will only record its address.
*/
trace_workqueue_execute_end(work);
lock_map_release(&lockdep_map);
lock_map_release(&pwq->wq->lockdep_map);

if (unlikely(in_atomic() || lockdep_depth(current) > 0)) {
pr_err("BUG: workqueue leaked lock or atomic: %s/0x%08x/%d\n"
      "     last function: %pf\n",
      current->comm, preempt_count(), task_pid_nr(current),
      worker->current_func);
debug_show_held_locks(current);
dump_stack();
}

/*
* The following prevents a kworker from hogging CPU on !PREEMPT
* kernels, where a requeueing work item waiting for something to
* happen could deadlock with stop_machine as such work item could
* indefinitely requeue itself while all other CPUs are trapped in
* stop_machine. At the same time, report a quiescent RCU state so
* the same condition doesn't freeze RCU.
*/
cond_resched_rcu_qs();

spin_lock_irq(&pool->lock);

/* clear cpu intensive status */
if (unlikely(cpu_intensive))
worker_clr_flags(worker, WORKER_CPU_INTENSIVE);

/* we're done with it, release */
hash_del(&worker->hentry);
worker->current_work = NULL;
worker->current_func = NULL;
worker->current_pwq = NULL;
worker->desc_valid = false;
pwq_dec_nr_in_flight(pwq, work_color);

}

1.2.2 worker_pool 动态管理 worker

worker_pool 怎么来动态增减 worker，这部分的算法是 CMWQ 的核心。其思想如下：

• worker_pool 中的 worker 有 3 种状态：idle、running、suspend；

• 如果 worker_pool 中有 work 需要处理，保持至少一个 running worker 来处理；

• running worker 在处理 work 的过程中进入了阻塞 suspend 状态，为了保持其他 work 的执行，需要唤醒新的 idle worker 来处理 work；

• 如果有 work 需要执行且 running worker 大于 1 个，会让多余的 running worker 进入 idle 状态；

• 如果没有 work 需要执行，会让所有 worker 进入 idle 状态；

• 如果创建的 worker 过多，destroy_worker 在 300s(IDLE_WORKER_TIMEOUT) 时间内没有再次运行的 idle worker。

详细代码可以参考上节 worker_thread() -> process_one_work() 的分析。

为了追踪 worker 的 running 和 suspend 状态，用来动态调整 worker 的数量。wq 使用在进程调度中加钩子函数的技巧：

• 追踪 worker 从 suspend 进入 running 状态：ttwu_activate() -> wq_worker_waking_up()

• 追踪 worker 从 running 进入 suspend 状态：__schedule() -> wq_worker_sleeping()

struct task_struct *wq_worker_sleeping(struct task_struct *task, int cpu){
struct worker *worker = kthread_data(task), *to_wakeup = NULL;
struct worker_pool *pool;

/*
* Rescuers, which may not have all the fields set up like normal
* workers, also reach here, let's not access anything before
* checking NOT_RUNNING.
*/
if (worker->flags & WORKER_NOT_RUNNING)
return NULL;

pool = worker->pool;

/* this can only happen on the local cpu */
if (WARN_ON_ONCE(cpu != raw_smp_processor_id() || pool->cpu != cpu))
return NULL;

/*
* The counterpart of the following dec_and_test, implied mb,
* worklist not empty test sequence is in insert_work().
* Please read comment there.
*
* NOT_RUNNING is clear.  This means that we're bound to and
* running on the local cpu w/ rq lock held and preemption
* disabled, which in turn means that none else could be
* manipulating idle_list, so dereferencing idle_list without pool
* lock is safe.
*/
// 减少 worker_pool 中 running 的 worker 数量// 如果 worklist 还有 work 需要处理，唤醒第一个 idle worker 进行处理if (atomic_dec_and_test(&pool->nr_running) &&
   !list_empty(&pool->worklist))
to_wakeup = first_idle_worker(pool);
return to_wakeup ? to_wakeup->task : NULL;

}

这里 worker_pool 的调度思想是：如果有 work 需要处理，保持一个 running 状态的 worker 处理，不多也不少。

但是这里有一个问题如果 work 是 CPU 密集型的，它虽然也没有进入 suspend 状态，但是会长时间的占用 CPU，让后续的 work 阻塞太长时间。

为了解决这个问题，CMWQ 设计了 WQ_CPU_INTENSIVE，如果一个 wq 声明自己是 CPU_INTENSIVE，则让当前 worker 脱离动态调度，像是进入了 suspend 状态，那么 CMWQ 会创建新的 worker，后续的 work 会得到执行。

• kernel/workqueue.c:

• worker_thread() -> process_one_work()

static void process_one_work(struct worker *worker, struct work_struct *work)__releases(&pool->lock)__acquires(&pool->lock){

bool cpu_intensive = pwq->wq->flags & WQ_CPU_INTENSIVE;

// (1) 设置当前 worker 的 WORKER_CPU_INTENSIVE 标志// nr_running 会被减 1// 对 worker_pool 来说，当前 worker 相当于进入了 suspend 状态/*
* CPU intensive works don't participate in concurrency management.
* They're the scheduler's responsibility.  This takes @worker out
* of concurrency management and the next code block will chain
* execution of the pending work items.
*/
if (unlikely(cpu_intensive))
worker_set_flags(worker, WORKER_CPU_INTENSIVE);

// (2) 接上一步，判断是否需要唤醒新的 worker 来处理 work/*
* Wake up another worker if necessary.  The condition is always
* false for normal per-cpu workers since nr_running would always
* be >= 1 at this point.  This is used to chain execution of the
* pending work items for WORKER_NOT_RUNNING workers such as the
* UNBOUND and CPU_INTENSIVE ones.
*/
if (need_more_worker(pool))
wake_up_worker(pool);

// (3) 执行 workworker->current_func(work);

// (4) 执行完，清理当前 worker 的 WORKER_CPU_INTENSIVE 标志// 当前 worker 重新进入 running 状态/* clear cpu intensive status */
if (unlikely(cpu_intensive))
worker_clr_flags(worker, WORKER_CPU_INTENSIVE);}

WORKER_NOT_RUNNING= WORKER_PREP | WORKER_CPU_INTENSIVE |
 WORKER_UNBOUND | WORKER_REBOUND,static inline void worker_set_flags(struct worker *worker, unsigned int flags){
struct worker_pool *pool = worker->pool;

WARN_ON_ONCE(worker->task != current);

/* If transitioning into NOT_RUNNING, adjust nr_running. */
if ((flags & WORKER_NOT_RUNNING) &&
   !(worker->flags & WORKER_NOT_RUNNING)) {
atomic_dec(&pool->nr_running);
}

worker->flags |= flags;}static inline void worker_clr_flags(struct worker *worker, unsigned int flags){
struct worker_pool *pool = worker->pool;
unsigned int oflags = worker->flags;

WARN_ON_ONCE(worker->task != current);

worker->flags &= ~flags;

/*
* If transitioning out of NOT_RUNNING, increment nr_running.  Note
* that the nested NOT_RUNNING is not a noop.  NOT_RUNNING is mask
* of multiple flags, not a single flag.
*/
if ((flags & WORKER_NOT_RUNNING) && (oflags & WORKER_NOT_RUNNING))
if (!(worker->flags & WORKER_NOT_RUNNING))
atomic_inc(&pool->nr_running);

}

1.2.3 CPU hotplug 处理

从上几节可以看到，系统会创建和 CPU 绑定的 normal worker_pool 和不绑定 CPU 的 unbound worker_pool，worker_pool 又会动态的创建 worker。

那么在 CPU hotplug 的时候，会怎么样动态的处理 worker_pool 和 worker 呢？来看具体的代码分析：

• kernel/workqueue.c:

• workqueue_cpu_up_callback()/workqueue_cpu_down_callback()

static int __init init_workqueues(void)

{

cpu_notifier(workqueue_cpu_up_callback, CPU_PRI_WORKQUEUE_UP);
hotcpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);}| →static int workqueue_cpu_down_callback(struct notifier_block *nfb,
unsigned long action,
void *hcpu){
int cpu = (unsigned long)hcpu;
struct work_struct unbind_work;
struct workqueue_struct *wq;

switch (action & ~CPU_TASKS_FROZEN) {
case CPU_DOWN_PREPARE:
/* unbinding per-cpu workers should happen on the local CPU */
INIT_WORK_ONSTACK(&unbind_work, wq_unbind_fn);
// (1) cpu down_prepare// 把和当前 cpu 绑定的 normal worker_pool 上的 worker 停工// 随着当前 cpu 被 down 掉，这些 worker 会迁移到其他 cpu 上queue_work_on(cpu, system_highpri_wq, &unbind_work);

// (2) unbound wq 对 cpu 变化的更新/* update NUMA affinity of unbound workqueues */
mutex_lock(&wq_pool_mutex);
list_for_each_entry(wq, &workqueues, list)
wq_update_unbound_numa(wq, cpu, false);
mutex_unlock(&wq_pool_mutex);

/* wait for per-cpu unbinding to finish */
flush_work(&unbind_work);
destroy_work_on_stack(&unbind_work);
break;
}
return NOTIFY_OK;}| →static int workqueue_cpu_up_callback(struct notifier_block *nfb,
unsigned long action, void *hcpu){
int CPU = (unsigned long)hcpu;
struct worker_pool *pool;
struct workqueue_struct *wq;
int pi;
switch (action & ~CPU_TASKS_FROZEN) {
case CPU_UP_PREPARE:
for_each_cpu_worker_pool(pool, CPU) {
if (pool->nr_workers)
continue;
if (!create_worker(pool))
return NOTIFY_BAD;
}
break;
case CPU_DOWN_FAILED:
case CPU_ONLINE:
mutex_lock(&wq_pool_mutex);
// (3) CPU upfor_each_pool(pool, pi) {
mutex_lock(&pool->attach_mutex);
// 如果和当前 CPU 绑定的 normal worker_pool 上，有 WORKER_UNBOUND 停工的 worker// 重新绑定 worker 到 worker_pool// 让这些 worker 开工，并绑定到当前 CPUif (pool->CPU == CPU)
rebind_workers(pool);
else if (pool->CPU < 0)
restore_unbound_workers_cpumask(pool, CPU);
mutex_unlock(&pool->attach_mutex);
}

/* update NUMA affinity of unbound workqueues */
list_for_each_entry(wq, &workqueues, list)
wq_update_unbound_numa(wq, CPU, true);
mutex_unlock(&wq_pool_mutex);
break;
}
return NOTIFY_OK;

}

1.3 workqueue

workqueue 就是存放一组 work 的集合，基本可以分为两类：一类系统创建的 workqueue，一类是用户自己创建的 workqueue。

不论是系统还是用户的 workqueue，如果没有指定 WQ_UNBOUND，默认都是和 normal worker_pool 绑定。

1.3.1 系统 workqueue

系统在初始化时创建了一批默认的 workqueue：system_wq、system_highpri_wq、system_long_wq、system_unbound_wq、system_freezable_wq、system_power_efficient_wq、system_freezable_power_efficient_wq。

像 system_wq，就是 schedule_work() 默认使用的。

• kernel/workqueue.c:

• init_workqueues()

static int __init init_workqueues(void)
{
system_wq = alloc_workqueue("events", 0, 0);
system_highpri_wq = alloc_workqueue("events_highpri", WQ_HIGHPRI, 0);
system_long_wq = alloc_workqueue("events_long", 0, 0);
system_unbound_wq = alloc_workqueue("events_unbound", WQ_UNBOUND,
   WQ_UNBOUND_MAX_ACTIVE);
system_freezable_wq = alloc_workqueue("events_freezable",
     WQ_FREEZABLE, 0);
system_power_efficient_wq = alloc_workqueue("events_power_efficient",
     WQ_POWER_EFFICIENT, 0);

system_freezable_power_efficient_wq = alloc_workqueue("events_freezable_power_efficient",
     WQ_FREEZABLE | WQ_POWER_EFFICIENT,
     0);
}

1.3.2 workqueue 创建

详细过程见上几节的代码分析：alloc_workqueue() -> __alloc_workqueue_key() -> alloc_and_link_pwqs()。

1.3.3 flush_workqueue()

这一部分的逻辑，wq->work_color、wq->flush_color 换来换去的逻辑实在看的头晕。看不懂暂时不想看，放着以后看吧，或者有谁看懂了教我一下。：）

1.4 pool_workqueue

pool_workqueue 只是一个中介角色。

详细过程见上几节的代码分析：alloc_workqueue() -> __alloc_workqueue_key() -> alloc_and_link_pwqs()。

1.5 work

描述一份待执行的工作。

1.5.1 queue_work()

将 work 压入到 workqueue 当中。

• kernel/workqueue.c:

• queue_work() -> queue_work_on() -> __queue_work()

static void __queue_work(int cpu, struct workqueue_struct *wq,
struct work_struct *work){
struct pool_workqueue *pwq;
struct worker_pool *last_pool;
struct list_head *worklist;
unsigned int work_flags;
unsigned int req_cpu = cpu;

/*
* While a work item is PENDING && off queue, a task trying to
* steal the PENDING will busy-loop waiting for it to either get
* queued or lose PENDING.  Grabbing PENDING and queueing should
* happen with IRQ disabled.
*/
WARN_ON_ONCE(!irqs_disabled());

debug_work_activate(work);

/* if draining, only works from the same workqueue are allowed */
if (unlikely(wq->flags & __WQ_DRAINING) &&
   WARN_ON_ONCE(!is_chained_work(wq)))
return;retry:
// (1) 如果没有指定 cpu，则使用当前 cpuif (req_cpu == WORK_CPU_UNBOUND)
cpu = raw_smp_processor_id();

/* pwq which will be used unless @work is executing elsewhere */
if (!(wq->flags & WQ_UNBOUND))
// (2) 对于 normal wq，使用当前 cpu 对应的 normal worker_poolpwq = per_cpu_ptr(wq->cpu_pwqs, cpu);
else
// (3) 对于 unbound wq，使用当前 cpu 对应 node 的 worker_poolpwq = unbound_pwq_by_node(wq, cpu_to_node(cpu));

// (4) 如果 work 在其他 worker 上正在被执行，把 work 压到对应的 worker 上去// 避免 work 出现重入的问题/*
* If @work was previously on a different pool, it might still be
* running there, in which case the work needs to be queued on that
* pool to guarantee non-reentrancy.
*/
last_pool = get_work_pool(work);
if (last_pool && last_pool != pwq->pool) {
struct worker *worker;

spin_lock(&last_pool->lock);

worker = find_worker_executing_work(last_pool, work);

if (worker && worker->current_pwq->wq == wq) {
pwq = worker->current_pwq;
} else {
/* meh... not running there, queue here */
spin_unlock(&last_pool->lock);
spin_lock(&pwq->pool->lock);
}
} else {
spin_lock(&pwq->pool->lock);
}

/*
* pwq is determined and locked.  For unbound pools, we could have
* raced with pwq release and it could already be dead.  If its
* refcnt is zero, repeat pwq selection.  Note that pwqs never die
* without another pwq replacing it in the numa_pwq_tbl or while
* work items are executing on it, so the retrying is guaranteed to
* make forward-progress.
*/
if (unlikely(!pwq->refcnt)) {
if (wq->flags & WQ_UNBOUND) {
spin_unlock(&pwq->pool->lock);
cpu_relax();
goto retry;
}
/* oops */
WARN_ONCE(true, "workqueue: per-cpu pwq for %s on cpu%d has 0 refcnt",
 wq->name, cpu);
}

/* pwq determined, queue */
trace_workqueue_queue_work(req_cpu, pwq, work);

if (WARN_ON(!list_empty(&work->entry))) {
spin_unlock(&pwq->pool->lock);
return;
}

pwq->nr_in_flight[pwq->work_color]++;
work_flags = work_color_to_flags(pwq->work_color);

// (5) 如果还没有达到 max_active，将 work 挂载到 pool->worklistif (likely(pwq->nr_active < pwq->max_active)) {
trace_workqueue_activate_work(work);
pwq->nr_active++;
worklist = &pwq->pool->worklist;
// 否则，将 work 挂载到临时队列 pwq->delayed_works} else {
work_flags |= WORK_STRUCT_DELAYED;
worklist = &pwq->delayed_works;
}

// (6) 将 work 压入 worklist 当中insert_work(pwq, work, worklist, work_flags);

spin_unlock(&pwq->pool->lock);

}

1.5.2 flush_work()

flush 某个 work，确保 work 执行完成。

怎么判断异步的 work 已经执行完成？这里面使用了一个技巧：在目标 work 的后面插入一个新的 work wq_barrier，如果 wq_barrier 执行完成，那么目标 work 肯定已经执行完成。

• kernel/workqueue.c:

• queue_work() -> queue_work_on() -> __queue_work()

/**
* flush_work - wait for a work to finish executing the last queueing instance
* @work: the work to flush
*
* Wait until @work has finished execution.  @work is guaranteed to be idle
* on return if it hasn't been requeued since flush started.
*
* Return:
* %true if flush_work() waited for the work to finish execution,
* %false if it was already idle.
*/bool flush_work(struct work_struct *work){
struct wq_barrier barr;

lock_map_acquire(&work->lockdep_map);
lock_map_release(&work->lockdep_map);

if (start_flush_work(work, &barr)) {
// 等待 barr work 执行完成的信号wait_for_completion(&barr.done);
destroy_work_on_stack(&barr.work);
return true;
} else {
return false;
}}| →static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr){
struct worker *worker = NULL;
struct worker_pool *pool;
struct pool_workqueue *pwq;

might_sleep();

// (1) 如果 work 所在 worker_pool 为 NULL，说明 work 已经执行完local_irq_disable();
pool = get_work_pool(work);
if (!pool) {
local_irq_enable();
return false;
}

spin_lock(&pool->lock);
/* see the comment in try_to_grab_pending() with the same code */
pwq = get_work_pwq(work);
if (pwq) {
// (2) 如果 work 所在 pwq 指向的 worker_pool 不等于上一步得到的 worker_pool，说明 work 已经执行完if (unlikely(pwq->pool != pool))
goto already_gone;
} else {
// (3) 如果 work 所在 pwq 为 NULL，并且也没有在当前执行的 work 中，说明 work 已经执行完worker = find_worker_executing_work(pool, work);
if (!worker)
goto already_gone;
pwq = worker->current_pwq;
}

// (4) 如果 work 没有执行完，向 work 的后面插入 barr workinsert_wq_barrier(pwq, barr, work, worker);
spin_unlock_irq(&pool->lock);

/*
* If @max_active is 1 or rescuer is in use, flushing another work
* item on the same workqueue may lead to deadlock.  Make sure the
* flusher is not running on the same workqueue by verifying write
* access.
*/
if (pwq->wq->saved_max_active == 1 || pwq->wq->rescuer)
lock_map_acquire(&pwq->wq->lockdep_map);
else
lock_map_acquire_read(&pwq->wq->lockdep_map);
lock_map_release(&pwq->wq->lockdep_map);

return true;already_gone:
spin_unlock_irq(&pool->lock);
return false;

}

|| →

static void insert_wq_barrier(struct pool_workqueue *pwq,
     struct wq_barrier *barr,
     struct work_struct *target, struct worker *worker){
struct list_head *head;
unsigned int linked = 0;

/*
* debugobject calls are safe here even with pool->lock locked
* as we know for sure that this will not trigger any of the
* checks and call back into the fixup functions where we
* might deadlock.
*/
// (4.1) barr work 的执行函数 wq_barrier_func()INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
__set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
init_completion(&barr->done);

/*
* If @target is currently being executed, schedule the
* barrier to the worker; otherwise, put it after @target.
*/
// (4.2) 如果 work 当前在 worker 中执行，则 barr work 插入 scheduled 队列if (worker)
head = worker->scheduled.next;
// 否则，则 barr work 插入正常的 worklist 队列中，插入位置在目标 work 后面// 并且置上 WORK_STRUCT_LINKED 标志else {
unsigned long *bits = work_data_bits(target);

head = target->entry.next;
/* there can already be other linked works, inherit and set */
linked = *bits & WORK_STRUCT_LINKED;
__set_bit(WORK_STRUCT_LINKED_BIT, bits);
}

debug_work_activate(&barr->work);
insert_work(pwq, &barr->work, head,
   work_color_to_flags(WORK_NO_COLOR) | linked);

}

||| →

static void wq_barrier_func(struct work_struct *work){
struct wq_barrier *barr = container_of(work, struct wq_barrier, work);
// (4.1.1) barr work 执行完成，发出 complete 信号。complete(&barr->done);

}

2.Workqueue 对外接口函数

CMWQ 实现的 workqueue 机制，被包装成相应的对外接口函数。

2.1 schedule_work()

把 work 压入系统默认 wq system_wq，WORK_CPU_UNBOUND 指定 worker 为当前 CPU 绑定的 normal worker_pool 创建的 worker。

• kernel/workqueue.c:

• schedule_work() -> queue_work_on() -> __queue_work()

2.2 schedule_work_on()

在 schedule_work() 基础上，可以指定 work 运行的 CPU。

• kernel/workqueue.c:

• schedule_work_on() -> queue_work_on() -> __queue_work()

2.3 schedule_delayed_work()

启动一个 timer，在 timer 定时到了以后调用 delayed_work_timer_fn() 把 work 压入系统默认 wq system_wq。

• kernel/workqueue.c:

• schedule_work_on() -> queue_work_on() -> __queue_work()

static inline bool schedule_delayed_work(struct delayed_work *dwork,
unsigned long delay)

{
return queue_delayed_work(system_wq, dwork, delay);

}

| →

static inline bool queue_delayed_work(struct workqueue_struct *wq,
     struct delayed_work *dwork,
     unsigned long delay)

{
return queue_delayed_work_on(WORK_CPU_UNBOUND, wq, dwork, delay);}|| →bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
  struct delayed_work *dwork, unsigned long delay){
struct work_struct *work = &dwork->work;
bool ret = false;
unsigned long flags;

/* read the comment in __queue_work() */
local_irq_save(flags);

if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
__queue_delayed_work(cpu, wq, dwork, delay);
ret = true;
}

local_irq_restore(flags);
return ret;

}

||| →

static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
struct delayed_work *dwork, unsigned long delay){
struct timer_list *timer = &dwork->timer;
struct work_struct *work = &dwork->work;

WARN_ON_ONCE(timer->function != delayed_work_timer_fn ||
    timer->data != (unsigned long)dwork);
WARN_ON_ONCE(timer_pending(timer));
WARN_ON_ONCE(!list_empty(&work->entry));

/*
* If @delay is 0, queue @dwork->work immediately.  This is for
* both optimization and correctness.  The earliest @timer can
* expire is on the closest next tick and delayed_work users depend
* on that there's no such delay when @delay is 0.
*/
if (!delay) {
__queue_work(cpu, wq, &dwork->work);
return;
}

timer_stats_timer_set_start_info(&dwork->timer);

dwork->wq = wq;
dwork->cpu = cpu;
timer->expires = jiffies + delay;

if (unlikely(cpu != WORK_CPU_UNBOUND))
add_timer_on(timer, cpu);
else
add_timer(timer);

}

|||| →

void delayed_work_timer_fn(unsigned long __data)

{
struct delayed_work *dwork = (struct delayed_work *)__data;

/* should have been called from irqsafe timer with irq already off */
__queue_work(dwork->cpu, dwork->wq, &dwork->work);

}

参考资料

1. Documentation/workqueue.txt