Linux Workqueue——魅族内核大神文章

http://kernel.meizu.com/linux-workqueue.html

21 August 2016

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。

所以 Workqueue 的主要设计思想：一个是并行，多个 work 不要相互阻塞；另外一个是节省资源，多个 work 尽量共享资源 ( 进程、调度、内存 )，不要造成系统过多的资源浪费。

为了实现的设计思想，workqueue 的设计实现也更新了很多版本。最新的 workqueue 实现叫做 CMWQ(Concurrency Managed Workqueue)，也就是用更加智能的算法来实现“并行和节省”。新版本的 workqueue 创建函数改成alloc_workqueue()，旧版本的函数 create_workqueue() 逐渐会被被废弃。

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。

normal wq_topology

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

normal worker_pool

对应的拓扑图如下：

normal worker_pool topology

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 cpupool->cpu = cpu;cpumask_copy(pool->attrs->cpumask, cpumask_of(cpu));// 指定进程优先级 nicepool->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 workhash_init(pool->busy_hash);// (1.4) 检查 idle 状态 worker 是否需要 destroy 的 timerinit_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 timersetup_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 niceset_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：

snprintf(id_buf, sizeof(id_buf), "u%d:%d", pool->id, id);worker->task = kthread_create_on_node(worker_thread, worker, pool->node,      "kworker/%s", id_buf);

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;

unbound worker_pool: unbound_std_wq

对应的拓扑图如下：

unbound_std_wq topology

• ordered_wq。所有 node 对应一个 default worker_pool；

unbound worker_pool: ordered_wq

对应的拓扑图如下：

ordered_wq topology

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

static int __init init_workqueues(void){// (1) 初始化 normal 和 high nice 对应的 unbound attrs/* create default unbound and ordered wq attrs */for (i = 0; i < NR_STD_WORKER_POOLS; i++) {struct workqueue_attrs *attrs;// (2) unbound_std_wq_attrsBUG_ON(!(attrs = alloc_workqueue_attrs(GFP_KERNEL)));attrs->nice = std_nice[i];unbound_std_wq_attrs[i] = attrs;/* * An ordered wq should have only one pwq as ordering is * guaranteed by max_active which is enforced by pwqs. * Turn off NUMA so that dfl_pwq is used for all nodes. */// (3) ordered_wq_attrs，no_numa = true;BUG_ON(!(attrs = alloc_workqueue_attrs(GFP_KERNEL)));attrs->nice = std_nice[i];attrs->no_numa = true;ordered_wq_attrs[i] = 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_pwqswq->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_workqueueinit_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_poolpool = 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。

worker status machine

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()

void wq_worker_waking_up(struct task_struct *task, int cpu){struct worker *worker = kthread_data(task);if (!(worker->flags & WORKER_NOT_RUNNING)) {WARN_ON_ONCE(worker->pool->cpu != cpu);// 增加 worker_pool 中 running 的 worker 数量atomic_inc(&worker->pool->nr_running);}}

• 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);elselock_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()

static inline bool schedule_work(struct work_struct *work){return queue_work(system_wq, work);}| →static inline bool queue_work(struct workqueue_struct *wq,      struct work_struct *work){return queue_work_on(WORK_CPU_UNBOUND, wq, work);}

2.2 sschedule_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()

static inline bool schedule_work_on(int cpu, struct work_struct *work){return queue_work_on(cpu, system_wq, 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);elseadd_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

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