Linux Workqueue

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workqueu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workque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。

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：

    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;

对应的拓扑图如下：

• 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

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_attrs        BUG_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_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 @att- kernel/workqueue.c:  - __alloc_workqueue_key() -> alloc_and_link_pwqs() -> apply_workqueue_attrs() -> alloc_unbound_pwq()/numa_pwq_tbl_install()rs->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work            process_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work            move_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runn- kernel/workqueue.c:
• worker_thread() -> process_one_work()
  ing 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()

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work    worker->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 up        for_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cpu            if (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cpu    if (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_pool        pwq = per_cpu_ptr(wq->cpu_pwqs, cpu);    else        // (3) 对于unbound wq，使用当前cpu对应node的worker_pool        pwq = 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->worklist    if (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 work    insert_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()

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);    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