Linux Workqueue 转载

转载来自：魅族内核组

　　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”

老版本workqueue：

1.支持单线程和多线程workqueue
2.单线程不绑定CPU，多线程每个CPU绑定一个线程
3.多线程workqueue中，在某个CPU上进行queue_work的work，一定会在该CPU上执行
4.queue_work时如果检测到该work处于pending状态，那么不会重复加入队列，防止重入

Linux kernel 2.6.36版本之后，内核加入了CMWQ，并发管理任务队列，内核已经实现了workqueue，为什么还要CMWQ呢？

　　当然是为了解决旧的workqueue遇到的一些缺点，那么旧的实现都有哪些问题呢？

内核线程过多

　　旧的workqueue实现，每个workqueue都会创建一系列per cpu的thread，这会大大增加系统中的进程数量，导致系统性能的降低

并发性较差

　　对于single thread类型的workqueue，其中的work是顺序执行，如果一个work阻塞，那么后面的都将收到影响，对于per cpu thread workqueue，情况会稍微好些，但是依然不够好，当一个CPU thread中已经加入了一个work时，由于queue_work只会在当前CPU上运行，后面再加入到该CPU thread的work依然需要等待，即使其他CPU thread是空闲的。

可能出现的死锁问题

　　在旧版本的workqueue中，已经加入到一个CPU上的work是不能转移到另一个CPU上的，那么假设有两个work 1和work 2，work1 依赖于work2，但是work1和work2被顺序加入到了同一个CPU thread上，那么work1将永远无法等到work2执行结束，这就形成了死锁问题。

CMWQ就是为了解决以上问题而引入内核的，它对workqueue的实现进行了优化，使得work的调度更加的灵活，为了兼容旧版本workqueue，接口基本保持了不变。

　　CMWQ把work的生产和消费分为了两个部分，对于生产者使用workqueue来管理产生的work，消费者是使用thread pool的形式来执行work。worker pool和workqueue是一对多的关系。worker pool和pwq（struct pool_workqueue）是一对一的关系。

生产者

　　内核在启动时会创建一些系统级的workqueue，比如system_wq，除了内核自带的一些workqueue，各个模块驱动可以自行创建workqueue，这些workqueue可以通过参数来选择用什么类型的work pool来运行work。

消费者

　　CMWQ中，把用于工作的线程叫做worker，并且建立两种worker pool，也就是thread pool线程池。第一种为绑定CPU的worker pool；第二种为unbound worker pool。系统初始化时会针对每个CPU创建绑定的worker pool，每个CPU创建两个类型的worker pool，一个是普通优先级的normal worker pool，另一个是highpri worker pool。每个worker pool中包含的worker线程数量是不定的。系统也会创建unbound worker pool，也有两个类型分别是noraml和highpri。

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 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;

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_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 @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 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);supposed
sleep:
    // (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 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 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
2. worker 做什么 怎样知道worker 在做什么》？ 也就是如何查看其stack 栈空间