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 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

Documentation/workqueue.txt

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