Linux Modules（7.3）- work queue

Work queue提供一個interface，讓使用者輕易的建立kernel thread並且將work綁在這個kernel thread上面，如下圖[1]所示。

由於work queue是建立一個kernel thread來執行，所以是在process context，不同於tasklet的interrupt context，因此，work queue可以sleep(設定semaphore或者執行block I/O等等)。

Creating Work
透過 DECLARE_WORK(name, void (work_func_t)(struct work_struct *work)); // statically
或者
INIT_WORK(struct work_struct*, void (work_func_t)(struct work_struct *work)); //dynamically
建立work，就是要執行的工作。
有了work還需要將它和work thread結合，您可以透過create_singlethread_workqueue("name")建立一個名為name的single thread(執行work的thread就稱為work thread)，或者create_workqueue("name")建立per cpu的thread。接著就是要將work和work thread做關聯了，透過queue_work(work_thread, work)就可以將work送給work thread執行了。

queue_delayed_work(work_thread, delayed_work, delay)為queue_work()的delay版本。
flush_workqueue(work_thread)會wait直到這個work_thread的work都做完。flush_workqueue()並不會取消任何被delay執行的work，如果要取消delayed的work則需要呼叫cancel_delayed_work(delayed_work)將delayed_work自某個work thread中移除。

最後，要將work_thread摧毀要呼叫destroy_workqueue(work_thread)。


event/n
除了自己建立work thread以外，kernel還建立一個公用的work thread稱為event

kernel/workqueue.c

void __init init_workqueues(void)
{
    …
    keventd_wq = create_workqueue("events");
    …
}

您可以透過schedule_work(&work)將，work送給"events"執行，flush_scheduled_work(void)等待"events"中所有的work執行完畢。

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/workqueue.h>
#include <linux/sched.h>
#include <linux/slab.h>

MODULE_LICENSE("GPL");

static void brook_1_routine(struct work_struct *);
static void brook_2_routine(struct work_struct *);
static void brook_3_routine(struct work_struct *);

static struct work_struct *brook_1_work; // for event
static DECLARE_WORK(brook_2_work, brook_2_routine);
static DECLARE_DELAYED_WORK(brook_3_work, brook_3_routine);
static struct workqueue_struct *brook_workqueue;
static int stop_wq;
module_param(stop_wq, int, S_IRUGO | S_IWUGO);

static int __init init_modules(void)
{
    // for event
    brook_1_work = kzalloc(sizeof(typeof(*brook_1_work)), GFP_KERNEL);
    INIT_WORK(brook_1_work, brook_1_routine);
    schedule_work(brook_1_work);

    // for brook_wq
    brook_workqueue = create_workqueue("brook_wq");
    queue_work(brook_workqueue, &brook_2_work);
    queue_delayed_work(brook_workqueue, &brook_3_work, 0);
    stop_wq = 0;
    return 0;
}

static void __exit exit_modules(void)
{
    cancel_delayed_work(&brook_3_work);
    flush_workqueue(brook_workqueue);
    stop_wq = 1;
    destroy_workqueue(brook_workqueue);
}

static void brook_1_routine(struct work_struct *ws)
{
    printk("%s(): on cpu:%d, pname:%s\n",
            __func__, smp_processor_id(), current->comm);
}

static void brook_2_routine(struct work_struct *ws)
{
    printk("%s(): on cpu:%d, pname:%s\n",
            __func__, smp_processor_id(), current->comm);
    // do something to block/sleep
    // the work in the same workqueue is also deferred.
    msleep(5000);
    if (!stop_wq) {
        queue_work(brook_workqueue, &brook_2_work);
    }
}

static void brook_3_routine(struct work_struct *ws)
{
    printk("%s(): on cpu:%d, pname:%s\n",
            __func__, smp_processor_id(), current->comm);
    queue_delayed_work(brook_workqueue, &brook_3_work, 50);
}

module_init(init_modules);
module_exit(exit_modules);

Kernel Version：2.6.35
參考資料：

1. http://www.embexperts.com/viewthread.php?tid=12&highlight=work%2Bqueue
2. Linux Kernel Development 2nd, Novell Press