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Asynchronous function calls for boot performance个人理解主要是通过函数异步调用还减少启动时间，即将没有依赖且没有严格串行的程序可以异步执行来减少启动时间。异步函数调用的具体实现在kernel/async.c中。我们举例说明async的用法.这个例子是在drivers/acpi/battery.c中static int __init acpi_battery_init(void){	if (acpi_disabled)		return -ENODEV;#可见在入口函数中，通过async_schedule来让acpi_battery_init_async 异步调用以#加快系统的启动时间，从这里可以知道acpi_battery_init_async和其他硬件没有依赖，且#没有严格的串行关系.注意这里会返回一个static async_cookie_t async_cookie;在模块的注销函数#中需要调用acpi_battery_init_async。	async_cookie = async_schedule(acpi_battery_init_async, NULL);	return 0;}static void __exit acpi_battery_exit(void){#	async_synchronize_cookie(async_cookie + 1);	if (battery_driver_registered) {		acpi_bus_unregister_driver(&acpi_battery_driver);		battery_hook_exit();	}#ifdef CONFIG_ACPI_PROCFS_POWER	if (acpi_battery_dir)		acpi_unlock_battery_dir(acpi_battery_dir);#endif}#定义模块的入口函数module_init(acpi_battery_init);module_exit(acpi_battery_exit);所以async的用法就是这样。下来我们看看acpi_battery_init_async的具体实现async_cookie_t async_schedule(async_func_t func, void *data){#默认的domain是async_dfl_domain	return __async_schedule(func, data, &async_dfl_domain);}static async_cookie_t __async_schedule(async_func_t func, void *data, struct async_domain *domain){	struct async_entry *entry;	unsigned long flags;	async_cookie_t newcookie;	/* allow irq-off callers */#注册一个entry	entry = kzalloc(sizeof(struct async_entry), GFP_ATOMIC);	/*	 * If we're out of memory or if there's too much work	 * pending already, we execute synchronously.	 */#异步执行的函数执行的个数有限制，不能超过MAX_WORK，这个限制应该和memory有关	if (!entry || atomic_read(&entry_count) > MAX_WORK) {		kfree(entry);		spin_lock_irqsave(&async_lock, flags);		newcookie = next_cookie++;		spin_unlock_irqrestore(&async_lock, flags);		/* low on memory.. run synchronously */		func(data, newcookie);		return newcookie;	}#初始化两个列表	INIT_LIST_HEAD(&entry->domain_list);	INIT_LIST_HEAD(&entry->global_list);#初始化一个workqueue，并设置回调函数为async_run_entry_fn	INIT_WORK(&entry->work, async_run_entry_fn);	entry->func = func;	entry->data = data;	entry->domain = domain;	spin_lock_irqsave(&async_lock, flags);	/* allocate cookie and queue */	newcookie = entry->cookie = next_cookie++;#将新建的entry添加到全局列表中	list_add_tail(&entry->domain_list, &domain->pending);	if (domain->registered)		list_add_tail(&entry->global_list, &async_global_pending);	atomic_inc(&entry_count);	spin_unlock_irqrestore(&async_lock, flags);	/* mark that this task has queued an async job, used by module init */	current->flags |= PF_USED_ASYNC;#开始调度workqueue，这里可以看明白async的精髓，以本例为例，也就是你调用module_init后你的函数不一定立刻运行了	/* schedule for execution */	queue_work(system_unbound_wq, &entry->work);	return newcookie;}static void async_run_entry_fn(struct work_struct *work){	struct async_entry *entry =		container_of(work, struct async_entry, work);	unsigned long flags;	ktime_t uninitialized_var(calltime), delta, rettime;	/* 1) run (and print duration) */	if (initcall_debug && system_state < SYSTEM_RUNNING) {		pr_debug("calling  %lli_%pF @ %i\n",			(long long)entry->cookie,			entry->func, task_pid_nr(current));		calltime = ktime_get();	}#执行用户调用__async_schedule时传入的回调函数	entry->func(entry->data, entry->cookie);	}

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