Linux网络开始收发包之前需要做的事情

1、创建ksoftirqd内核进程

2、网络子系统初始化

3、内核网络协议栈注册

4、网卡驱动初始化

5、启动网卡

以上5步就是我们在收发包之前内核要做的事情。

1、创建ksoftirqd内核进程

ksoftirqd内核进程也称为 per_cpu进程(每一个cpu核都有一个)。其由smpboot_register_percpu_thread函数创建。
在kernel/smpboot.c中调用smpboot_register_percpu_thread函数

int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
{unsigned int cpu;int ret = 0;get_online_cpus();mutex_lock(&smpboot_threads_lock);for_each_online_cpu(cpu) {ret = __smpboot_create_thread(plug_thread, cpu);if (ret) {smpboot_destroy_threads(plug_thread);goto out;}smpboot_unpark_thread(plug_thread, cpu);}list_add(&plug_thread->list, &hotplug_threads);
out:mutex_unlock(&smpboot_threads_lock);put_online_cpus();return ret;
}
EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);

在这里我们要搞懂俩个事情，这个函数是被谁调用的，它最终做了什么，至于里面的细节暂时先不谈。
一、这个函数是谁调用的：
下面一张截图是其调用流程

early_initcall(spawn_ksoftirqd) 中调用smpboot_register_percpu_thread函数。内核启动过程中会有start_kernel --> rest_init这个流程，在reset_init会创建一个内核线程，执行kernel_init，early_initcall需要在SMP初始化之前被调用，其在do_pre_smp_initcalls函数中被调用。early_initcall宏的调用调用时刻有点复杂，暂时先不深入了解。
ksoftirq进程其实是由kthreadd进程创建的，其是所有Linux内核线程的鼻祖，swapper进程是init进程和kthreadd进程的父进程，是Linux启动的第一个进程；

kernel_init --> kernel_init_freeable --> do_pre_smp_initcalls|---> do_basic_setup --> do_initcalls

二、这个函数执行完后发生了什么：
1、为每一个cpu核都创建了一个ksoftirqd进程*(kthread_create_on_cpu)，回调函数是smpboot_thread_fn。
追踪代码发现实际上是调用到了__kthread_create_on_node函数创建ksoftirqd进程*，创建的过程如图所示。

分配一个kthread_create_info结构体，并初始化它，然后加入到kthread_create_list全局链表中，此链表是内核线程链表，所有的内核线程都在里面。并使用wake_up_process唤醒ksoftirqd进程。

2、线程被唤醒后会执行回调函数，在回调函数smpboot_thread_fn会阻塞休眠，一旦有软中断需要处理(进程run)，就会调用softirq_threads->thread_fn。

3、在softirq_threads->thread_fn中开始执行软中断处理函数。

下面是完整的代码流程，涉及到/kernel/softirq.c和/kernel/smpboot.c。

在/kernel/softirq.c中初始化softirq_threads结构体，并调用smpboot_register_percpu_thread
static struct smp_hotplug_thread softirq_threads = {.store         = &ksoftirqd,.thread_should_run    = ksoftirqd_should_run,.thread_fn      = run_ksoftirqd,.thread_comm       = "ksoftirqd/%u",
};
BUG_ON(smpboot_register_percpu_thread(&softirq_threads));/kernel/smpboot.c
int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
{unsigned int cpu;int ret = 0;get_online_cpus();mutex_lock(&smpboot_threads_lock);for_each_online_cpu(cpu) {ret = __smpboot_create_thread(plug_thread, cpu);if (ret) {smpboot_destroy_threads(plug_thread);goto out;}smpboot_unpark_thread(plug_thread, cpu);}list_add(&plug_thread->list, &hotplug_threads);
out:mutex_unlock(&smpboot_threads_lock);put_online_cpus();return ret;
}
EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);__smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
{struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);struct smpboot_thread_data *td;if (tsk)return 0;td = kzalloc_node(sizeof(*td), GFP_KERNEL, cpu_to_node(cpu));if (!td)return -ENOMEM;td->cpu = cpu;td->ht = ht;在这里创建kthread进程，实际最终调用的是上面讲的__kthread_create_on_node函数，回调函数为smpboot_thread_fn。tsk = kthread_create_on_cpu(smpboot_thread_fn, td, cpu,ht->thread_comm);if (IS_ERR(tsk)) {kfree(td);return PTR_ERR(tsk);}kthread_set_per_cpu(tsk, cpu);/** Park the thread so that it could start right on the CPU* when it is available.*/kthread_park(tsk);get_task_struct(tsk);*per_cpu_ptr(ht->store, cpu) = tsk;if (ht->create) {/** Make sure that the task has actually scheduled out* into park position, before calling the create* callback. At least the migration thread callback* requires that the task is off the runqueue.*/if (!wait_task_inactive(tsk, TASK_PARKED))WARN_ON(1);elseht->create(cpu);}return 0;
}在回调函数smpboot_thread_fn会一直循环去检测是否需要执行软中断处理，一旦有软中断需要处理，就会调用smp_hotplug_thread->thread_fn。
static int smpboot_thread_fn(void *data)
{struct smpboot_thread_data *td = data;struct smp_hotplug_thread *ht = td->ht;while (1) {set_current_state(TASK_INTERRUPTIBLE);preempt_disable();if (kthread_should_stop()) {__set_current_state(TASK_RUNNING);preempt_enable();/* cleanup must mirror setup */if (ht->cleanup && td->status != HP_THREAD_NONE)ht->cleanup(td->cpu, cpu_online(td->cpu));kfree(td);return 0;}if (kthread_should_park()) {__set_current_state(TASK_RUNNING);preempt_enable();if (ht->park && td->status == HP_THREAD_ACTIVE) {BUG_ON(td->cpu != smp_processor_id());ht->park(td->cpu);td->status = HP_THREAD_PARKED;}kthread_parkme();/* We might have been woken for stop */continue;}BUG_ON(td->cpu != smp_processor_id());/* Check for state change setup */switch (td->status) {case HP_THREAD_NONE:__set_current_state(TASK_RUNNING);preempt_enable();if (ht->setup)ht->setup(td->cpu);td->status = HP_THREAD_ACTIVE;continue;case HP_THREAD_PARKED:__set_current_state(TASK_RUNNING);preempt_enable();if (ht->unpark)ht->unpark(td->cpu);td->status = HP_THREAD_ACTIVE;continue;}if (!ht->thread_should_run(td->cpu)) {preempt_enable_no_resched();schedule();} else {__set_current_state(TASK_RUNNING);preempt_enable();ht->thread_fn(td->cpu);}}
}
static void run_ksoftirqd(unsigned int cpu)
{local_irq_disable();if (local_softirq_pending()) {/** We can safely run softirq on inline stack, as we are not deep* in the task stack here.*/__do_softirq();local_irq_enable();cond_resched();return;}local_irq_enable();
}

从数据结构的角度来看，总共涉及到三个结构体和一个链表，他们的关系如截图所示。