Table of Contents
接收到帧时通知驱动程序
轮询
例如，内核可以持续读取设备上的一个内存寄存器，或者当一个定时器到期时就回头检查哪个寄存器。
中断
此时，当特定事件发生时，设备驱动程序会代表内核指示设备产生硬件中断。内核将中断其他活动，然后调用一个驱动程序
所注册的处理函数，以满足设备的需要。当事件是接收到一个帧时，处理函数就会把该帧排入队列某处，然后通知内核。
中断处理程序
| 函数/宏 | 描述 |
|---|---|
| in_interrupt | 处于软硬件中断中，且抢占功能是关闭的 |
| in_softirq | 处于软件中断中 |
| in_irq | 处于硬件中断中 |
| softirq_pending | 软件中断未决 |
| local_softirq_pending | 本地软件中断未决 |
| __raise_softirq_irqoff | 设置与输入的软IRQ类型相关联的标识，将该软IRQ标记为未决 |
| raise_softirq_irqoff | 先关闭硬件中断，再调用__raise_softirq_irqoff,再恢复其原有状态 |
| raise_softirq | |
| __local_bh_enable | |
| local_bh_enable | |
| local_bh_disable | |
| local_irq_disable | |
| local_irq_enable | |
| local_irq_save | |
| local_irq_restore | |
| spin_lock_bh | |
| spin_unlock_bh |
抢占功能
- preempt_disable()
为当前任务关闭抢占功能。可以重复调用，递增一个引用计数器。 - preempt_enable()
- preempt_enable_no_resched()
开启抢占功能。preempt_enable_no_reched()只是递减一个引用计数器，使得其值为0时，可以让抢占再度开启。
下半部函数
内核2.4版本以后的下半部函数: 引入软IRQ
对并发的唯一限制就是何时，在一个CPU上每个软IRQ都只能有一个实例运行。
新式的软IRQ模型只有10种模型(include/linux/interrupt.h):
1: /* PLEASE, avoid to allocate new softirqs, if you need not _really_ high
2: frequency threaded job scheduling. For almost all the purposes
3: tasklets are more than enough. F.e. all serial device BHs et
4: al. should be converted to tasklets, not to softirqs.
5: */
6:
7: enum
8: {
9: HI_SOFTIRQ=,
10: TIMER_SOFTIRQ,
11: NET_TX_SOFTIRQ,
12: NET_RX_SOFTIRQ,
13: BLOCK_SOFTIRQ,
14: BLOCK_IOPOLL_SOFTIRQ,
15: TASKLET_SOFTIRQ,
16: SCHED_SOFTIRQ,
17: HRTIMER_SOFTIRQ,
18: RCU_SOFTIRQ, /* Preferable RCU should always be the last softirq */
19:
20: NR_SOFTIRQS
21: };
网络代码如何使用软IRQ
网络子系统在net/core/dev.c中注册接收发送软中断：
open_softirq(NET_TX_SOFTIRQ, net_tx_action);
open_softirq(NET_RX_SOFTIRQ, net_rx_action);
1: /*
2: * Initialize the DEV module. At boot time this walks the device list and
3: * unhooks any devices that fail to initialise (normally hardware not
4: * present) and leaves us with a valid list of present and active devices.
5: *
6: */
7:
8: /*
9: * This is called single threaded during boot, so no need
10: * to take the rtnl semaphore.
11: */
12: static int __init net_dev_init(void)
13: {
14: int i, rc = -ENOMEM;
15:
16: BUG_ON(!dev_boot_phase);
17:
18: if (dev_proc_init())
19: goto out;
20:
21: if (netdev_kobject_init())
22: goto out;
23:
24: INIT_LIST_HEAD(&ptype_all);
25: for (i = ; i < PTYPE_HASH_SIZE; i++)
26: INIT_LIST_HEAD(&ptype_base[i]);
27:
28: if (register_pernet_subsys(&netdev_net_ops))
29: goto out;
30:
31: /*
32: * Initialise the packet receive queues.
33: */
34:
35: for_each_possible_cpu(i) {
36: struct softnet_data *sd = &per_cpu(softnet_data, i);
37:
38: memset(sd, , sizeof(*sd));
39: skb_queue_head_init(&sd->input_pkt_queue);
40: skb_queue_head_init(&sd->process_queue);
41: sd->completion_queue = NULL;
42: INIT_LIST_HEAD(&sd->poll_list);
43: sd->output_queue = NULL;
44: sd->output_queue_tailp = &sd->output_queue;
45: #ifdef CONFIG_RPS
46: sd->csd.func = rps_trigger_softirq;
47: sd->csd.info = sd;
48: sd->csd.flags = ;
49: sd->cpu = i;
50: #endif
51:
52: sd->backlog.poll = process_backlog;
53: sd->backlog.weight = weight_p;
54: sd->backlog.gro_list = NULL;
55: sd->backlog.gro_count = ;
56: }
57:
58: dev_boot_phase = ;
59:
60: /* The loopback device is special if any other network devices
61: * is present in a network namespace the loopback device must
62: * be present. Since we now dynamically allocate and free the
63: * loopback device ensure this invariant is maintained by
64: * keeping the loopback device as the first device on the
65: * list of network devices. Ensuring the loopback devices
66: * is the first device that appears and the last network device
67: * that disappears.
68: */
69: if (register_pernet_device(&loopback_net_ops))
70: goto out;
71:
72: if (register_pernet_device(&default_device_ops))
73: goto out;
74:
75: open_softirq(NET_TX_SOFTIRQ, net_tx_action);
76: open_softirq(NET_RX_SOFTIRQ, net_rx_action);
77:
78: hotcpu_notifier(dev_cpu_callback, );
79: dst_init();
80: dev_mcast_init();
81: rc = ;
82: out:
83: return rc;
84: }
85:
86: subsys_initcall(net_dev_init);
softnet_data结构
每个CPU都有其队列，用来接收进来的帧。数据结构为：
1: /*
2: * Incoming packets are placed on per-cpu queues
3: */
4: struct softnet_data {
5: struct Qdisc *output_queue;
6: struct Qdisc **output_queue_tailp;
7: struct list_head poll_list;
8: struct sk_buff *completion_queue;
9: struct sk_buff_head process_queue;
10:
11: /* stats */
12: unsigned int processed;
13: unsigned int time_squeeze;
14: unsigned int cpu_collision;
15: unsigned int received_rps;
16:
17: #ifdef CONFIG_RPS
18: struct softnet_data *rps_ipi_list;
19:
20: /* Elements below can be accessed between CPUs for RPS */
21: struct call_single_data csd ____cacheline_aligned_in_smp;
22: struct softnet_data *rps_ipi_next;
23: unsigned int cpu;
24: unsigned int input_queue_head;
25: unsigned int input_queue_tail;
26: #endif
27: unsigned dropped;
28: struct sk_buff_head input_pkt_queue;
29: struct napi_struct backlog;
30: };