summary
softirq 真正干活的函数是__do_softirq。 linuxv3.11内核里能够执行__do_softirq,有如下调用, 这里指真正执行softirq的地方，不是触发(设置)softirq标志 !!!
- 每个硬中断退出的时。
- 当bh使能的时。
- 发送回环报文时。
case1 每个硬中断退出的时候。
call trace
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> irq_exit> > invoke_softirq> > > __do_softirq
irq_exit
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350 /*351 * Exit an interrupt context. Process softirqs if needed and possible:352 */353 void irq_exit(void)354 {355 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED356 local_irq_disable();357 #else358 WARN_ON_ONCE(!irqs_disabled());359 #endif360361 account_irq_exit_time(current);362 trace_hardirq_exit();363 sub_preempt_count(HARDIRQ_OFFSET);364 if (!in_interrupt() && local_softirq_pending())365 invoke_softirq();366367 tick_irq_exit();368 rcu_irq_exit();369 }
invoke_softirq
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329 static inline void invoke_softirq(void)330 {331 if (!force_irqthreads)332 __do_softirq();333 else334 wakeup_softirqd();335 }
case2 使能bh的时候
call trace
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> local_bh_enable> > _local_bh_enable_ip> > > do_softirq> > > > (x86_64特有的)call_softirq> > > > __do_softirq
local_bh_enable
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184 void local_bh_enable(void)185 {186 _local_bh_enable_ip(_RET_IP_);187 }188 EXPORT_SYMBOL(local_bh_enable);
_local_bh_enable_ip
NOTE: 使能bh的时候，刚开始抢占还是禁止的。只有软中断处理完了后，抢占才使能的。 顺便说一下，使能bh时，有可能发生进程切换(见preempt_check_resched).
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157 static inline void _local_bh_enable_ip(unsigned long ip)158 {159 WARN_ON_ONCE(in_irq() || irqs_disabled());160 #ifdef CONFIG_TRACE_IRQFLAGS161 local_irq_disable();162 #endif163 /*164 * Are softirqs going to be turned on now:165 */166 if (softirq_count() == SOFTIRQ_DISABLE_OFFSET)167 trace_softirqs_on(ip);168 /*169 * Keep preemption disabled until we are done with170 * softirq processing:171 */172 sub_preempt_count(SOFTIRQ_DISABLE_OFFSET - 1);173174 if (unlikely(!in_interrupt() && local_softirq_pending()))175 do_softirq();176177 dec_preempt_count();178 #ifdef CONFIG_TRACE_IRQFLAGS179 local_irq_enable();180 #endif181 preempt_check_resched();182 }
do_softirq根据不同的体系机构有不同的版本。
当对应的硬件体系里没有定义do_softirq时，使用 kernel/softirq.c中通用的do_softirq`.
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286 #ifndef __ARCH_HAS_DO_SOFTIRQ287288 asmlinkage void do_softirq(void)289 {290 __u32 pending;291 unsigned long flags;292293 if (in_interrupt())294 return;295296 local_irq_save(flags);297298 pending = local_softirq_pending();299300 if (pending)301 __do_softirq();302303 local_irq_restore(flags);304 }305306 #endif
而对于X86_64,有其自己的定义。
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92 extern void call_softirq(void); 93 94 asmlinkage void do_softirq(void) 95 { 96 __u32 pending; 97 unsigned long flags; 98 99 if (in_interrupt())100 return;101102 local_irq_save(flags);103 pending = local_softirq_pending();104 /* Switch to interrupt stack */105 if (pending) {106 call_softirq();107 WARN_ON_ONCE(softirq_count());108 }109 local_irq_restore(flags);110 }
其中call_softirq是个用汇编语言写的一个函数定义在 arch/x86/kernel/entry_64.S
netif_rx_ni
这一块的理解不是很透，仅以一个具体的例子来说明。发送回环报文.
发送回环报文
call trace
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> dev_loopback_xmit> > netif_rx_ni> > do_softirq> > > __do_softirq
loopback 报文会通过netif_rx函数，被重新放回到 per_cpu变量softnet_data下的队列input_pkt_queue里， 同时激发napi调用，进而激发软中断调用(只是设置标志位，不是真正调用）
另外，注意netif_rx_ni里没有禁止bh调度，而是只是禁止了抢占(preempt_disable).
dev_loopback_xmit
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2765 int dev_loopback_xmit(struct sk_buff *skb)2766 {2767 skb_reset_mac_header(skb);2768 __skb_pull(skb, skb_network_offset(skb));2769 skb->pkt_type = PACKET_LOOPBACK;2770 skb->ip_summed = CHECKSUM_UNNECESSARY;2771 WARN_ON(!skb_dst(skb));2772 skb_dst_force(skb);2773 netif_rx_ni(skb);2774 return 0;2775 }
netif_rx_ni
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3267 int netif_rx_ni(struct sk_buff *skb)3268 {3269 int err;32703271 preempt_disable();3272 err = netif_rx(skb);3273 if (local_softirq_pending())3274 do_softirq();3275 preempt_enable();32763277 return err;3278 }3279 EXPORT_SYMBOL(netif_rx_ni);
