kernel/softirq.c:irq_exit()→do_softirq();
kernel/softirq.c:ksoftirqd()→do_softirq();
net/core/dev.c:netif_rx_ni()→do_softirq();
net/core/pktgen.c:pktgen_thread_worker()→do_softirq();
__do_softirq(void)
While block scan pending interrupts.
if (pending)
rescan interrupt occurred during softirq. so MAX_SOFTIRQ_RESTART
specify maximum rescan trial.