What's the concern about setting irq thread's policy as SCHED_FIFO

[Qin Chuanyu]

I am doing network performance test under suse11sp3 and intel 82599 nic,
Becasuse the softirq is out of schedule policy's control, so netserver 
thread couldn't always get 100% cpu usage, then packet dropped in kernel 
udp socket's receive queue.

In order to get a stable result, I did some patch in ixgbe driver and 
then use irq_thread instead of softirq to handle rx.
It seems work well, but irq_thread's SCHED_FIFO schedule policy cause 
that when the cpu is limited, netserver couldn't work at all.

So I change the irq_thread's schedule policy from SCHED_FIFO to 
SCHED_NORMAL, then the irq_thread could share the cpu usage with 
netserver thread.

the question is:
What's the concrete reason about setting irq thread's policy as SCHED_FIFO?
Except the priority affecting the cpu usage, any function would be 
broken if irq thread change to SCHED_NORMAL?

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