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SMP IRQ affinity, started by Ingo Molnar 

/proc/irq/IRQ#/smp_affinity specifies which target CPUs are permitted

for a given IRQ source. It's a bitmask of allowed CPUs. It's not allowed

to turn off all CPUs, and if an IRQ controller does not support IRQ

affinity then the value will not change from the default 0xffffffff.

Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting

the IRQ to CPU4-7 (this is an 8-CPU SMP box):

[root@moon 44]# cat smp_affinity

ffffffff

[root@moon 44]# echo 0f > smp_affinity

[root@moon 44]# cat smp_affinity

0000000f

[root@moon 44]# ping -f h

PING hell (195.4.7.3): 56 data bytes

...

--- hell ping statistics ---

6029 packets transmitted, 6027 packets received, 0% packet loss

round-trip min/avg/max = 0.1/0.1/0.4 ms

[root@moon 44]# cat /proc/interrupts | grep 44:

 44:          0       1785       1785       1783       1783          1

1          0   IO-APIC-level  eth1

[root@moon 44]# echo f0 > smp_affinity

[root@moon 44]# ping -f h

PING hell (195.4.7.3): 56 data bytes

..

--- hell ping statistics ---

2779 packets transmitted, 2777 packets received, 0% packet loss

round-trip min/avg/max = 0.1/0.5/585.4 ms

[root@moon 44]# cat /proc/interrupts | grep 44:

 44:       1068       1785       1785       1784       1784       1069       1070       1069   IO-APIC-level  eth1

[root@moon 44]#