diff --git a/Documentation/IRQ-affinity.txt b/Documentation/IRQ-affinity.txt
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+
+SMP IRQ affinity, started by Ingo Molnar <mingo@redhat.com>
+
+
+/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]#
+