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+ChangeLog:
+ Started by Ingo Molnar <mingo@redhat.com>
+ Update by Max Krasnyansky <maxk@qualcomm.com>
+
+SMP IRQ affinity
+
+/proc/irq/IRQ#/smp_affinity and /proc/irq/IRQ#/smp_affinity_list specify
+which target CPUs are permitted for a given IRQ source. It's a bitmask
+(smp_affinity) or cpu list (smp_affinity_list) 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 of all cpus.
+
+/proc/irq/default_smp_affinity specifies default affinity mask that applies
+to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
+will be set to the default mask. It can then be changed as described above.
+Default mask is 0xffffffff.
+
+Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
+it to CPU4-7 (this is an 8-CPU SMP box):
+
+[root@moon 44]# cd /proc/irq/44
+[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 'CPU\|44:'
+ CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
+ 44: 1068 1785 1785 1783 0 0 0 0 IO-APIC-level eth1
+
+As can be seen from the line above IRQ44 was delivered only to the first four
+processors (0-3).
+Now lets restrict that IRQ to CPU(4-7).
+
+[root@moon 44]# echo f0 > smp_affinity
+[root@moon 44]# cat smp_affinity
+000000f0
+[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 | 'CPU\|44:'
+ CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
+ 44: 1068 1785 1785 1783 1784 1069 1070 1069 IO-APIC-level eth1
+
+This time around IRQ44 was delivered only to the last four processors.
+i.e counters for the CPU0-3 did not change.
+
+Here is an example of limiting that same irq (44) to cpus 1024 to 1031:
+
+[root@moon 44]# echo 1024-1031 > smp_affinity
+[root@moon 44]# cat smp_affinity
+1024-1031
+
+Note that to do this with a bitmask would require 32 bitmasks of zero
+to follow the pertinent one.