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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 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.

/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.
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