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_list
  [root@moon 44]# cat smp_affinity_list

  1024-1031
  
  Note that to do this with a bitmask would require 32 bitmasks of zero
  to follow the pertinent one.