linux/Documentation/IRQ-affinity.txt
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   1ChangeLog:
   2        Started by Ingo Molnar <mingo@redhat.com>
   3        Update by Max Krasnyansky <maxk@qualcomm.com>
   4
   5SMP IRQ affinity
   6
   7/proc/irq/IRQ#/smp_affinity specifies which target CPUs are permitted
   8for a given IRQ source. It's a bitmask of allowed CPUs. It's not allowed
   9to turn off all CPUs, and if an IRQ controller does not support IRQ
  10affinity then the value will not change from the default 0xffffffff.
  11
  12/proc/irq/default_smp_affinity specifies default affinity mask that applies
  13to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
  14will be set to the default mask. It can then be changed as described above.
  15Default mask is 0xffffffff.
  16
  17Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
  18it to CPU4-7 (this is an 8-CPU SMP box):
  19
  20[root@moon 44]# cd /proc/irq/44
  21[root@moon 44]# cat smp_affinity
  22ffffffff
  23
  24[root@moon 44]# echo 0f > smp_affinity
  25[root@moon 44]# cat smp_affinity
  260000000f
  27[root@moon 44]# ping -f h
  28PING hell (195.4.7.3): 56 data bytes
  29...
  30--- hell ping statistics ---
  316029 packets transmitted, 6027 packets received, 0% packet loss
  32round-trip min/avg/max = 0.1/0.1/0.4 ms
  33[root@moon 44]# cat /proc/interrupts | grep 'CPU\|44:'
  34           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
  35 44:       1068       1785       1785       1783         0          0           0         0    IO-APIC-level  eth1
  36
  37As can be seen from the line above IRQ44 was delivered only to the first four
  38processors (0-3).
  39Now lets restrict that IRQ to CPU(4-7).
  40
  41[root@moon 44]# echo f0 > smp_affinity
  42[root@moon 44]# cat smp_affinity
  43000000f0
  44[root@moon 44]# ping -f h
  45PING hell (195.4.7.3): 56 data bytes
  46..
  47--- hell ping statistics ---
  482779 packets transmitted, 2777 packets received, 0% packet loss
  49round-trip min/avg/max = 0.1/0.5/585.4 ms
  50[root@moon 44]# cat /proc/interrupts |  'CPU\|44:'
  51           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
  52 44:       1068       1785       1785       1783      1784       1069        1070       1069   IO-APIC-level  eth1
  53
  54This time around IRQ44 was delivered only to the last four processors.
  55i.e counters for the CPU0-3 did not change.
  56
  57
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