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 and /proc/irq/IRQ#/smp_affinity_list specify
   8which target CPUs are permitted for a given IRQ source.  It's a bitmask
   9(smp_affinity) or cpu list (smp_affinity_list) of allowed CPUs.  It's not
  10allowed to turn off all CPUs, and if an IRQ controller does not support
  11IRQ affinity then the value will not change from the default of all cpus.
  12
  13/proc/irq/default_smp_affinity specifies default affinity mask that applies
  14to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
  15will be set to the default mask. It can then be changed as described above.
  16Default mask is 0xffffffff.
  17
  18Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
  19it to CPU4-7 (this is an 8-CPU SMP box):
  20
  21[root@moon 44]# cd /proc/irq/44
  22[root@moon 44]# cat smp_affinity
  23ffffffff
  24
  25[root@moon 44]# echo 0f > smp_affinity
  26[root@moon 44]# cat smp_affinity
  270000000f
  28[root@moon 44]# ping -f h
  29PING hell (195.4.7.3): 56 data bytes
  30...
  31--- hell ping statistics ---
  326029 packets transmitted, 6027 packets received, 0% packet loss
  33round-trip min/avg/max = 0.1/0.1/0.4 ms
  34[root@moon 44]# cat /proc/interrupts | grep 'CPU\|44:'
  35           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
  36 44:       1068       1785       1785       1783         0          0           0         0    IO-APIC-level  eth1
  37
  38As can be seen from the line above IRQ44 was delivered only to the first four
  39processors (0-3).
  40Now lets restrict that IRQ to CPU(4-7).
  41
  42[root@moon 44]# echo f0 > smp_affinity
  43[root@moon 44]# cat smp_affinity
  44000000f0
  45[root@moon 44]# ping -f h
  46PING hell (195.4.7.3): 56 data bytes
  47..
  48--- hell ping statistics ---
  492779 packets transmitted, 2777 packets received, 0% packet loss
  50round-trip min/avg/max = 0.1/0.5/585.4 ms
  51[root@moon 44]# cat /proc/interrupts |  'CPU\|44:'
  52           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
  53 44:       1068       1785       1785       1783      1784       1069        1070       1069   IO-APIC-level  eth1
  54
  55This time around IRQ44 was delivered only to the last four processors.
  56i.e counters for the CPU0-3 did not change.
  57
  58Here is an example of limiting that same irq (44) to cpus 1024 to 1031:
  59
  60[root@moon 44]# echo 1024-1031 > smp_affinity
  61[root@moon 44]# cat smp_affinity
  621024-1031
  63
  64Note that to do this with a bitmask would require 32 bitmasks of zero
  65to follow the pertinent one.
  66
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