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Linux/Documentation/IRQ-affinity.txt

  1 ChangeLog:
  2         Started by Ingo Molnar <mingo@redhat.com>
  3         Update by Max Krasnyansky <maxk@qualcomm.com>
  4 
  5 SMP IRQ affinity
  6 
  7 /proc/irq/IRQ#/smp_affinity and /proc/irq/IRQ#/smp_affinity_list specify
  8 which 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
 10 allowed to turn off all CPUs, and if an IRQ controller does not support
 11 IRQ 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
 14 to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
 15 will be set to the default mask. It can then be changed as described above.
 16 Default mask is 0xffffffff.
 17 
 18 Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
 19 it 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
 23 ffffffff
 24 
 25 [root@moon 44]# echo 0f > smp_affinity
 26 [root@moon 44]# cat smp_affinity
 27 0000000f
 28 [root@moon 44]# ping -f h
 29 PING hell (195.4.7.3): 56 data bytes
 30 ...
 31 --- hell ping statistics ---
 32 6029 packets transmitted, 6027 packets received, 0% packet loss
 33 round-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 
 38 As can be seen from the line above IRQ44 was delivered only to the first four
 39 processors (0-3).
 40 Now lets restrict that IRQ to CPU(4-7).
 41 
 42 [root@moon 44]# echo f0 > smp_affinity
 43 [root@moon 44]# cat smp_affinity
 44 000000f0
 45 [root@moon 44]# ping -f h
 46 PING hell (195.4.7.3): 56 data bytes
 47 ..
 48 --- hell ping statistics ---
 49 2779 packets transmitted, 2777 packets received, 0% packet loss
 50 round-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 
 55 This time around IRQ44 was delivered only to the last four processors.
 56 i.e counters for the CPU0-3 did not change.
 57 
 58 Here is an example of limiting that same irq (44) to cpus 1024 to 1031:
 59 
 60 [root@moon 44]# echo 1024-1031 > smp_affinity_list
 61 [root@moon 44]# cat smp_affinity_list
 62 1024-1031
 63 
 64 Note that to do this with a bitmask would require 32 bitmasks of zero
 65 to follow the pertinent one.

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