0001 ================
0002 SMP IRQ affinity
0003 ================
0004
0005 ChangeLog:
0006 - Started by Ingo Molnar <mingo@redhat.com>
0007 - Update by Max Krasnyansky <maxk@qualcomm.com>
0008
0009
0010 /proc/irq/IRQ#/smp_affinity and /proc/irq/IRQ#/smp_affinity_list specify
0011 which target CPUs are permitted for a given IRQ source. It's a bitmask
0012 (smp_affinity) or cpu list (smp_affinity_list) of allowed CPUs. It's not
0013 allowed to turn off all CPUs, and if an IRQ controller does not support
0014 IRQ affinity then the value will not change from the default of all cpus.
0015
0016 /proc/irq/default_smp_affinity specifies default affinity mask that applies
0017 to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
0018 will be set to the default mask. It can then be changed as described above.
0019 Default mask is 0xffffffff.
0020
0021 Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
0022 it to CPU4-7 (this is an 8-CPU SMP box)::
0023
0024 [root@moon 44]# cd /proc/irq/44
0025 [root@moon 44]# cat smp_affinity
0026 ffffffff
0027
0028 [root@moon 44]# echo 0f > smp_affinity
0029 [root@moon 44]# cat smp_affinity
0030 0000000f
0031 [root@moon 44]# ping -f h
0032 PING hell (195.4.7.3): 56 data bytes
0033 ...
0034 --- hell ping statistics ---
0035 6029 packets transmitted, 6027 packets received, 0% packet loss
0036 round-trip min/avg/max = 0.1/0.1/0.4 ms
0037 [root@moon 44]# cat /proc/interrupts | grep 'CPU\|44:'
0038 CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
0039 44: 1068 1785 1785 1783 0 0 0 0 IO-APIC-level eth1
0040
0041 As can be seen from the line above IRQ44 was delivered only to the first four
0042 processors (0-3).
0043 Now lets restrict that IRQ to CPU(4-7).
0044
0045 ::
0046
0047 [root@moon 44]# echo f0 > smp_affinity
0048 [root@moon 44]# cat smp_affinity
0049 000000f0
0050 [root@moon 44]# ping -f h
0051 PING hell (195.4.7.3): 56 data bytes
0052 ..
0053 --- hell ping statistics ---
0054 2779 packets transmitted, 2777 packets received, 0% packet loss
0055 round-trip min/avg/max = 0.1/0.5/585.4 ms
0056 [root@moon 44]# cat /proc/interrupts | 'CPU\|44:'
0057 CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
0058 44: 1068 1785 1785 1783 1784 1069 1070 1069 IO-APIC-level eth1
0059
0060 This time around IRQ44 was delivered only to the last four processors.
0061 i.e counters for the CPU0-3 did not change.
0062
0063 Here is an example of limiting that same irq (44) to cpus 1024 to 1031::
0064
0065 [root@moon 44]# echo 1024-1031 > smp_affinity_list
0066 [root@moon 44]# cat smp_affinity_list
0067 1024-1031
0068
0069 Note that to do this with a bitmask would require 32 bitmasks of zero
0070 to follow the pertinent one.
