Root/Documentation/IRQ-affinity.txt

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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