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Documentation / IRQ-affinity.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	================
2	SMP IRQ affinity
3	================
4	
5	ChangeLog:
6		- Started by Ingo Molnar <mingo@redhat.com>
7		- Update by Max Krasnyansky <maxk@qualcomm.com>
8	
9	
10	/proc/irq/IRQ#/smp_affinity and /proc/irq/IRQ#/smp_affinity_list specify
11	which target CPUs are permitted for a given IRQ source.  It's a bitmask
12	(smp_affinity) or cpu list (smp_affinity_list) of allowed CPUs.  It's not
13	allowed to turn off all CPUs, and if an IRQ controller does not support
14	IRQ affinity then the value will not change from the default of all cpus.
15	
16	/proc/irq/default_smp_affinity specifies default affinity mask that applies
17	to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
18	will be set to the default mask. It can then be changed as described above.
19	Default mask is 0xffffffff.
20	
21	Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
22	it to CPU4-7 (this is an 8-CPU SMP box)::
23	
24		[root@moon 44]# cd /proc/irq/44
25		[root@moon 44]# cat smp_affinity
26		ffffffff
27	
28		[root@moon 44]# echo 0f > smp_affinity
29		[root@moon 44]# cat smp_affinity
30		0000000f
31		[root@moon 44]# ping -f h
32		PING hell (195.4.7.3): 56 data bytes
33		...
34		--- hell ping statistics ---
35		6029 packets transmitted, 6027 packets received, 0% packet loss
36		round-trip min/avg/max = 0.1/0.1/0.4 ms
37		[root@moon 44]# cat /proc/interrupts | grep 'CPU\|44:'
38			CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
39		44:       1068       1785       1785       1783         0          0           0         0    IO-APIC-level  eth1
40	
41	As can be seen from the line above IRQ44 was delivered only to the first four
42	processors (0-3).
43	Now lets restrict that IRQ to CPU(4-7).
44	
45	::
46	
47		[root@moon 44]# echo f0 > smp_affinity
48		[root@moon 44]# cat smp_affinity
49		000000f0
50		[root@moon 44]# ping -f h
51		PING hell (195.4.7.3): 56 data bytes
52		..
53		--- hell ping statistics ---
54		2779 packets transmitted, 2777 packets received, 0% packet loss
55		round-trip min/avg/max = 0.1/0.5/585.4 ms
56		[root@moon 44]# cat /proc/interrupts |  'CPU\|44:'
57			CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
58		44:       1068       1785       1785       1783      1784       1069        1070       1069   IO-APIC-level  eth1
59	
60	This time around IRQ44 was delivered only to the last four processors.
61	i.e counters for the CPU0-3 did not change.
62	
63	Here is an example of limiting that same irq (44) to cpus 1024 to 1031::
64	
65		[root@moon 44]# echo 1024-1031 > smp_affinity_list
66		[root@moon 44]# cat smp_affinity_list
67		1024-1031
68	
69	Note that to do this with a bitmask would require 32 bitmasks of zero
70	to follow the pertinent one.
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