1 IRQ affinity on IA64 platforms
2 ------------------------------
3 07.01.2002, Erich Focht <efocht@ess.nec.de>
4
5
6 By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be
7 controlled. The behavior on IA64 platforms is slightly different from
8 that described in Documentation/IRQ-affinity.txt for i386 systems.
9
10 Because of the usage of SAPIC mode and physical destination mode the
11 IRQ target is one particular CPU and cannot be a mask of several
12 CPUs. Only the first non-zero bit is taken into account.
13
14
15 Usage examples:
16
17 The target CPU has to be specified as a hexadecimal CPU mask. The
18 first non-zero bit is the selected CPU. This format has been kept for
19 compatibility reasons with i386.
20
21 Set the delivery mode of interrupt 41 to fixed and route the
22 interrupts to CPU #3 (logical CPU number) (2^3=0x08):
23 echo "8" >/proc/irq/41/smp_affinity
24
25 Set the default route for IRQ number 41 to CPU 6 in lowest priority
26 delivery mode (redirectable):
27 echo "r 40" >/proc/irq/41/smp_affinity
28
29 The output of the command
30 cat /proc/irq/IRQ#/smp_affinity
31 gives the target CPU mask for the specified interrupt vector. If the CPU
32 mask is preceded by the character "r", the interrupt is redirectable
33 (i.e. lowest priority mode routing is used), otherwise its route is
34 fixed.
35
36
37
38 Initialization and default behavior:
39
40 If the platform features IRQ redirection (info provided by SAL) all
41 IO-SAPIC interrupts are initialized with CPU#0 as their default target
42 and the routing is the so called "lowest priority mode" (actually
43 fixed SAPIC mode with hint). The XTP chipset registers are used as hints
44 for the IRQ routing. Currently in Linux XTP registers can have three
45 values:
46 - minimal for an idle task,
47 - normal if any other task runs,
48 - maximal if the CPU is going to be switched off.
49 The IRQ is routed to the CPU with lowest XTP register value, the
50 search begins at the default CPU. Therefore most of the interrupts
51 will be handled by CPU #0.
52
53 If the platform doesn't feature interrupt redirection IOSAPIC fixed
54 routing is used. The target CPUs are distributed in a round robin
55 manner. IRQs will be routed only to the selected target CPUs. Check
56 with
57 cat /proc/interrupts
58
59
60
61 Comments:
62
63 On large (multi-node) systems it is recommended to route the IRQs to
64 the node to which the corresponding device is connected.
65 For systems like the NEC AzusA we get IRQ node-affinity for free. This
66 is because usually the chipsets on each node redirect the interrupts
67 only to their own CPUs (as they cannot see the XTP registers on the
68 other nodes).
69
This page was automatically generated by the
LXR engine.
Visit the LXR main site for more
information.