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IRQ affinity on IA64 platforms
07.01.2002, Erich Focht <>
By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be
controlled. The behavior on IA64 platforms is slightly different from
that described in Documentation/IRQ-affinity.txt for i386 systems.
Because of the usage of SAPIC mode and physical destination mode the
IRQ target is one particular CPU and cannot be a mask of several
CPUs. Only the first non-zero bit is taken into account.
Usage examples
The target CPU has to be specified as a hexadecimal CPU mask. The
first non-zero bit is the selected CPU. This format has been kept for
compatibility reasons with i386.
Set the delivery mode of interrupt 41 to fixed and route the
interrupts to CPU #3 (logical CPU number) (2^3=0x08)::
echo "8" >/proc/irq/41/smp_affinity
Set the default route for IRQ number 41 to CPU 6 in lowest priority
delivery mode (redirectable)::
echo "r 40" >/proc/irq/41/smp_affinity
The output of the command::
cat /proc/irq/IRQ#/smp_affinity
gives the target CPU mask for the specified interrupt vector. If the CPU
mask is preceded by the character "r", the interrupt is redirectable
(i.e. lowest priority mode routing is used), otherwise its route is
Initialization and default behavior
If the platform features IRQ redirection (info provided by SAL) all
IO-SAPIC interrupts are initialized with CPU#0 as their default target
and the routing is the so called "lowest priority mode" (actually
fixed SAPIC mode with hint). The XTP chipset registers are used as hints
for the IRQ routing. Currently in Linux XTP registers can have three
- minimal for an idle task,
- normal if any other task runs,
- maximal if the CPU is going to be switched off.
The IRQ is routed to the CPU with lowest XTP register value, the
search begins at the default CPU. Therefore most of the interrupts
will be handled by CPU #0.
If the platform doesn't feature interrupt redirection IOSAPIC fixed
routing is used. The target CPUs are distributed in a round robin
manner. IRQs will be routed only to the selected target CPUs. Check
cat /proc/interrupts
On large (multi-node) systems it is recommended to route the IRQs to
the node to which the corresponding device is connected.
For systems like the NEC AzusA we get IRQ node-affinity for free. This
is because usually the chipsets on each node redirect the interrupts
only to their own CPUs (as they cannot see the XTP registers on the
other nodes).