IRQ vectors are displayed only when relevent. For example, the threshold vector does not exist on x86_64 platforms. Others are suppressed when the system is a uniprocessor. As of this writing, only i386 and x86_64 platforms support the new IRQ vector displays.

Of some interest is the introduction of the /proc/irq directory to 2.4. It could be used to set IRQ to CPU affinity, this means that you can “hook” an IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the irq subdir is one subdir for each IRQ, and two files; default_smp_affinity and prof_cpu_mask.

For example:

ls /proc/irq/
 
0  10  12  14  16  18  2  4  6  8  prof_cpu_mask
1  11  13  15  17  19  3  5  7  9  default_smp_affinity
 
 
ls /proc/irq/0/
 
smp_affinity

smp_affinity is a bitmask, in which you can specify which CPUs can handle the IRQ, you can set it by doing:

echo 1 > /proc/irq/10/smp_affinity

This means that only the first CPU will handle the IRQ, but you can also echo 5 which means that only the first and fourth CPU can handle the IRQ.

The contents of each smp_affinity file is the same by default:

cat /proc/irq/0/smp_affinity
 
ffffffff

The default_smp_affinity mask applies to all non-active IRQs, which are the IRQs which have not yet been allocated/activated, and hence which lack a /proc/irq/[0-9]* directory.