The /proc/interrupts file provides a real-time view of interrupt request (IRQ) statistics on Linux systems. Each line represents a unique interrupt source, showing how many times it's been triggered per CPU.

Here's what each column represents:

1. IRQ number
2-N. Interrupt counts per CPU core
N+1. Interrupt controller type (IRQ-IO-APIC, IR-PCI-MSI, etc.)
N+2. IRQ number and trigger mode
N+3. Device name associated with the interrupt

The number you see (like 524288-edge) actually contains two critical pieces of information:

• First part (numeric): This is the interrupt vector number used by the system. For PCI MSI (Message Signaled Interrupts), this represents the allocated message data.
• Second part (edge/level): Indicates the trigger mode:
  • edge: Interrupt triggered on signal transition (rising or falling edge)
  • level: Interrupt remains active while signal is at specified level
  • fasteoi: Fast End Of Interrupt handling

Let's examine two common scenarios:

1. Traditional IO-APIC interrupt:

  0:         22          0  IR-IO-APIC   2-edge      timer

Here '2-edge' means:
- Vector number: 2
- Trigger mode: edge-sensitive

2. PCI MSI interrupt:

123:      25164    5760490  IR-PCI-MSI 1048576-edge      enp2s0

Here '1048576-edge' means:
- Message data: 1048576 (used for MSI/MSI-X)
- Trigger mode: edge-sensitive

The trigger mode affects how your system handles interrupts:

• Edge-triggered: Better for high-performance devices as they don't need continuous line assertion
• Level-triggered: Simpler but may require masking during handling

You can view and modify IRQ affinity with:

# Check current affinity
cat /proc/irq/123/smp_affinity

# Set affinity to CPU 0
echo 1 > /proc/irq/123/smp_affinity

For deeper analysis:

# View detailed MSI information
lspci -vvv | grep -A 5 MSI

# Check interrupt distribution
watch -n 1 "cat /proc/interrupts | grep enp2s0"

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The /proc/interrupts file in Linux provides crucial information about interrupt request (IRQ) statistics. Here's the complete breakdown of each column:

IRQ_NUM : CPU0_CNT CPU1_CNT ... : IRQ_TYPE   : IRQ_CONFIG   : DEVICE_NAME

The fifth column represents the interrupt configuration, consisting of two parts:

• Numerical part (e.g., 524288): This is the interrupt vector or identifier specific to the IRQ source
• Trigger type (e.g., edge): Indicates the interrupt triggering mode (edge-triggered or level-triggered)

Here are some common patterns you'll encounter:

# MSI (Message Signaled Interrupts) configuration
IRQ123: 25164 5760490 IR-PCI-MSI 1048576-edge enp2s0
# APIC configuration 
IRQ0: 22 0 IR-IO-APIC 2-edge timer
# Legacy ISA configuration
IRQ1: 2 0 IR-IO-APIC 1-edge i8042

Let's examine some real-world cases:

Network Interface (enp2s0)

IRQ123: 25164 5760490 IR-PCI-MSI 1048576-edge enp2s0

This shows a high-performance network interface using MSI with:

• Vector: 1048576 (0x100000)
• Trigger: Edge
• Balanced across both CPUs

GPU Interrupt (amdgpu)

IRQ124: 17 5424414 IR-PCI-MSI 524288-edge amdgpu

This AMD GPU interrupt shows:

• Vector: 524288 (0x80000)
• Trigger: Edge
• Mostly handled by CPU1

You can verify CPU affinity for specific IRQs:

# cat /proc/irq/123/smp_affinity
3

Where '3' in binary (11) means both CPU0 and CPU1 can handle this interrupt.

To change the affinity for IRQ 123 to CPU0 only:

echo 1 > /proc/irq/123/smp_affinity

For high-performance applications:

• Use irqbalance for automatic distribution
• Consider manual pinning for latency-sensitive applications
• MSI-X generally provides better performance than legacy interrupts

Use these commands for deeper analysis:

# View detailed IRQ info
lspci -vvv | grep -A 30 MSI
# Monitor interrupt rates
watch -d cat /proc/interrupts