pin-irq.sh

#!/usr/bin/env bash
# pin-irq.sh — Pin NIC TX/RX IRQs and the scancraft process to the same CPU core.
#
# Usage:
#   sudo ./pin-irq.sh [OPTIONS] -- [SCANCRAFT ARGS]
#
# Options:
#   -i IFACE   Network interface to optimise (default: auto-detect default route iface)
#   -c CPU     CPU core number to pin to (default: 2)
#   -n         Dry run — print commands without executing them
#   -h         Show this help
#
# Examples:
#   sudo ./pin-irq.sh -i eth0 -c 3 -- --rate 10000000
#   sudo ./pin-irq.sh -n -i eth0 -c 2 -- --rate 5000000

set -euo pipefail

# ---- defaults ----------------------------------------------------------------
IFACE=""
CPU=2
DRY_RUN=0
BINARY="./target/release/scancraft"

# ---- argument parsing --------------------------------------------------------
while getopts ":i:c:nh" opt; do
    case $opt in
        i) IFACE="$OPTARG" ;;
        c) CPU="$OPTARG" ;;
        n) DRY_RUN=1 ;;
        h)
            sed -n '2,/^$/p' "$0"
            exit 0
            ;;
        :) echo "Option -$OPTARG requires an argument." >&2; exit 1 ;;
        \?) echo "Unknown option: -$OPTARG" >&2; exit 1 ;;
    esac
done
shift $((OPTIND - 1))

# Remaining args after -- are passed to scancraft
SCANCRAFT_ARGS=("$@")

# ---- helpers -----------------------------------------------------------------
run() {
    if [[ $DRY_RUN -eq 1 ]]; then
        echo "[dry-run] $*"
    else
        echo "+ $*"
        eval "$*"
    fi
}

require_root() {
    if [[ $EUID -ne 0 && $DRY_RUN -eq 0 ]]; then
        echo "Error: must be run as root (use sudo, or -n for a dry run)." >&2
        exit 1
    fi
}

# ---- resolve interface -------------------------------------------------------
if [[ -z "$IFACE" ]]; then
    IFACE=$(ip route show default | awk '/default/ { print $5; exit }')
    if [[ -z "$IFACE" ]]; then
        echo "Error: could not auto-detect default interface; use -i IFACE." >&2
        exit 1
    fi
    echo "Auto-detected interface: $IFACE"
fi

# ---- validate CPU ------------------------------------------------------------
NUM_CPUS=$(nproc)
if (( CPU < 0 || CPU >= NUM_CPUS )); then
    echo "Error: CPU $CPU is out of range (0–$((NUM_CPUS - 1)))." >&2
    exit 1
fi

# IRQ affinity bitmask: bit N set = CPU N.  CPU 2 → 0x4, CPU 3 → 0x8, etc.
IRQ_MASK=$(printf '%x' $((1 << CPU)))

require_root

# ---- show NUMA topology (informational) -------------------------------------
echo ""
echo "=== NUMA / PCI topology ==="
if command -v lscpu &>/dev/null; then
    lscpu | grep -E "NUMA|Socket|Core" || true
fi
if [[ -f /sys/class/net/"$IFACE"/device/numa_node ]]; then
    NUMA_NODE=$(cat /sys/class/net/"$IFACE"/device/numa_node)
    if [[ "$NUMA_NODE" == "-1" || "$NUMA_NODE" == "" ]]; then
        echo "NIC $IFACE: single-socket / non-NUMA system — any CPU is equidistant."
    else
        echo "NIC $IFACE is on NUMA node: $NUMA_NODE"
        if command -v numactl &>/dev/null; then
            echo "CPUs on that node: $(numactl --hardware | grep "node $NUMA_NODE cpus" | cut -d: -f2)"
        fi
    fi
fi
echo ""

# ---- warn if this looks like a WiFi interface --------------------------------
if [[ -d /sys/class/net/"$IFACE"/wireless ]]; then
    echo "WARNING: $IFACE appears to be a WiFi interface."
    echo "         WiFi cannot sustain high pps due to CSMA/CA, ACK overhead, and"
    echo "         half-duplex behaviour.  Use a wired (Ethernet) interface instead."
    echo ""
fi

# ---- find IRQs for the interface --------------------------------------------
echo "=== IRQs for $IFACE ==="

# /proc/interrupts truncates driver names, so try progressively shorter prefixes.
# E.g. "enp6s0" may appear as "enp6s0", "enp6s", or just "enp6".
mapfile -t IRQ_LINES < <(grep -E "\b${IFACE}\b" /proc/interrupts || true)

if [[ ${#IRQ_LINES[@]} -eq 0 ]]; then
    echo "Note: exact match not found; trying truncated name prefixes..."
    for prefix_len in ${#IFACE} $((${#IFACE}-1)) $((${#IFACE}-2)) 5 4; do
        [[ $prefix_len -lt 3 ]] && break
        prefix="${IFACE:0:$prefix_len}"
        mapfile -t IRQ_LINES < <(grep -E "${prefix}" /proc/interrupts || true)
        if [[ ${#IRQ_LINES[@]} -gt 0 ]]; then
            echo "  Matched using prefix '${prefix}'."
            break
        fi
    done
fi

if [[ ${#IRQ_LINES[@]} -eq 0 ]]; then
    echo "Error: no IRQ lines matched for $IFACE." >&2
    echo "       Check /proc/interrupts manually — relevant lines:" >&2
    grep -E "eth|eno|enp|ens|wlp|wlan" /proc/interrupts | head -20 || true
    exit 1
fi

declare -a IRQS=()
for line in "${IRQ_LINES[@]}"; do
    irq_num=$(echo "$line" | awk -F: '{ gsub(/ /, "", $1); print $1 }')
    echo "  IRQ $irq_num: $(echo "$line" | awk '{ $1=$2=$3=$4=$5=$6=$7=$8=$9=$10=$11=""; print }' | xargs)"
    IRQS+=("$irq_num")
done
echo ""

# ---- pin each IRQ to the chosen CPU -----------------------------------------
echo "=== Pinning ${#IRQS[@]} IRQ(s) to CPU $CPU (affinity mask 0x$IRQ_MASK) ==="
for irq in "${IRQS[@]}"; do
    affinity_file="/proc/irq/$irq/smp_affinity"
    if [[ ! -f "$affinity_file" ]]; then
        echo "  Skipping IRQ $irq — $affinity_file not found."
        continue
    fi
    run "echo $IRQ_MASK > $affinity_file"
    if [[ $DRY_RUN -eq 0 ]]; then
        actual=$(cat "$affinity_file")
        echo "  IRQ $irq affinity is now: $actual"
    fi
done
echo ""

# ---- disable IRQ coalescing --------------------------------------------------
echo "=== Disabling IRQ coalescing on $IFACE ==="
if command -v ethtool &>/dev/null; then
    run "ethtool -C $IFACE tx-usecs 0 tx-frames 1 rx-usecs 0 rx-frames 1"
else
    echo "  Warning: ethtool not found; skipping coalescing configuration."
fi
echo ""

# ---- optionally set RPS/RFS --------------------------------------------------
# Receive Packet Steering — steer all RX softirqs to our chosen CPU.
RPS_FILE="/sys/class/net/$IFACE/queues/rx-0/rps_cpus"
if [[ -f "$RPS_FILE" ]]; then
    echo "=== Setting RPS on $IFACE rx-0 to CPU $CPU ==="
    run "echo $IRQ_MASK > $RPS_FILE"
    echo ""
fi

# ---- run scancraft pinned to the chosen CPU ----------------------------------
if [[ ${#SCANCRAFT_ARGS[@]} -gt 0 ]]; then
    if [[ ! -x "$BINARY" ]]; then
        echo "Warning: $BINARY not found or not executable; skipping launch."
        echo "         Build with: cargo build --release"
    else
        echo "=== Launching scancraft on CPU $CPU ==="
        run "taskset -c $CPU $BINARY ${SCANCRAFT_ARGS[*]}"
    fi
else
    echo "No scancraft arguments given — skipping launch."
    echo "To run: sudo ./pin-irq.sh -i $IFACE -c $CPU -- --rate 10000000"
fi