#!/bin/bash

# NAME
#   ./run-with-governor.sh - temporarily use specific CPU governor while a
#   command is running
#
# SYNOPSIS
#   run-with-governor GOVERNOR COMMAND [ARGS...]
#
# EXAMPLE
#   run-with-governor performance make -j8
#
#   Runs `make -j8` with the CPU governor set to "performance".
#
# EXIT STATUS
#   Exits with the status of COMMAND, or 1 if parameters are wrong GOVERNOR is
#   invalid.
#
# HINT
#   The sudoers file can be changed to allow cpupower without a sudo password
#   prompt (at your own risk):
#
#   1. Run `sudo visudo`
#
#   2. Add the following:
#      ```
#      # allow cpupower without password
#      myusername ALL=(root) NOPASSWD: /usr/bin/cpupower frequency-set -g *
#      myusername ALL=(root) NOPASSWD: /usr/bin/cpupower ^-c [0-9]+ frequency-set -g .+$
#      ```
#
# ---
#
# References:
#   https://man7.org/linux/man-pages/man1/cpupower.1.html
#   https://man7.org/linux/man-pages/man5/sudoers.5.html
#   https://docs.kernel.org/admin-guide/pm/cpufreq.html

set -euo pipefail

die () {
	echo $@ > /dev/stderr
	exit 1
}

if [[ $# -lt 2 ]]; then
    die "Usage: ${0} [governor] [command]"
fi

desired_governor=$1
shift

if ! grep -Fw "${desired_governor}" \
	/sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors >/dev/null;
then
    die "Governor doesn't exist"
fi

declare -A cpus=()
for f in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do
    num=${f//[^0-9]/}
    cpus["${num}"]=$(cat "${f}")
done

restore_governor() {
    for i in "${!cpus[@]}"; do
        sudo cpupower -c "${i}" frequency-set -g "${cpus[${i}]}" >/dev/null
    done
}
trap restore_governor EXIT

sudo cpupower frequency-set -g "${desired_governor}" >/dev/null

"$@"