#!/usr/bin/env bash
# Written by Draco (tytydraco @ GitHub)

write() {
	# Bail out if file does not exist
	[[ ! -f "$1" ]] && return 1

	# Make file writable in case it is not already
	chmod +w "$1" 2> /dev/null

	# Write the new value and bail if there's an error
	if ! echo "$2" > "$1" 2> /dev/null
	then
		echo "Failed: $1 → $2"
		return 1
	fi

	# Log the success
	echo "$1 → $2"
}

# Check for root permissions and bail if not granted
if [[ "$(id -u)" -ne 0 ]]
then
	echo "No root permissions. Exiting."
	exit 1
fi


# Log the date and time for records sake
echo "Time of execution: $(date)"

# We need to execute this multiple times because
# sched_downmirate must be less than sched_upmigrate, and
# sched_upmigrate must be greater than sched_downmigrate
for _ in $(seq 2)
do
	# Migrate tasks down at this much load
	write /proc/sys/kernel/sched_downmigrate 80
	write /proc/sys/kernel/sched_group_downmigrate "80 80"

	# Migrate tasks up at this much load
	write /proc/sys/kernel/sched_upmigrate 80
	write /proc/sys/kernel/sched_group_upmigrate "80 80"
done

# Limit max perf event processing time to this much CPU usage
write /proc/sys/kernel/perf_cpu_time_max_percent 5

# Do not group task groups automatically
write /proc/sys/kernel/sched_autogroup_enabled 0

# Execute child process before parent after fork
write /proc/sys/kernel/sched_child_runs_first 1

# Preliminary requirement for the following values
write /proc/sys/kernel/sched_tunable_scaling 0

# Reduce the maximum scheduling period for lower latency
write /proc/sys/kernel/sched_latency_ns 2000000

# Schedule 10 tasks in the guarenteed sched period
write /proc/sys/kernel/sched_min_granularity_ns 200000

# Require preeptive tasks to surpass half of a sched period in vmruntime
write /proc/sys/kernel/sched_wakeup_granularity_ns 1000000

# Reduce the frequency of task migrations
write /proc/sys/kernel/sched_migration_cost_ns 5000000

# Always allow sched boosting on top-app tasks
write /proc/sys/kernel/sched_min_task_util_for_colocation 0

# Improve real time latencies by reducing the scheduler migration time
write /proc/sys/kernel/sched_nr_migrate 8

# Disable scheduler statistics to reduce overhead
write /proc/sys/kernel/sched_schedstats 0

# Start non-blocking writeback later
write /proc/sys/vm/dirty_background_ratio 10

# Start blocking writeback later
write /proc/sys/vm/dirty_ratio 30

# Require dirty memory to stay in memory for longer
write /proc/sys/vm/dirty_expire_centisecs 3000

# Run the dirty memory flusher threads less often
write /proc/sys/vm/dirty_writeback_centisecs 3000

# Disable read-ahead for swap devices
write /proc/sys/vm/page-cluster 0

# Update /proc/stat less often to reduce jitter
write /proc/sys/vm/stat_interval 10

# Swap to the swap device at a fair rate
write /proc/sys/vm/swappiness 100

# Allow more inodes and dentries to be cached
write /proc/sys/vm/vfs_cache_pressure 60

if [[ -f "/sys/kernel/debug/sched_features" ]]
then
	# Consider scheduling tasks that are eager to run
	write /sys/kernel/debug/sched_features NEXT_BUDDY

	# Schedule tasks on their origin CPU if possible
	write /sys/kernel/debug/sched_features TTWU_QUEUE

	# Use a high resolution timer for the scheduler
	write /sys/kernel/debug/sched_features HRTICK
fi

if [[ -d "/dev/stune/" ]]
then
	# Artificially inflate top-app task load
	write /dev/stune/top-app/schedtune.boost 10

	# Prefer to schedule top-app tasks on idle CPUs
	write /dev/stune/top-app/schedtune.prefer_idle 1
fi

for cpu in /sys/devices/system/cpu/cpu*/cpufreq/
do
	avail_govs="$(cat "${cpu}scaling_available_governors")"
	case "$avail_govs" in
		*schedutil*)
			write "${cpu}scaling_governor" schedutil

			# Consider changing frequencies once per scheduling period
			write "${cpu}schedutil/up_rate_limit_us" 5000
			write "${cpu}schedutil/down_rate_limit_us" 5000
			write "${cpu}schedutil/rate_limit_us" 5000

			# Jump to max frequency at 90% load
			write "${cpu}schedutil/hispeed_load" 90
			write "${cpu}schedutil/hispeed_freq" "$(cat "${cpu}cpuinfo_max_freq")"
			;;
		*interactive*)
			write "${cpu}scaling_governor" interactive
		
			# Consider changing frequencies once per scheduling period
			write "${cpu}interactive/timer_rate" 5000
			write "${cpu}interactive/min_sample_time" 5000

			# Jump to max frequency at 90% load
			write "${cpu}interactive/go_hispeed_load" 90
			write "${cpu}interactive/hispeed_freq" "$(cat "${cpu}cpuinfo_max_freq")"
			;;
	esac
done

for queue in /sys/block/*/queue/
do
	# Choose the first governor available
	avail_scheds="$(cat "${queue}scheduler")"
	for sched in cfq noop kyber bfq mq-deadline none
	do
		if [[ "$avail_scheds" == *"$sched"* ]]
		then
			write "${queue}scheduler" "$sched"
			break
		fi
	done

	# Do not use I/O as a source of randomness
	write "${queue}add_random" 0

	# Disable I/O statistics accounting
	write "${queue}iostats" 0

	# Only attempt simple merges
	write "${queue}nomerges" 1

	# Force request completion on the origin CPU for maximum cache locality
	write "${queue}rq_affinity" 2

	# Reduce heuristic read-ahead in exchange for I/O latency
	write "${queue}read_ahead_kb" 16

	# Reduce the maximum number of I/O requests in exchange for latency
	write "${queue}nr_requests" 64
done

# Always return success, even if the last write fails
exit 0