Linux Notes: CPU Control

1. The information presented here is intended for educational use by qualified computer technologists.
2. The information presented here is provided free of charge, as-is, with no warranty of any kind.

Edit: 2022-01-12 (updated the script)

BASH script to control CPU cores

Notes:

• this is an unfinished script (the NUMA stuff was never implemented)
  • I used this on a 2-socket HPE DL385p_gen8 on June-2021 to disable all the cores associated with the second CPU
    • this saved me from having to drive 160 km to swap out a box and/or unplug one CPU chip
    • also, I did this on a running system so there is the convenience of not having to shutdown etc.
  • this script should work on a 2-socket DL380p (but never tested by me)
  • this script should work on a 4-socket DL580 or DL585 (but never tested by me)
  • caveats:
    • this script begins with three safety parameters (SOCKETS, CORES, THREADS). These must be changed for any platform other than the one I wrote this for so BE VERY CAREFUL.
    • If you do not know what you are doing then please call your resident computer technologist.
    • changes made by this script are not permanent (so it you screw up then just power off followed by a reboot)

#!/bin/sh
# ====================================================
# file    : neil_cpu_control.sh
# author  : Neil Rieck
# edit    : 2021-06-18
# edit    : 2021-06-19
# edit    : 2022-01-12
# notes   : USE AT YOUR OWN RISK!
# hardware:
# 1) display hardware:
#	lscpu | grep 'Socket(s)'
#	lscpu | grep 'Core(s)'
#	lscpu | grep 'Thread(s)'
#	lscpu -e (or lscpu -p)
#	lscpu -y (default)
# 2) examples for a typical 2-socket DL385p
# a) display CPUs
#	ls -la /sys/devices/system/cpu
# b) displays the current CPU configuration
#	lscpu
#    yields:
#	NUMA node0 CPU(s): 0,2,4,6,8,10
#	NUMA node1 CPU(s): 12,14,16,18,20,22
#	NUMA node2 CPU(s): 1,3,5,7,9,11
#	NUMA node3 CPU(s): 13,15,17,19,21,23
# c) after running this tool
#    yields
#	NUMA node0 CPU(s): 0,2,4,6,8,10
#	NUMA node1 CPU(s): 12,14,16,18,20,22
#	NUMA node2 CPU(s):     
#	NUMA node3 CPU(s):     
# ====================================================
set -e					# stop on error
SOCKETS=2				# number of CPU chips
CORES=6					# cores per chip
THREADS=2				# threads per core
MAXPR=$((SOCKETS * CORES * THREADS))	# max processors
HELP1="   example:  "$0" noun unit verb optional-safeword"
HELP2="   example:  "$0" socket 2 enable"
HELP=${HELP1}$'\n'${HELP2}
PASS="ncc1701d"
# ====================================================
echo ""
echo "pgm: "$0
echo "note: the first chip is in socket #1"
echo "===================================="
echo "-i-hardware:"
lscpu | grep 'Socket(s)'
lscpu | grep 'Core(s)'
lscpu | grep 'Thread(s)'
echo "-i-script constants:"
echo "   sockets         : "${SOCKETS}
echo "   cores-per-socket: "${CORES}
echo "   max-processors  : "${MAXPR}
# ====================================================
if [ "$1" == "" ]
then
    NOUN="blank"
else
    NOUN=$1
fi
if [ "$2" == "" ]
then
    UNIT=-1
else
    UNIT=$2
fi
if [ "$3" == "" ]
then
    VERB=-1
else
    VERB=$3
fi
if [ "$4" == "" ]
then
    SAFEWORD="test"
else
    SAFEWORD=$4
fi
# ====================================================
if [ $SOCKETS != 2 ]; then
    echo "-e-only for use on 2-socket systems"
    exit -1
fi
if [[ ( $NOUN != "socket" ) && ( $NOUN != "numa" ) ]]; then
    echo "-e-bad NOUN (not one of: socket, numa)"
    printf "%s\n\n" "${HELP}"
    exit -1
fi
if [[ (( $UNIT -le 0 )) || (( $UNIT -gt $SOCKETS )) ]]; then
    echo "-e-bad UNIT (not legal or not in range)"
    printf "%s\n\n" "${HELP}"
    exit -1
fi
if [[ ( $VERB != "enable" ) && ( $VERB != "disable" ) ]]; then
   echo "-e-bad VERB. (not one of: enable, disable)"
   printf "%s\n\n" "${HELP}"
   exit -1
fi
if [[ ( $SAFEWORD != $PASS )  && ( $SAFEWORD != "test" ) ]]; then
   echo "-e-bad SAFEWORD. (defaults to test mode)";
   printf "%s\n\n" "${HELP}"
   exit -1
fi
#======================================
#	doit()
#	entry:	i = CPU#
#		NOUN
#		VERB
#======================================
function doit() {
    if [[ $VERB = "disable" ]]; then
	STATE="0"
    else
	STATE="1"
    fi
    # we always skip processor-0 just in case
    if [[ ${i} = 0 ]]; then
	echo "skip" ${STATE} "> /sys/devices/system/cpu/cpu"${i}"/online skip" 
	return 0
    fi
    if [[ $SAFEWORD = $PASS ]]; then
	echo "writing" $STATE "> /sys/devices/system/cpu/cpu"${i}"/online"
	echo ${STATE} > /sys/devices/system/cpu/cpu${i}/online
    else
	echo "script-test" ${STATE} "> /sys/devices/system/cpu/cpu"${i}"/online"
    fi
    return 0
}
echo "-i-processing begins:"
case $NOUN in

"socket" )
    start=$((UNIT-1))
    stop=$((MAXPR))
    step=$((SOCKETS))
    echo "start: "${start}" stop: "${stop}" step: "${step}
    for (( i=${start}; i<${stop}; i=i+${step} )); do
	doit;
    done
    ;;

"numa")
    echo "-w-not implemented"

esac
#
echo "done"
exit 0

---

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Neil Rieck
Waterloo, Ontario, Canada.