chcpu can disable and enable CPU cores online

chcpu -d 4,5,6,7,8,9,10,11,12,13,14,15
chcpu -e 4,5,6,7,8,9,10,11,12,13,14,15

To disable or enable SMT / Hyperthreading online

echo off | sudo tee /sys/devices/system/cpu/smt/control
echo on | sudo tee /sys/devices/system/cpu/smt/control