Configuration examples on the internets for configuring GRUB to boot serial console tend to turn off the regular console, for GRUB and for Linux. Here’s how you get serial console and regular keyboard/monitor console for both GRUB and Linux:
# Tell GRUB to use both consoles
echo '#! /bin/sh
echo "
serial --unit=0 --speed=115200 --word=8 --parity=no --stop=0
terminal_input --append serial
terminal_output --append serial
"' > /etc/grub.d/01_serial
chmod 755 /etc/grub.d/01_serial # Tell GRUB to have the kernel use both consoles
grep console=tty /etc/default/grub ||
echo >> /etc/default/grub 'GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0,115200n8"'# Make new GRUB config with those settings
grub2-mkconfig > /etc/grub2.cfg # CentOS
update-grub2# /dev/ttyS0 serial login – on older systems
systemctl enable serial-getty@ttyS0.service
systemctl start serial-getty@ttyS0.service