I often go out then remotely access to my home PC to work. Sometimes I use that PC at home, running a desktop session then forget to logout when leaving home. At a consequence, many programs run and waste the CPU, RAM. How to logout that session when I'm not at home?

There is a command to do that, loginctl. So, from the coffee shop, I just SSH to my home PC, then run this:

loginctl terminate-user $USER

It will logout the desktop session, and also current SSH session. There are other loginctl subcommands to logout only a specific session. Just discover yourselves!