To reboot a node, follow this recipe:
 go down to the computer room (A+Manu's internal phone number)
 try to log on the corresponding node(s), and to reboot it properly (command reboot)
 if you cannot log on, try the SysRq command sequence: press simultaneously the keys <alt>+<SysRq>+<###> where <###> is in the following order (mnemomic: Raising Skinny Elephants Is Utterly Boring, cf. /usr/src/linux/Documentation/sysrq.txt on any well installed linux system for more information):

• 'r' - Turns off keyboard raw mode and sets it to XLATE.
• 's' - Will attempt to sync all mounted filesystems.
• 'e' - Send a SIGTERM to all processes, except for init.
• 'i' - Send a SIGKILL to all processes, except for init.
• 'u' - Will attempt to remount all mounted filesystems read-only.
• 'b' - Will immediately reboot the system without syncing or unmounting your disks.

 If none of the above is possible, reset the computer by pressing the "reset" button, or power switch the computer by pressing the "power" button.