Red Hat Knowledgebase

In addition to the usual suspects such as a system report and a detailed description of the situation (for example, "My system started to randomly hang after I applied a firmware update for my RAID card"), the information produced by the SysRq keys will be extremely helpful. To gather this information, you will need to be able to:

1. Edit /etc/grub.conf
2. Edit /etc/sysctl.conf
3. Attach a serial console to the serial port of the system which is hanging. In this case, a "serial console" is commonly another computer attached via a null modem cable and running a terminal emulator such as minicom, which is included with Red Hat Enterprise Linux.

First, you will need to edit /etc/grub.conf to use the serial port as a console. Open the file in your favorite text editor and find the kernel line which corresponds to your currently running kernel. Add the following at the end of that line:

console=tty0 console=ttyS0,9600n8

Second, open /etc/sysctl.conf in your favorite text editor and add the following line:

kernel.sysrq = 1

Also ensure that there are no lines in that file which say kernel.sysrq = 0. If there are, remove them or comment them out.

Finally, attach the serial console to the system and restart the system. You should see the kernel messages appear on the console as the system starts up. If you do not, then the console is not connected correctly. Ensure that it is connected to the correct serial port on the system, that it is connected using a null modem cable, and that all flow control on the serial console is off.

The next time your system hangs, press the following key combinations:

For Red Hat Enterprise Linux 2.1:

• Alt-SysRq-T
• Alt-SysRq-P (press this key combination at least twice for each CPU that is present in the system
• Alt-SysRq-M

For Red Hat Enterprise Linux 3 and 4:

• Alt-SysRq-T
• Alt-SysRq-W
• Alt-SysRq-M

After you press each key combination, information should appear on the serial console. Attach this information into your support ticket.