Troubleshoot your Linux instance using SysRq

The System Request (SysRq) key, which is sometimes referred to as "magic SysRq", can be used to directly send the kernel a command, outside of a shell, and the kernel will respond, regardless of what the kernel is doing. For example, if the instance has stopped responding, you can use the SysRq key to tell the kernel to crash or reboot. For more information, see Magic SysRq key in Wikipedia.

Prerequisites

Before you can configure and use SysRq, you must grant access to the serial console. For more information, see Configure access to the EC2 Serial Console.

Configure SysRq

To configure SysRq, you enable the SysRq commands for the current boot cycle. To make the configuration persistent, you can also enable the SysRq commands for subsequent boots.

To enable all SysRq commands for the current boot cycle

1. Connect to your instance.

2. Run the following command.

   [ec2-user ~]$ sudo sysctl -w kernel.sysrq=1

   Note

   This setting will clear on the next reboot.

To enable all SysRq commands for subsequent boots

1. Create the file /etc/sysctl.d/99-sysrq.conf and open it in your favorite editor.

   [ec2-user ~]$ sudo vi /etc/sysctl.d/99-sysrq.conf

2. Add the following line.

   kernel.sysrq=1

3. Reboot the instance to apply the changes.

   [ec2-user ~]$ sudo reboot

4. At the login prompt, enter the user name of the password-based user that you set up previously, and then press Enter.

5. At the Password prompt, enter the password, and then press Enter.

Use SysRq

You can use SysRq commands in the EC2 Serial Console browser-based client or in an SSH client. The command to send a break request is different for each client.

To use SysRq, choose one of the following procedures based on the client that you are using.

Browser-based client

To use SysRq in the serial console browser-based client

1. Connect to the instance's serial console.

2. To send a break request, press CTRL+0 (zero). If your keyboard supports it, you can also send a break request using the Pause or Break key.

   [ec2-user ~]$ CTRL+0

3. To issue a SysRq command, press the key on your keyboard that corresponds to the required command. For example, to display a list of SysRq commands, press h.

   [ec2-user ~]$ h

   The h command outputs something similar to the following.

   [ 1169.389495] sysrq: HELP : loglevel(0-9) reboot(b) crash(c) terminate-all-tasks(e) memory-full-oom-kill(f) kill-all-tasks(i) thaw-filesystems
   (j) sak(k) show-backtrace-all-active-cpus(l) show-memory-usage(m) nice-all-RT-tasks(n) poweroff(o) show-registers(p) show-all-timers(q) unraw(r
   ) sync(s) show-task-states(t) unmount(u) show-blocked-tasks(w) dump-ftrace-buffer(z)

SSH client

To use SysRq in an SSH client

1. Connect to the instance's serial console.

2. To send a break request, press ~B (tilde, followed by uppercase B).

   [ec2-user ~]$ ~B

3. To issue a SysRq command, press the key on your keyboard that corresponds to the required command. For example, to display a list of SysRq commands, press h.

   [ec2-user ~]$ h

   The h command outputs something similar to the following.

   [ 1169.389495] sysrq: HELP : loglevel(0-9) reboot(b) crash(c) terminate-all-tasks(e) memory-full-oom-kill(f) kill-all-tasks(i) thaw-filesystems
   (j) sak(k) show-backtrace-all-active-cpus(l) show-memory-usage(m) nice-all-RT-tasks(n) poweroff(o) show-registers(p) show-all-timers(q) unraw(r
   ) sync(s) show-task-states(t) unmount(u) show-blocked-tasks(w) dump-ftrace-buffer(z)

   Note

   The command that you use for sending a break request might be different depending on the SSH client that you're using.