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This is a quick example of how to use kdb.
Boot kernel with arguments:
console=ttyS0,115200 kgdboc=ttyS0,115200
OR
Configure kgdboc after the kernel booted; assuming you are using a serial port console:
echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
Enter the kernel debugger manually or by waiting for an oops or fault. There are several ways you can enter the kernel debugger manually; all involve using the sysrq-g, which means you must have enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.
When logged in as root or with a super user session you can run:
echo g > /proc/sysrq-triggerExample using minicom 2.2
Press:
Control-aPress:
fPress:
gWhen you have telneted to a terminal server that supports sending a remote break
Press:
Control-]Type in:
send breakPress:
EnterPress:
g
From the kdb prompt you can run the "help" command to see a complete list of the commands that are available.
Some useful commands in kdb include:
lsmod -- Shows where kernel modules are loaded
ps -- Displays only the active processes
ps A -- Shows all the processes
summary -- Shows kernel version info and memory usage
bt -- Get a backtrace of the current process using dump_stack()
dmesg -- View the kernel syslog buffer
go -- Continue the system
When you are done using kdb you need to consider rebooting the system or using the "go" command to resuming normal kernel execution. If you have paused the kernel for a lengthy period of time, applications that rely on timely networking or anything to do with real wall clock time could be adversely affected, so you should take this into consideration when using the kernel debugger.