klogd: The Kernel Logging Dæmon
Linux beginners are unlikely to encounter this, but more seasoned users will often have more than one bootable kernel on their system at a time. If the box is a hobbyist or kernel hacker's box, it is likely to have a number of stable and a number of development series kernels on it. I myself always keep three generations of stable kernels on my systems, so that if a bug should show up, I can immediately reboot into the older kernel.
When klogd starts, it identifies the kernel version (all kernels since 1.3.43 put version information in the map files) and then looks at:
/boot/System.map /System.map /usr/src/linux/System.map
It will use the kernel version information to choose the correct one, if possible. When I have a development series kernel on a box, I leave the stable kernel map in /boot and I leave the development kernel map in /usr/src/linux. As for my two “old” stable kernels, I just live with the fact that klogd will not be able to resolve addresses in the event of a fault if I boot into them. Remember, you can use the -k switch on klogd to force it to use a particular map file if you build an archive of them.
Just bear in mind as you read this discussion that these events are so rare that in 15 machine years (three machines running Linux 24x7 for five years), I have seen this happen twice, and both times it was due to failing hardware.
In addition to the command-line switches, klogd will respond to certain signals. You send signals with the kill command.
The signals klogd responds to are:
SIGTSTP suspends and SIGCONT resumes kernel logging. The resuming includes re-initialization, so you can use this to, for example, unmount the /proc file system without killing klogd:
kill -TSTP <pid> umount /proc kill -CONT <pid>
See the Memory Address Resolution section for more information.
These signals all gracefully shut down klogd.
The klogd works with syslogd to handle the dispatch of kernel messages. It exists solely because the kernel itself is unable to use the syslog API directly. Klogd provides for resolving raw memory addresses into kernel symbol names.