When kernel developers want to communicate something about the state of a
running kernel, they tend to use printk(); that results in a log
entry that is intended — with varying success — to be human-readable. As it
happens, though, the
consumers of that information are often not human; the kernel’s log output
is also read by automated monitoring systems that are looking for
problems. The result is an impedance mismatch that often ends with the
monitoring system missing important messages. The printk()
format indexing patch set is the latest of many attempts to improve
this situation.

Source: LWN.net – [$] printk() indexing