Selecting this option causes printk() to add a caller "thread id" (if
in task context) or a caller "processor id" (if not in task context)
to every message.

This option is intended for environments where multiple threads
concurrently call printk() for many times, for it is difficult to
interpret without knowing where these lines (or sometimes individual
line which was divided into multiple lines due to race) came from.

Since toggling after boot makes the code racy, currently there is
no option to enable/disable at the kernel command line parameter or
sysfs interface.