Modern Linux kernels (from 2.6.27) embed a very powerful tracing engine called Ftrace. Ftrace is the rough kernel equivalent of the user space strace(1) utility, but that would be short-selling it! Ftrace allows the sysad (or developer, tester, or anyone with root privileges really) to literally look under the hood, seeing every single function being executed in kernel space, who (which thread) executed it, how long it ran for, what APIs it invoked, with interrupts (hard and soft) included as they occur, various types of latency measurements, and more. You can use Ftrace to learn about how system utilities, applications, and the kernel actually work, as well as to perform deep tracing at the level of the OS.
Here, in this book, we refrain from delving into the depths of raw Ftrace usage (as it deviates from the subject at hand); instead, it is just quicker and easier to use a user space wrapper over Ftrace, a more convenient interface to it, called...