This tracer helps developers to optimize boot times: it records
the timings of the initcalls and traces key events and the identity
of tasks that can cause boot delays, such as context-switches.

Its aim is to be parsed by the scripts/bootgraph.pl tool to
produce pretty graphics about boot inefficiencies, giving a visual
representation of the delays during initcalls - but the raw
/debug/tracing/trace text output is readable too.

You must pass in initcall_debug and ftrace=initcall to the kernel
command line to enable this on bootup.

source code:

kernel/trace/trace_boot.c