ftrace (Function Tracer) is a tracing framework for the

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

. Although its original name, Function Tracer, came from ftrace's ability to record information related to various

function call In computer programming, a function (also procedure, method, subroutine, routine, or subprogram) is a callable unit of software logic that has a well-defined interface and behavior and can be invoked multiple times. Callable units provide a p ...

s performed while the kernel is running, ftrace's tracing capabilities cover a much broader range of kernel's internal operations.

Details

With its various tracer plugins, ftrace can be targeted at different static tracepoints, such as

scheduling A schedule (, ) or a timetable, as a basic time-management tool, consists of a list of times at which possible tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order in which such things ...

events,

interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted ...

memory-mapped I/O Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer (often mediating access via chipset). An altern ...

, CPU power state transitions, and operations related to file systems and

virtualization In computing, virtualization (abbreviated v12n) is a series of technologies that allows dividing of physical computing resources into a series of virtual machines, operating systems, processes or containers. Virtualization began in the 1960s wit ...

. Also, dynamic tracking of kernel function calls is available, optionally restrictable to a subset of functions by using globs, and with the possibility to generate

call graph A call graph (also known as a call multigraph) is a control-flow graph, which represents calling relationships between subroutines in a computer program. Each node represents a procedure and each edge ''(f, g)'' indicates that procedure ''f'' c ...

s and provide

stack Stack may refer to: Places * Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group * Blue Stack Mountains, in Co. Donegal, Ireland People * Stack (surname) (including a list of people ...

usage reports. At the same time, ftrace can be used to measure various latencies within the Linux kernel, such as for how long interrupts or preemption are disabled. An ftrace-enabled Linux kernel is built by enabling the kernel configuration option. The entire runtime interaction with ftrace is performed through readable and writable virtual files contained in a specifically mounted debugfs file system; as a result, ftrace requires no specialized

userspace A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware prote ...

utilities to operate. However, there are additional userspace utilities that provide more advanced features for data recording, analysis and visualization; examples of such utilities are ''trace-cmd'' and ''KernelShark''. Internally, ftrace relies on the gcc's profiling mechanism to prepend

machine instruction In computer programming, machine code is computer code consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). For conventional binary computers, machine code is the binaryOn nonbi ...

s to the compiled versions of all source-level kernel functions, which redirect the execution of functions to the ftrace's trampolines and tracer plugins that perform the actual tracing. These "

entry point In computer programming, an entry point is the place in a program where the execution of a program begins, and where the program has access to command line arguments. To start a program's execution, the loader or operating system passes co ...

" instructions created by gcc are altered by ftrace when the kernel is booted, and varied later at runtime by ftrace between NOPs and actual jumps to the tracing trampolines, depending on the tracing types and options configured at runtime. ftrace is developed primarily by Steven Rostedt, and it was merged into the Linux kernel mainline in kernel version 2.6.27, which was released on October 9, 2008.

References

External links

Debugging the kernel using Ftrace part 1
an
part 2
LWN.net, December 2009, by Steven Rostedt
Secrets of the Ftrace function tracer
LWN.net, January 20, 2010, by Steven Rostedt
Ftrace: The hidden light switch
LWN.net, August 13, 2014, by Brendan Gregg
The source of the e1000e corruption bug
LWN.net, October 21, 2008, by Jonathan Corbet {{Operating system Debugging Free software programmed in C Linux kernel features Unix programming tools

Details

See also

References

External links