ftrace (Function Tracer) is a
tracing framework for the
Linux kernel. Although its original name, Function Tracer, came from ftrace's ability to record information related to various
function call
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions may ...
s performed while the
kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel (image processing), a matrix used for image convolution
* Compute kernel, in GPGPU programming
* Kernel method, in machine learn ...
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 are ...
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, ...
s,
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. An alternative approach is using dedicated I/O pr ...
,
CPU power state transitions, and operations related to
file systems and
virtualization
In computing, virtualization or virtualisation (sometimes abbreviated v12n, a numeronym) is the act of creating a virtual (rather than actual) version of something at the same abstraction level, including virtual computer hardware platforms, stor ...
. 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'' cal ...
s and provide
stack 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 file
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
s contained in a specifically mounted
debugfs file system; as a result, ftrace requires no specialized
userspace
A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.
Kernel ...
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 any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
s to the
compiled versions of all
source
Source may refer to:
Research
* Historical document
* Historical source
* Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence
* Source (journalism), a person, publication, publishing institute o ...
-level kernel functions, which redirect the execution of functions to the ftrace's
trampolines
A trampoline is a device consisting of a piece of taut, strong fabric stretched between a steel frame using many coiled springs. Not all trampolines have springs, as the Springfree Trampoline uses glass-reinforced plastic rods. People bounce on ...
and tracer plugins that perform the actual tracing. These "
entry point
In computer programming
Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programmin ...
" 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
The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU o ...
in kernel version 2.6.27, which was released on October 9, 2008.
See also
*
DTrace
DTrace is a comprehensive dynamic tracing framework originally created by Sun Microsystems for troubleshooting kernel and application problems on production systems in real time.
Originally developed for Solaris, it has since been released und ...
a tracing framework for troubleshooting kernel and application problems, originally developed for Solaris
*
ktrace a BSD Unix and Mac OS X utility that traces kernel–program interactions
*
ltrace
ltrace is a debugging utility in Linux, used to display the calls a userspace application makes to shared libraries. It does this by hooking into the dynamic loading system, allowing it to insert shims which display the parameters which the appl ...
a Linux debugging utility, displays the calls a userland application makes to shared libraries
*
strace
strace is a diagnostic, debugging and instructional userspace utility for Linux. It is used to monitor and tamper with interactions between processes and the Linux kernel, which include system calls, signal deliveries, and changes of proce ...
a debugging utility for Linux and some other Unix-like systems, monitors system calls used by a program and all received signals
*
SystemTap a scripting language and utility used for instrumenting Linux systems
References
External links
Debugging the kernel using Ftrace part 1an
part 2 LWN.net
LWN.net is a computing webzine with an emphasis on free software and software for Linux and other Unix-like operating systems. It consists of a weekly issue, separate stories which are published most days, and threaded discussion attached to ...
, 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