"Yes," he said, "they are quite plain: a hobbit's footprints. Pippin's I think. He is smaller than the other. And look at this!" He held up a thing that glittered in the sunlight. It looked like the new-opened leaf of a beech-tree, fair and strange in that treeless plain.
"The brooch of an elven-cloak!" cried Legolas and Gimli together.
"Not idly do the leaves of Lórien fall," said Aragorn. "This did not drop by chance: it was cast away as a token to any that might follow."
—The Two Towers, Book III, Chapter 2
One reality of working with the Linux kernel is that traditional
troubleshooting tools such as debuggers are much less accessible. Even the
printk function may be too heavyweight to be used in
time sensitive code paths such as interrupt handlers and scheduler code.
However, there are very robust tracing utilities available to the kernel
developer that allow you to record system behavior and review it after
In this studio, you will:
ftrace) via two utilities called
Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete.
As you work through these exercises, please record your answers, and when finished email your results to firstname.lastname@example.org with the phrase Kernel Tracing in the subject line.
Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.
strace ./do_nothing". This shows you all of the system calls that are made in the process of executing the program (in this case, all the effort it takes to simply do nothing). You can get information about interpreting the output of
straceon the manual page.
Pick out two system calls and describe what you think they are accomplishing
in this context. Every system call listed here has a manual page entry that
will help you decode what is going on. In this case, you should specify the
second chapter of the man pages, which covers system calls, otherwise it's
possible you might get the wrong page. For example, "
will bring up the documentation for the program called
man 2 write" gives you the documentation for the system
write. See the manual pages documentation
man man") for details.
straceto trace the
singprogram from last time. What system call is invoked by the call to
strace ./dense_mm 100" and compare that to the results of "
strace ./dense_mm 300". What is different about the execution of the program? Which two system calls are involved? What do you think is the motivation for using one over the other?
malloc. Allocate ten memory regions of 100 bytes each. Profile this program with
strace, how are those ten calls to
malloctranslated to system calls?
mallocin the standard C library?
ftrace(short for Function Tracer) and is a part of the Linux kernel. We will be using two utilities called
trace-cmdand Kernelshark to access the function tracer.
Install these utilites with the command "
sudo apt-get install
trace-cmd kernelshark" and leave this answer blank.
sudo trace-cmd record -e sched_switch ./dense_mm 100". Make sure that in the output for each CPU there are zero dropped events.
To explain that command a little further,
trace-cmd is the
tracing utility that lets us work with traces. The
specifies that we want to record a new trace. The option
-e sched_switch specifies that the event we want to trace is
invocations to the kernel function
sched_switch (which is the
function responsible for switching processes). Finally, the program to
record is given at the end.
As a result, a file called "
trace.dat" will be generated.
For more information, you can see the man
pages at "
man trace-cmd" and "
Leave this question blank.
kernelshark trace.dat". By default you will be looking at a timeline of each CPU in the system. Each process in the system will be given a unique color so you can track individual processes as they are scheduled on and off of processors as well as migrated between processors.
Start by zooming in on the trace until you can make out discrete events. To zoom in: press and hold the left mouse button; drag the cursor to the right; and then release to define a zoom window. Zooming out is the reverse: press and hold the left mouse button; drag the cursor to the left; and then release the mouse button.
Try zooming around your trace. Leave this answer blank.
dense_mmand activate it. Scroll down or enlarge the viewing window until you see the process timeline at the bottom. This timeline only shows the activity of this process, and different colors represent execution time on different processors. Red boxes on this timeline represent time where this task was not scheduled on any processor. Leave this answer blank.
dense_mm)? If you zoom in to where you can see discrete events, you can mouse over those events to see exactly which processes are switched.
sched_switchevents the same across all processors at all points in time? What do you think is happening?
dense_mmdoesn't create a really interesting trace because it can only use one processor at a time. Generate a new parallel program trace with the command "
sudo trace-cmd record -e sched_switch ./parallel_dense_mm 200" and open it with Kernelshark. Identify a spot in the timeline where you feel the system is not using CPU resources efficiently, what do you think is happening?
stracecan be used on any binary. Try using it on common programs such as
su. You can even attach it to running programs such as the ssh authentication daemon with the
This is a great first step in reverse engineering a binary or looking for security vulnerabilities as well, though we don't cover that in this course.
listoption can be used with
ftraceabout what kinds of tracers are currently available on the system. The command "
sudo trace-cmd list -e" will print out all available events. You can narrow the possible events with simple pattern matching: the command "
sudo trace-cmd list -e sched" will list all events with "sched" in their name, which includes every event in the scheduler subsystem as well as a few others. See the man page at "
man trace-cmd-list" for more info.
trace-cmd, and Kernelshark is extremely powerful. Try searching the documentation or online to find examples for doing all of the above, as well as other neat things people have done.