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ReStructuredText
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219 lines
8.2 KiB
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==============================
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Profiling/Debugging the kernel
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==============================
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--------
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Overview
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--------
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Kernel development often involves writing code without an IDE. Even if an IDE is used,
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proper profiling support is often lacking. Luckily, FreeBSD includes DTrace.
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Since OPNsense runs on a fork of FreeBSD, DTrace is natively available on the
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system for developers to use in debugging and profiling. To quote the FreeBSD handbook on DTrace:
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::
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“DTrace, also known as Dynamic Tracing, was developed by Sun™ as a tool
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for locating performance bottlenecks in production and pre-production
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systems. In addition to diagnosing performance problems, DTrace can be used
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to help investigate and debug unexpected behavior in both the FreeBSD kernel
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and in userland programs.
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DTrace is a remarkable profiling tool, with an impressive array of features
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for diagnosing system issues. It may also be used to run pre-written scripts
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to take advantage of its capabilities. Users can author their own utilities
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using the DTrace D Language, allowing them to customize their profiling based
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on specific needs.”
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We will be looking at setting up DTrace on OPNsense, and running a sample script
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to perform some useful analysis on kernel space. An example for generating a FlameGraph
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will also be presented. Keep in mind that user space can also be analyzed using DTrace,
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but this is beyond the scope of this document.
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Although detailed steps for enabling DTrace in a custom kernel are widely available,
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this document provides an explanation in combination with the use of OPNsense-specific tools.
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Further resources will be provided at the end of this document.
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.. note::
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All generic DTrace concepts, as well as default probe providers are
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documented `here <http://dtrace.org/guide/preface.html#preface>`__.
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If you are interested in using DTrace to profile performance issues, we suggest reading
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the documentation before proceeding. We will not go into too much detail here to prevent
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cluttering the document and keep it simple and to-the-point.
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---------------
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Enabling DTrace
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---------------
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Make sure the OPNsense/src repository is present on the system and navigate to
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:code:`/usr/src/sys/[ARCHITECTURE, e.g. amd64]/conf` and open up :code:`GENERIC`.
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Add the following options:
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.. code-block:: sh
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makeoptions DEBUG=-g # Will build a debug kernel (using gdb debug symbols)
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makeoptions WITH_CTF=1 # Adds CTF type data to built executables
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options KDTRACE_FRAME # Ensure frames are compiled in
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.. note::
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CTF type data is described
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`here <https://www.freebsd.org/cgi/man.cgi?query=ctf&sektion=5&manpath=freebsd-release-ports>`__.
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The idea is to represent the types used in a C program within the executable,
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allowing users of DTrace to access symbols and function parameters during runtime.
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As a best practice, under stock FreeBSD you would save the file under a different name
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(e.g. DTRACE) and build the kernel by specifying :code:`KERNCONF=DTRACE`.
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The opnsense-tools require the name to be :code:`GENERIC`, so we will leave it as is.
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It is advisable to create a backup of the original :code:`GENERIC` file though.
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Make sure the OPNsense-tools repository is installed and navigate to :code:`/usr/tools/config/[VERSION]`.
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Open up :code:`SMP` and make sure the following line is either removed or commented out:
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.. code-block:: sh
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nomakeoptions DEBUG
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Furthermore, should you wish to enable DTrace in Userspace, it is imperative that you also
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add the following line to :code:`/usr/tools/conf/[VERSION]/make.conf`
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.. code-block:: sh
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STRIP=
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CFLAGS+=-fno-omit-frame-pointer
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Now clean and build the new kernel
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.. code-block:: sh
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# make clean-obj,kernel kernel
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A kernel package will be available at the following location:
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:code:`/usr/local/opnsense/build/[Version]/[Architecture]/sets/kernel*.txz`
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Install the package using our :code:`opnsense-update` command:
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.. code-block:: sh
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# opnsense-update -ikfr [version] -l /location/from/
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Where [version] is the version part of the kernel package, such as :code:`21.1.a_83`
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---------------------
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Test the installation
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---------------------
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Follow the steps described in the section 'Getting Started' `here <https://wiki.freebsd.org/DTrace/KernelSupport>`__
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--------------
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DTrace program
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--------------
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Although it is possible to instruct DTrace to execute a certain function on the
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command line (and there are a lot of one-liners for this `here <https://wiki.freebsd.org/DTrace/One-Liners>`__), instead we present a D script that measures some statistics on the ixgbe 10GbE driver and
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shows some interesting capabilities of DTrace in one go. Specifically, it measures the amount
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of interrupts fired on the packet receive side, as well as the time spent in the relevant functions
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in nanoseconds. It also measures how many threads on average are waiting to be serviced because
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another thread is busy.
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.. literalinclude:: ./files/example.d
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The one-line command to pre-process, compile and run the script is:
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.. code-block:: sh
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# dtrace -C -s example.d
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The :code:`-C` option specifies that the standard C preprocessor should evaluate the file, ensuring
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that the C-specific code like macro definitions are defined.
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The output (when pinging the device):
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.. image:: ./images/dtrace_output_docs.png
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:alt: Dtrace Output
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Notice how the queue behaviour section shows no output, this is because there is no actual
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queue behaviour going on. There is simply too little traffic for the kernel to be queueing threads.
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When stressing the driver, it might look something like this:
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.. image:: ./images/queue_behaviour.png
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:alt: Queue behaviour
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------------
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Flame Graphs
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------------
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If you run the following command:
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.. code-block:: sh
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# dtrace -n 'uiomove:entry{ @[stack()] = count(); }'
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Let it run for a while, and exit using :code:`CTRL-C`
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You will see all `stack frames <https://en.wikipedia.org/wiki/Call_stack>`__ associated with
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:code:`uiomove()` that have taken place, aggregated to only unique frames. The number below the individual
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frame represents the amount of times that particular frame was executed. Although this is nice,
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it isn't particulary visually pleasing or helpful in spotting potential bottlenecks.
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A senior performance analyst at Netflix, named Brendan Gregg, developed the
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`FlameGraph <https://github.com/brendangregg/FlameGraph>`__ for use with DTrace (and other profiling tools).
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The program consists of a set of Perl files that take in the data produced by a command
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similar to the one we executed, and generate a graph that helps spot 'hot code paths'.
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This time however, we will look at the kernel as a whole and sample it in a set interval.
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1. Clone the `repository <https://github.com/brendangregg/FlameGraph>`__ and instruct dtrace to
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sample kernel stacks at a rate of 997 Hertz for 60 seconds:
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.. code-block:: sh
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# dtrace -x stackframes=100 -n 'profile-997 /arg0/ { @[stack()] = count(); } tick-60s { exit(0); }' -o out.kern_stacks
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2. Collapse the individual lines in the output file by folding it into single lines:
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.. code-block:: sh
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# ./stackcollapse.pl out.kern_stacks > out.kern_folded
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Having a separate folded input file allows for grepping functions of interest:
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.. code-block:: sh
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# grep cpuid out.kern_folded | ./flamegraph.pl > cpuid.svg
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.. note::
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If you get the message "Command not found", use the command
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:code:`perl stackcollapse.pl out.kern_stacks > out.kern_folded`
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3. Generate a flame graph:
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.. code-block:: sh
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# ./flamegraph.pl out.kern_folded > kernel.svg
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.. image:: ./images/example-dtrace.svg
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:alt: FlameGraph
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The output image is an SVG file, enabling interaction within a browser.
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---------------
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Other Resources
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---------------
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Brendan Gregg's collection of DTrace programs:
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http://www.brendangregg.com/dtrace.html
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Using the right datatypes in DTrace to increase profiling performance:
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http://dtrace.org/blogs/brendan/2011/11/25/dtrace-variable-types/
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FreeBSD DTrace manual page:
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https://www.freebsd.org/cgi/man.cgi?query=dtrace
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FreeBSD DTrace one-liners:
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https://wiki.freebsd.org/DTrace/One-Liners
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