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Update of supported oses. Reviewers: dberris Reviewed By: dberris Differential Revision: https://reviews.llvm.org/D49789 llvm-svn: 338003
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====================
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XRay Instrumentation
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====================
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:Version: 1 as of 2016-11-08
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.. contents::
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:local:
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Introduction
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============
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XRay is a function call tracing system which combines compiler-inserted
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instrumentation points and a runtime library that can dynamically enable and
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disable the instrumentation.
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More high level information about XRay can be found in the `XRay whitepaper`_.
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This document describes how to use XRay as implemented in LLVM.
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XRay in LLVM
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============
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XRay consists of three main parts:
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- Compiler-inserted instrumentation points.
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- A runtime library for enabling/disabling tracing at runtime.
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- A suite of tools for analysing the traces.
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**NOTE:** As of July 25, 2018 , XRay is only available for the following
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architectures running Linux: x86_64, arm7 (no thumb), aarch64, powerpc64le,
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mips, mipsel, mips64, mips64el, NetBSD: x86_64, FreeBSD: x86_64 and
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OpenBSD: x86_64.
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The compiler-inserted instrumentation points come in the form of nop-sleds in
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the final generated binary, and an ELF section named ``xray_instr_map`` which
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contains entries pointing to these instrumentation points. The runtime library
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relies on being able to access the entries of the ``xray_instr_map``, and
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overwrite the instrumentation points at runtime.
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Using XRay
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==========
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You can use XRay in a couple of ways:
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- Instrumenting your C/C++/Objective-C/Objective-C++ application.
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- Generating LLVM IR with the correct function attributes.
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The rest of this section covers these main ways and later on how to customise
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what XRay does in an XRay-instrumented binary.
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Instrumenting your C/C++/Objective-C Application
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------------------------------------------------
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The easiest way of getting XRay instrumentation for your application is by
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enabling the ``-fxray-instrument`` flag in your clang invocation.
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For example:
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::
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clang -fxray-instrument ...
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By default, functions that have at least 200 instructions will get XRay
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instrumentation points. You can tweak that number through the
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``-fxray-instruction-threshold=`` flag:
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::
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clang -fxray-instrument -fxray-instruction-threshold=1 ...
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You can also specifically instrument functions in your binary to either always
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or never be instrumented using source-level attributes. You can do it using the
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GCC-style attributes or C++11-style attributes.
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.. code-block:: c++
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[[clang::xray_always_instrument]] void always_instrumented();
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[[clang::xray_never_instrument]] void never_instrumented();
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void alt_always_instrumented() __attribute__((xray_always_instrument));
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void alt_never_instrumented() __attribute__((xray_never_instrument));
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When linking a binary, you can either manually link in the `XRay Runtime
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Library`_ or use ``clang`` to link it in automatically with the
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``-fxray-instrument`` flag. Alternatively, you can statically link-in the XRay
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runtime library from compiler-rt -- those archive files will take the name of
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`libclang_rt.xray-{arch}` where `{arch}` is the mnemonic supported by clang
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(x86_64, arm7, etc.).
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LLVM Function Attribute
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-----------------------
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If you're using LLVM IR directly, you can add the ``function-instrument``
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string attribute to your functions, to get the similar effect that the
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C/C++/Objective-C source-level attributes would get:
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.. code-block:: llvm
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define i32 @always_instrument() uwtable "function-instrument"="xray-always" {
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; ...
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}
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define i32 @never_instrument() uwtable "function-instrument"="xray-never" {
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; ...
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}
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You can also set the ``xray-instruction-threshold`` attribute and provide a
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numeric string value for how many instructions should be in the function before
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it gets instrumented.
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.. code-block:: llvm
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define i32 @maybe_instrument() uwtable "xray-instruction-threshold"="2" {
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; ...
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}
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Special Case File
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-----------------
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Attributes can be imbued through the use of special case files instead of
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adding them to the original source files. You can use this to mark certain
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functions and classes to be never, always, or instrumented with first-argument
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logging from a file. The file's format is described below:
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.. code-block:: bash
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# Comments are supported
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[always]
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fun:always_instrument
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fun:log_arg1=arg1 # Log the first argument for the function
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[never]
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fun:never_instrument
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These files can be provided through the ``-fxray-attr-list=`` flag to clang.
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You may have multiple files loaded through multiple instances of the flag.
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XRay Runtime Library
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--------------------
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The XRay Runtime Library is part of the compiler-rt project, which implements
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the runtime components that perform the patching and unpatching of inserted
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instrumentation points. When you use ``clang`` to link your binaries and the
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``-fxray-instrument`` flag, it will automatically link in the XRay runtime.
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The default implementation of the XRay runtime will enable XRay instrumentation
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before ``main`` starts, which works for applications that have a short
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lifetime. This implementation also records all function entry and exit events
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which may result in a lot of records in the resulting trace.
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Also by default the filename of the XRay trace is ``xray-log.XXXXXX`` where the
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``XXXXXX`` part is randomly generated.
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These options can be controlled through the ``XRAY_OPTIONS`` environment
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variable, where we list down the options and their defaults below.
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+-------------------+-----------------+---------------+------------------------+
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| Option | Type | Default | Description |
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+===================+=================+===============+========================+
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| patch_premain | ``bool`` | ``false`` | Whether to patch |
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| | | | instrumentation points |
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| | | | before main. |
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+-------------------+-----------------+---------------+------------------------+
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| xray_mode | ``const char*`` | ``""`` | Default mode to |
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| | | | install and initialize |
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| | | | before ``main``. |
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+-------------------+-----------------+---------------+------------------------+
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| xray_logfile_base | ``const char*`` | ``xray-log.`` | Filename base for the |
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| | | | XRay logfile. |
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+-------------------+-----------------+---------------+------------------------+
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| verbosity | ``int`` | ``0`` | Runtime verbosity |
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| | | | level. |
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+-------------------+-----------------+---------------+------------------------+
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If you choose to not use the default logging implementation that comes with the
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XRay runtime and/or control when/how the XRay instrumentation runs, you may use
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the XRay APIs directly for doing so. To do this, you'll need to include the
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``xray_log_interface.h`` from the compiler-rt ``xray`` directory. The important API
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functions we list below:
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- ``__xray_log_register_mode(...)``: Register a logging implementation against
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a string Mode identifier. The implementation is an instance of
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``XRayLogImpl`` defined in ``xray/xray_log_interface.h``.
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- ``__xray_log_select_mode(...)``: Select the mode to install, associated with
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a string Mode identifier. Only implementations registered with
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``__xray_log_register_mode(...)`` can be chosen with this function.
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- ``__xray_log_init_mode(...)``: This function allows for initializing and
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re-initializing an installed logging implementation. See
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``xray/xray_log_interface.h`` for details, part of the XRay compiler-rt
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installation.
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Once a logging implementation has been initialized, it can be "stopped" by
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finalizing the implementation through the ``__xray_log_finalize()`` function.
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The finalization routine is the opposite of the initialization. When finalized,
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an implementation's data can be cleared out through the
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``__xray_log_flushLog()`` function. For implementations that support in-memory
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processing, these should register an iterator function to provide access to the
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data via the ``__xray_log_set_buffer_iterator(...)`` which allows code calling
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the ``__xray_log_process_buffers(...)`` function to deal with the data in
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memory.
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All of this is better explained in the ``xray/xray_log_interface.h`` header.
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Basic Mode
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----------
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XRay supports a basic logging mode which will trace the application's
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execution, and periodically append to a single log. This mode can be
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installed/enabled by setting ``xray_mode=xray-basic`` in the ``XRAY_OPTIONS``
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environment variable. Combined with ``patch_premain=true`` this can allow for
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tracing applications from start to end.
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Like all the other modes installed through ``__xray_log_select_mode(...)``, the
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implementation can be configured through the ``__xray_log_init_mode(...)``
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function, providing the mode string and the flag options. Basic-mode specific
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defaults can be provided in the ``XRAY_BASIC_OPTIONS`` environment variable.
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Flight Data Recorder Mode
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-------------------------
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XRay supports a logging mode which allows the application to only capture a
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fixed amount of memory's worth of events. Flight Data Recorder (FDR) mode works
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very much like a plane's "black box" which keeps recording data to memory in a
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fixed-size circular queue of buffers, and have the data available
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programmatically until the buffers are finalized and flushed. To use FDR mode
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on your application, you may set the ``xray_mode`` variable to ``xray-fdr`` in
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the ``XRAY_OPTIONS`` environment variable. Additional options to the FDR mode
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implementation can be provided in the ``XRAY_FDR_OPTIONS`` environment
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variable. Programmatic configuration can be done by calling
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``__xray_log_init_mode("xray-fdr", <configuration string>)`` once it has been
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selected/installed.
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When the buffers are flushed to disk, the result is a binary trace format
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described by `XRay FDR format <XRayFDRFormat.html>`_
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When FDR mode is on, it will keep writing and recycling memory buffers until
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the logging implementation is finalized -- at which point it can be flushed and
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re-initialised later. To do this programmatically, we follow the workflow
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provided below:
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.. code-block:: c++
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// Patch the sleds, if we haven't yet.
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auto patch_status = __xray_patch();
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// Maybe handle the patch_status errors.
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// When we want to flush the log, we need to finalize it first, to give
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// threads a chance to return buffers to the queue.
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auto finalize_status = __xray_log_finalize();
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if (finalize_status != XRAY_LOG_FINALIZED) {
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// maybe retry, or bail out.
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}
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// At this point, we are sure that the log is finalized, so we may try
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// flushing the log.
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auto flush_status = __xray_log_flushLog();
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if (flush_status != XRAY_LOG_FLUSHED) {
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// maybe retry, or bail out.
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}
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The default settings for the FDR mode implementation will create logs named
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similarly to the basic log implementation, but will have a different log
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format. All the trace analysis tools (and the trace reading library) will
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support all versions of the FDR mode format as we add more functionality and
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record types in the future.
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**NOTE:** We do not promise perpetual support for when we update the log
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versions we support going forward. Deprecation of the formats will be
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announced and discussed on the developers mailing list.
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Trace Analysis Tools
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--------------------
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We currently have the beginnings of a trace analysis tool in LLVM, which can be
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found in the ``tools/llvm-xray`` directory. The ``llvm-xray`` tool currently
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supports the following subcommands:
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- ``extract``: Extract the instrumentation map from a binary, and return it as
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YAML.
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- ``account``: Performs basic function call accounting statistics with various
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options for sorting, and output formats (supports CSV, YAML, and
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console-friendly TEXT).
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- ``convert``: Converts an XRay log file from one format to another. We can
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convert from binary XRay traces (both basic and FDR mode) to YAML,
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`flame-graph <https://github.com/brendangregg/FlameGraph>`_ friendly text
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formats, as well as `Chrome Trace Viewer (catapult)
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<https://github.com/catapult-project/catapult>` formats.
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- ``graph``: Generates a DOT graph of the function call relationships between
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functions found in an XRay trace.
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- ``stack``: Reconstructs function call stacks from a timeline of function
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calls in an XRay trace.
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These subcommands use various library components found as part of the XRay
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libraries, distributed with the LLVM distribution. These are:
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- ``llvm/XRay/Trace.h`` : A trace reading library for conveniently loading
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an XRay trace of supported forms, into a convenient in-memory representation.
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All the analysis tools that deal with traces use this implementation.
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- ``llvm/XRay/Graph.h`` : A semi-generic graph type used by the graph
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subcommand to conveniently represent a function call graph with statistics
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associated with edges and vertices.
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- ``llvm/XRay/InstrumentationMap.h``: A convenient tool for analyzing the
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instrumentation map in XRay-instrumented object files and binaries. The
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``extract`` and ``stack`` subcommands uses this particular library.
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Future Work
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===========
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There are a number of ongoing efforts for expanding the toolset building around
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the XRay instrumentation system.
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Trace Analysis Tools
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--------------------
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- Work is in progress to integrate with or develop tools to visualize findings
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from an XRay trace. Particularly, the ``stack`` tool is being expanded to
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output formats that allow graphing and exploring the duration of time in each
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call stack.
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- With a large instrumented binary, the size of generated XRay traces can
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quickly become unwieldy. We are working on integrating pruning techniques and
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heuristics for the analysis tools to sift through the traces and surface only
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relevant information.
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More Platforms
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--------------
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We're looking forward to contributions to port XRay to more architectures and
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operating systems.
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.. References...
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.. _`XRay whitepaper`: http://research.google.com/pubs/pub45287.html
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