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====================
XRay Instrumentation
====================

:Version: 1 as of 2016-11-08

.. contents::
   :local:


Introduction
============

XRay is a function call tracing system which combines compiler-inserted
instrumentation points and a runtime library that can dynamically enable and
disable the instrumentation.

More high level information about XRay can be found in the `XRay whitepaper`_.

This document describes how to use XRay as implemented in LLVM.

XRay in LLVM
============

XRay consists of three main parts:

- Compiler-inserted instrumentation points.
- A runtime library for enabling/disabling tracing at runtime.
- A suite of tools for analysing the traces.

  **NOTE:** As of the time of this writing, XRay is only available for x86_64
  and arm7 32-bit (no-thumb) Linux.

The compiler-inserted instrumentation points come in the form of nop-sleds in
the final generated binary, and an ELF section named ``xray_instr_map`` which
contains entries pointing to these instrumentation points. The runtime library
relies on being able to access the entries of the ``xray_instr_map``, and
overwrite the instrumentation points at runtime.

Using XRay
==========

You can use XRay in a couple of ways:

- Instrumenting your C/C++/Objective-C/Objective-C++ application.
- Generating LLVM IR with the correct function attributes.

The rest of this section covers these main ways and later on how to customise
what XRay does in an XRay-instrumented binary.

Instrumenting your C/C++/Objective-C Application
------------------------------------------------

The easiest way of getting XRay instrumentation for your application is by
enabling the ``-fxray-instrument`` flag in your clang invocation.

For example:

::

  clang -fxray-instrument ..

By default, functions that have at least 200 instructions will get XRay
instrumentation points. You can tweak that number through the
``-fxray-instruction-threshold=`` flag:

::

  clang -fxray-instrument -fxray-instruction-threshold=1 ..

You can also specifically instrument functions in your binary to either always
or never be instrumented using source-level attributes. You can do it using the
GCC-style attributes or C++11-style attributes.

.. code-block:: c++

    [[clang::xray_always_intrument]] void always_instrumented();

    [[clang::xray_never_instrument]] void never_instrumented();

    void alt_always_instrumented() __attribute__((xray_always_intrument));

    void alt_never_instrumented() __attribute__((xray_never_instrument));

When linking a binary, you can either manually link in the `XRay Runtime
Library`_ or use ``clang`` to link it in automatically with the
``-fxray-instrument`` flag.

LLVM Function Attribute
-----------------------

If you're using LLVM IR directly, you can add the ``function-instrument``
string attribute to your functions, to get the similar effect that the
C/C++/Objective-C source-level attributes would get:

.. code-block:: llvm

    define i32 @always_instrument() uwtable "function-instrument"="xray-always" {
      ; ...
    }

    define i32 @never_instrument() uwtable "function-instrument"="xray-never" {
      ; ...
    }

You can also set the ``xray-instruction-threshold`` attribute and provide a
numeric string value for how many instructions should be in the function before
it gets instrumented.

.. code-block:: llvm

    define i32 @maybe_instrument() uwtable "xray-instruction-threshold"="2" {
      ; ...
    }

XRay Runtime Library
--------------------

The XRay Runtime Library is part of the compiler-rt project, which implements
the runtime components that perform the patching and unpatching of inserted
instrumentation points. When you use ``clang`` to link your binaries and the
``-fxray-instrument`` flag, it will automatically link in the XRay runtime.

The default implementation of the XRay runtime will enable XRay instrumentation
before ``main`` starts, which works for applications that have a short
lifetime. This implementation also records all function entry and exit events
which may result in a lot of records in the resulting trace.

Also by default the filename of the XRay trace is ``xray-log.XXXXXX`` where the
``XXXXXX`` part is randomly generated.

These options can be controlled through the ``XRAY_OPTIONS`` environment
variable, where we list down the options and their defaults below.

+-------------------+-----------------+---------------+------------------------+
| Option            | Type            | Default       | Description            |
+===================+=================+===============+========================+
| patch_premain     | ``bool``        | ``true``      | Whether to patch       |
|                   |                 |               | instrumentation points |
|                   |                 |               | before main.           |
+-------------------+-----------------+---------------+------------------------+
| xray_naive_log    | ``bool``        | ``true``      | Whether to install     |
|                   |                 |               | the naive log          |
|                   |                 |               | implementation.        |
+-------------------+-----------------+---------------+------------------------+
| xray_logfile_base | ``const char*`` | ``xray-log.`` | Filename base for the  |
|                   |                 |               | XRay logfile.          |
+-------------------+-----------------+---------------+------------------------+

If you choose to not use the default logging implementation that comes with the
XRay runtime and/or control when/how the XRay instrumentation runs, you may use
the XRay APIs directly for doing so. To do this, you'll need to include the
``xray_interface.h`` from the compiler-rt ``xray`` directory. The important API
functions we list below:

- ``__xray_set_handler(void (*entry)(int32_t, XRayEntryType))``: Install your
  own logging handler for when an event is encountered. See
  ``xray/xray_interface.h`` for more details.
- ``__xray_remove_handler()``: Removes whatever the installed handler is.
- ``__xray_patch()``: Patch all the instrumentation points defined in the
  binary.
- ``__xray_unpatch()``: Unpatch the instrumentation points defined in the
  binary.

There are some requirements on the logging handler to be installed for the
thread-safety of operations to be performed by the XRay runtime library:

- The function should be thread-safe, as multiple threads may be invoking the
  function at the same time. If the logging function needs to do
  synchronisation, it must do so internally as XRay does not provide any
  synchronisation guarantees outside from the atomicity of updates to the
  pointer.
- The pointer provided to ``__xray_set_handler(...)`` must be live even after
  calls to ``__xray_remove_handler()`` and ``__xray_unpatch()`` have succeeded.
  XRay cannot guarantee that all threads that have ever gotten a copy of the
  pointer will not invoke the function.


Trace Analysis Tools
--------------------

We currently have the beginnings of a trace analysis tool in LLVM, which can be
found in the ``tools/llvm-xray`` directory. The ``llvm-xray`` tool currently
supports the following subcommands:

- ``extract``: Extract the instrumentation map from a binary, and return it as
  YAML.


Future Work
===========

There are a number of ongoing efforts for expanding the toolset building around
the XRay instrumentation system.

Flight Data Recorder Mode
-------------------------

The `XRay whitepaper`_ mentions a mode for when events are kept in memory, and
have the traces be dumped on demand through a triggering API. This work is
currently ongoing.

Trace Analysis
--------------

There are a few more subcommands making its way to the ``llvm-xray`` tool, that
are currently under review:

- ``convert``: Turns an XRay trace from one format to another. Currently
  supporting conversion from the binary XRay log to YAML.
- ``account``: Do function call accounting based on data in the XRay log.

We have more subcommands and modes that we're thinking of developing, in the
following forms:

- ``stack``: Reconstruct the function call stacks in a timeline.
- ``convert``: Converting from one version of the XRay log to another (higher)
  version, and converting to other trace formats (i.e. Chrome Trace Viewer,
  pprof, etc.).
- ``graph``: Generate a function call graph with relative timings and distributions.

More Platforms
--------------

Since XRay is only currently available in x86_64 and arm7 32-bit (no-thumb)
running Linux, we're looking to supporting more platforms (architectures and
operating systems).

.. References...

.. _`XRay whitepaper`: http://research.google.com/pubs/pub45287.html