mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-24 19:52:54 +01:00
517c1e5898
In an example like "clang -fxray-instrument .." the .. could be confused with a literal .. (parent directory), which is used in commands like "cmake -GNinja .." llvm-svn: 327000
333 lines
14 KiB
ReStructuredText
333 lines
14 KiB
ReStructuredText
====================
|
|
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 February 27, 2017 , XRay is only available for the following
|
|
architectures running Linux: x86_64, arm7 (no thumb), aarch64, powerpc64le,
|
|
mips, mipsel, mips64, mips64el.
|
|
|
|
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_instrument]] void always_instrumented();
|
|
|
|
[[clang::xray_never_instrument]] void never_instrumented();
|
|
|
|
void alt_always_instrumented() __attribute__((xray_always_instrument));
|
|
|
|
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. Alternatively, you can statically link-in the XRay
|
|
runtime library from compiler-rt -- those archive files will take the name of
|
|
`libclang_rt.xray-{arch}` where `{arch}` is the mnemonic supported by clang
|
|
(x86_64, arm7, etc.).
|
|
|
|
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`` | ``false`` | Whether to patch |
|
|
| | | | instrumentation points |
|
|
| | | | before main. |
|
|
+-------------------+-----------------+---------------+------------------------+
|
|
| xray_mode | ``const char*`` | ``""`` | Default mode to |
|
|
| | | | install and initialize |
|
|
| | | | before ``main``. |
|
|
+-------------------+-----------------+---------------+------------------------+
|
|
| xray_logfile_base | ``const char*`` | ``xray-log.`` | Filename base for the |
|
|
| | | | XRay logfile. |
|
|
+-------------------+-----------------+---------------+------------------------+
|
|
| xray_naive_log | ``bool`` | ``false`` | **DEPRECATED:** Use |
|
|
| | | | xray_mode=xray-basic |
|
|
| | | | instead. Whether to |
|
|
| | | | install the basic log |
|
|
| | | | the naive log |
|
|
| | | | implementation. |
|
|
+-------------------+-----------------+---------------+------------------------+
|
|
| xray_fdr_log | ``bool`` | ``false`` | **DEPRECATED:** Use |
|
|
| | | | xray_mode=xray-fdr |
|
|
| | | | instead. Whether to |
|
|
| | | | install the Flight |
|
|
| | | | Data Recorder |
|
|
| | | | (FDR) mode. |
|
|
+-------------------+-----------------+---------------+------------------------+
|
|
| verbosity | ``int`` | ``0`` | Runtime verbosity |
|
|
| | | | level. |
|
|
+-------------------+-----------------+---------------+------------------------+
|
|
|
|
|
|
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.
|
|
|
|
Flight Data Recorder Mode
|
|
-------------------------
|
|
|
|
XRay supports a logging mode which allows the application to only capture a
|
|
fixed amount of memory's worth of events. Flight Data Recorder (FDR) mode works
|
|
very much like a plane's "black box" which keeps recording data to memory in a
|
|
fixed-size circular queue of buffers, and have the data available
|
|
programmatically until the buffers are finalized and flushed. To use FDR mode
|
|
on your application, you may set the ``xray_fdr_log`` option to ``true`` in the
|
|
``XRAY_OPTIONS`` environment variable (while also optionally setting the
|
|
``xray_naive_log`` to ``false``).
|
|
|
|
When the buffers are flushed to disk, the result is a binary trace format
|
|
described by `XRay FDR format <XRayFDRFormat.html>`_
|
|
|
|
When FDR mode is on, it will keep writing and recycling memory buffers until
|
|
the logging implementation is finalized -- at which point it can be flushed and
|
|
re-initialised later. To do this programmatically, we follow the workflow
|
|
provided below:
|
|
|
|
.. code-block:: c++
|
|
|
|
// Patch the sleds, if we haven't yet.
|
|
auto patch_status = __xray_patch();
|
|
|
|
// Maybe handle the patch_status errors.
|
|
|
|
// When we want to flush the log, we need to finalize it first, to give
|
|
// threads a chance to return buffers to the queue.
|
|
auto finalize_status = __xray_log_finalize();
|
|
if (finalize_status != XRAY_LOG_FINALIZED) {
|
|
// maybe retry, or bail out.
|
|
}
|
|
|
|
// At this point, we are sure that the log is finalized, so we may try
|
|
// flushing the log.
|
|
auto flush_status = __xray_log_flushLog();
|
|
if (flush_status != XRAY_LOG_FLUSHED) {
|
|
// maybe retry, or bail out.
|
|
}
|
|
|
|
The default settings for the FDR mode implementation will create logs named
|
|
similarly to the naive log implementation, but will have a different log
|
|
format. All the trace analysis tools (and the trace reading library) will
|
|
support all versions of the FDR mode format as we add more functionality and
|
|
record types in the future.
|
|
|
|
**NOTE:** We do not however promise perpetual support for when we update the
|
|
log versions we support going forward. Deprecation of the formats will be
|
|
announced and discussed on the developers mailing list.
|
|
|
|
XRay allows for replacing the default FDR mode logging implementation using the
|
|
following API:
|
|
|
|
- ``__xray_set_log_impl(...)``: This function takes a struct of type
|
|
``XRayLogImpl``, which is defined in ``xray/xray_log_interface.h``, part of
|
|
the XRay compiler-rt installation.
|
|
- ``__xray_log_register_mode(...)``: Register a logging implementation against
|
|
a string Mode. The implementation is an instance of ``XRayLogImpl`` defined
|
|
in ``xray/xray_log_interface.h``.
|
|
- ``__xray_log_select_mode(...)``: Select the mode to install, associated with
|
|
a string Mode. Only implementations registered with
|
|
``__xray_log_register_mode(...)`` can be chosen with this function. When
|
|
successful, has the same effects as calling ``__xray_set_log_impl(...)`` with
|
|
the registered logging implementation.
|
|
- ``__xray_log_init(...)``: This function allows for initializing and
|
|
re-initializing an installed logging implementation. See
|
|
``xray/xray_log_interface.h`` for details, part of the XRay compiler-rt
|
|
installation.
|
|
|
|
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.
|
|
- ``account``: Performs basic function call accounting statistics with various
|
|
options for sorting, and output formats (supports CSV, YAML, and
|
|
console-friendly TEXT).
|
|
- ``convert``: Converts an XRay log file from one format to another. We can
|
|
convert from binary XRay traces (both naive and FDR mode) to YAML,
|
|
`flame-graph <https://github.com/brendangregg/FlameGraph>`_ friendly text
|
|
formats, as well as `Chrome Trace Viewer (catapult)
|
|
<https://github.com/catapult-project/catapult>` formats.
|
|
- ``graph``: Generates a DOT graph of the function call relationships between
|
|
functions found in an XRay trace.
|
|
- ``stack``: Reconstructs function call stacks from a timeline of function
|
|
calls in an XRay trace.
|
|
|
|
These subcommands use various library components found as part of the XRay
|
|
libraries, distributed with the LLVM distribution. These are:
|
|
|
|
- ``llvm/XRay/Trace.h`` : A trace reading library for conveniently loading
|
|
an XRay trace of supported forms, into a convenient in-memory representation.
|
|
All the analysis tools that deal with traces use this implementation.
|
|
- ``llvm/XRay/Graph.h`` : A semi-generic graph type used by the graph
|
|
subcommand to conveniently represent a function call graph with statistics
|
|
associated with edges and vertices.
|
|
- ``llvm/XRay/InstrumentationMap.h``: A convenient tool for analyzing the
|
|
instrumentation map in XRay-instrumented object files and binaries. The
|
|
``extract`` and ``stack`` subcommands uses this particular library.
|
|
|
|
Future Work
|
|
===========
|
|
|
|
There are a number of ongoing efforts for expanding the toolset building around
|
|
the XRay instrumentation system.
|
|
|
|
Trace Analysis Tools
|
|
--------------------
|
|
|
|
- Work is in progress to integrate with or develop tools to visualize findings
|
|
from an XRay trace. Particularly, the ``stack`` tool is being expanded to
|
|
output formats that allow graphing and exploring the duration of time in each
|
|
call stack.
|
|
- With a large instrumented binary, the size of generated XRay traces can
|
|
quickly become unwieldy. We are working on integrating pruning techniques and
|
|
heuristics for the analysis tools to sift through the traces and surface only
|
|
relevant information.
|
|
|
|
More Platforms
|
|
--------------
|
|
|
|
We're looking forward to contributions to port XRay to more architectures and
|
|
operating systems.
|
|
|
|
.. References...
|
|
|
|
.. _`XRay whitepaper`: http://research.google.com/pubs/pub45287.html
|
|
|