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350 lines
17 KiB
ReStructuredText
===================
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Debugging with XRay
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===================
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This document shows an example of how you would go about analyzing applications
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built with XRay instrumentation. Here we will attempt to debug ``llc``
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compiling some sample LLVM IR generated by Clang.
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.. contents::
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:local:
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Building with XRay
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------------------
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To debug an application with XRay instrumentation, we need to build it with a
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Clang that supports the ``-fxray-instrument`` option. See `XRay <XRay.html>`_
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for more technical details of how XRay works for background information.
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In our example, we need to add ``-fxray-instrument`` to the list of flags
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passed to Clang when building a binary. Note that we need to link with Clang as
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well to get the XRay runtime linked in appropriately. For building ``llc`` with
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XRay, we do something similar below for our LLVM build:
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::
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$ mkdir -p llvm-build && cd llvm-build
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# Assume that the LLVM sources are at ../llvm
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$ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
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-DCMAKE_C_FLAGS_RELEASE="-fxray-instrument" -DCMAKE_CXX_FLAGS="-fxray-instrument" \
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# Once this finishes, we should build llc
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$ ninja llc
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To verify that we have an XRay instrumented binary, we can use ``objdump`` to
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look for the ``xray_instr_map`` section.
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::
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$ objdump -h -j xray_instr_map ./bin/llc
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./bin/llc: file format elf64-x86-64
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Sections:
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Idx Name Size VMA LMA File off Algn
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14 xray_instr_map 00002fc0 00000000041516c6 00000000041516c6 03d516c6 2**0
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CONTENTS, ALLOC, LOAD, READONLY, DATA
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Getting Traces
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--------------
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By default, XRay does not write out the trace files or patch the application
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before main starts. If we run ``llc`` it should work like a normally built
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binary. If we want to get a full trace of the application's operations (of the
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functions we do end up instrumenting with XRay) then we need to enable XRay
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at application start. To do this, XRay checks the ``XRAY_OPTIONS`` environment
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variable.
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::
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# The following doesn't create an XRay trace by default.
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$ ./bin/llc input.ll
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# We need to set the XRAY_OPTIONS to enable some features.
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$ XRAY_OPTIONS="patch_premain=true xray_mode=xray-basic verbosity=1" ./bin/llc input.ll
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==69819==XRay: Log file in 'xray-log.llc.m35qPB'
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At this point we now have an XRay trace we can start analysing.
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The ``llvm-xray`` Tool
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----------------------
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Having a trace then allows us to do basic accounting of the functions that were
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instrumented, and how much time we're spending in parts of the code. To make
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sense of this data, we use the ``llvm-xray`` tool which has a few subcommands
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to help us understand our trace.
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One of the things we can do is to get an accounting of the functions that have
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been instrumented. We can see an example accounting with ``llvm-xray account``:
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::
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$ llvm-xray account xray-log.llc.m35qPB -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
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Functions with latencies: 29
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funcid count [ min, med, 90p, 99p, max] sum function
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187 360 [ 0.000000, 0.000001, 0.000014, 0.000032, 0.000075] 0.001596 LLLexer.cpp:446:0: llvm::LLLexer::LexIdentifier()
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85 130 [ 0.000000, 0.000000, 0.000018, 0.000023, 0.000156] 0.000799 X86ISelDAGToDAG.cpp:1984:0: (anonymous namespace)::X86DAGToDAGISel::Select(llvm::SDNode*)
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138 130 [ 0.000000, 0.000000, 0.000017, 0.000155, 0.000155] 0.000774 SelectionDAGISel.cpp:2963:0: llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*, unsigned int)
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188 103 [ 0.000000, 0.000000, 0.000003, 0.000123, 0.000214] 0.000737 LLParser.cpp:2692:0: llvm::LLParser::ParseValID(llvm::ValID&, llvm::LLParser::PerFunctionState*)
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88 1 [ 0.000562, 0.000562, 0.000562, 0.000562, 0.000562] 0.000562 X86ISelLowering.cpp:83:0: llvm::X86TargetLowering::X86TargetLowering(llvm::X86TargetMachine const&, llvm::X86Subtarget const&)
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125 102 [ 0.000001, 0.000003, 0.000010, 0.000017, 0.000049] 0.000471 Verifier.cpp:3714:0: (anonymous namespace)::Verifier::visitInstruction(llvm::Instruction&)
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90 8 [ 0.000023, 0.000035, 0.000106, 0.000106, 0.000106] 0.000342 X86ISelLowering.cpp:3363:0: llvm::X86TargetLowering::LowerCall(llvm::TargetLowering::CallLoweringInfo&, llvm::SmallVectorImpl<llvm::SDValue>&) const
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124 32 [ 0.000003, 0.000007, 0.000016, 0.000041, 0.000041] 0.000310 Verifier.cpp:1967:0: (anonymous namespace)::Verifier::visitFunction(llvm::Function const&)
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123 1 [ 0.000302, 0.000302, 0.000302, 0.000302, 0.000302] 0.000302 LLVMContextImpl.cpp:54:0: llvm::LLVMContextImpl::~LLVMContextImpl()
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139 46 [ 0.000000, 0.000002, 0.000006, 0.000008, 0.000019] 0.000138 TargetLowering.cpp:506:0: llvm::TargetLowering::SimplifyDemandedBits(llvm::SDValue, llvm::APInt const&, llvm::APInt&, llvm::APInt&, llvm::TargetLowering::TargetLoweringOpt&, unsigned int, bool) const
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This shows us that for our input file, ``llc`` spent the most cumulative time
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in the lexer (a total of 1 millisecond). If we wanted for example to work with
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this data in a spreadsheet, we can output the results as CSV using the
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``-format=csv`` option to the command for further analysis.
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If we want to get a textual representation of the raw trace we can use the
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``llvm-xray convert`` tool to get YAML output. The first few lines of that
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output for an example trace would look like the following:
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::
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$ llvm-xray convert -f yaml -symbolize -instr_map=./bin/llc xray-log.llc.m35qPB
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---
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header:
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version: 1
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type: 0
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constant-tsc: true
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nonstop-tsc: true
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cycle-frequency: 2601000000
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records:
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- { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426023268520 }
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- { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426023523052 }
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- { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426029925386 }
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- { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426030031128 }
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- { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426046951388 }
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- { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047282020 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426047857332 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047984152 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048036584 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048042292 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048055056 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048067316 }
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Controlling Fidelity
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--------------------
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So far in our examples, we haven't been getting full coverage of the functions
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we have in the binary. To get that, we need to modify the compiler flags so
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that we can instrument more (if not all) the functions we have in the binary.
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We have two options for doing that, and we explore both of these below.
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Instruction Threshold
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`````````````````````
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The first "blunt" way of doing this is by setting the minimum threshold for
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function bodies to 1. We can do that with the
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``-fxray-instruction-threshold=N`` flag when building our binary. We rebuild
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``llc`` with this option and observe the results:
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::
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$ rm CMakeCache.txt
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$ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
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-DCMAKE_C_FLAGS_RELEASE="-fxray-instrument -fxray-instruction-threshold=1" \
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-DCMAKE_CXX_FLAGS="-fxray-instrument -fxray-instruction-threshold=1"
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$ ninja llc
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$ XRAY_OPTIONS="patch_premain=true" ./bin/llc input.ll
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==69819==XRay: Log file in 'xray-log.llc.5rqxkU'
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$ llvm-xray account xray-log.llc.5rqxkU -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
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Functions with latencies: 36652
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funcid count [ min, med, 90p, 99p, max] sum function
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75 1 [ 0.672368, 0.672368, 0.672368, 0.672368, 0.672368] 0.672368 llc.cpp:271:0: main
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78 1 [ 0.626455, 0.626455, 0.626455, 0.626455, 0.626455] 0.626455 llc.cpp:381:0: compileModule(char**, llvm::LLVMContext&)
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139617 1 [ 0.472618, 0.472618, 0.472618, 0.472618, 0.472618] 0.472618 LegacyPassManager.cpp:1723:0: llvm::legacy::PassManager::run(llvm::Module&)
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139610 1 [ 0.472618, 0.472618, 0.472618, 0.472618, 0.472618] 0.472618 LegacyPassManager.cpp:1681:0: llvm::legacy::PassManagerImpl::run(llvm::Module&)
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139612 1 [ 0.470948, 0.470948, 0.470948, 0.470948, 0.470948] 0.470948 LegacyPassManager.cpp:1564:0: (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&)
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139607 2 [ 0.147345, 0.315994, 0.315994, 0.315994, 0.315994] 0.463340 LegacyPassManager.cpp:1530:0: llvm::FPPassManager::runOnModule(llvm::Module&)
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139605 21 [ 0.000002, 0.000002, 0.102593, 0.213336, 0.213336] 0.463331 LegacyPassManager.cpp:1491:0: llvm::FPPassManager::runOnFunction(llvm::Function&)
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139563 26096 [ 0.000002, 0.000002, 0.000037, 0.000063, 0.000215] 0.225708 LegacyPassManager.cpp:1083:0: llvm::PMDataManager::findAnalysisPass(void const*, bool)
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108055 188 [ 0.000002, 0.000120, 0.001375, 0.004523, 0.062624] 0.159279 MachineFunctionPass.cpp:38:0: llvm::MachineFunctionPass::runOnFunction(llvm::Function&)
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62635 22 [ 0.000041, 0.000046, 0.000050, 0.126744, 0.126744] 0.127715 X86TargetMachine.cpp:242:0: llvm::X86TargetMachine::getSubtargetImpl(llvm::Function const&) const
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Instrumentation Attributes
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``````````````````````````
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The other way is to use configuration files for selecting which functions
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should always be instrumented by the compiler. This gives us a way of ensuring
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that certain functions are either always or never instrumented by not having to
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add the attribute to the source.
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To use this feature, you can define one file for the functions to always
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instrument, and another for functions to never instrument. The format of these
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files are exactly the same as the SanitizerLists files that control similar
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things for the sanitizer implementations. For example:
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::
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# xray-attr-list.txt
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# always instrument functions that match the following filters:
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[always]
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fun:main
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# never instrument functions that match the following filters:
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[never]
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fun:__cxx_*
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Given the file above we can re-build by providing it to the
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``-fxray-attr-list=`` flag to clang. You can have multiple files, each defining
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different sets of attribute sets, to be combined into a single list by clang.
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The XRay stack tool
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-------------------
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Given a trace, and optionally an instrumentation map, the ``llvm-xray stack``
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command can be used to analyze a call stack graph constructed from the function
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call timeline.
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The way to use the command is to output the top stacks by call count and time spent.
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::
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$ llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc
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Unique Stacks: 3069
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Top 10 Stacks by leaf sum:
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Sum: 9633790
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lvl function count sum
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#0 main 1 58421550
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#1 compileModule(char**, llvm::LLVMContext&) 1 51440360
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#2 llvm::legacy::PassManagerImpl::run(llvm::Module&) 1 40535375
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#3 llvm::FPPassManager::runOnModule(llvm::Module&) 2 39337525
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#4 llvm::FPPassManager::runOnFunction(llvm::Function&) 6 39331465
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#5 llvm::PMDataManager::verifyPreservedAnalysis(llvm::Pass*) 399 16628590
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#6 llvm::PMTopLevelManager::findAnalysisPass(void const*) 4584 15155600
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#7 llvm::PMDataManager::findAnalysisPass(void const*, bool) 32088 9633790
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..etc..
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In the default mode, identical stacks on different threads are independently
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aggregated. In a multithreaded program, you may end up having identical call
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stacks fill your list of top calls.
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To address this, you may specify the ``-aggregate-threads`` or
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``-per-thread-stacks`` flags. ``-per-thread-stacks`` treats the thread id as an
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implicit root in each call stack tree, while ``-aggregate-threads`` combines
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identical stacks from all threads.
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Flame Graph Generation
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----------------------
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The ``llvm-xray stack`` tool may also be used to generate flamegraphs for
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visualizing your instrumented invocations. The tool does not generate the graphs
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themselves, but instead generates a format that can be used with Brendan Gregg's
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FlameGraph tool, currently available on `github
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<https://github.com/brendangregg/FlameGraph>`_.
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To generate output for a flamegraph, a few more options are necessary.
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- ``-all-stacks`` - Emits all of the stacks.
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- ``-stack-format`` - Choose the flamegraph output format 'flame'.
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- ``-aggregation-type`` - Choose the metric to graph.
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You may pipe the command output directly to the flamegraph tool to obtain an
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svg file.
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::
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$llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc -stack-format=flame -aggregation-type=time -all-stacks | \
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/path/to/FlameGraph/flamegraph.pl > flamegraph.svg
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If you open the svg in a browser, mouse events allow exploring the call stacks.
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Chrome Trace Viewer Visualization
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---------------------------------
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We can also generate a trace which can be loaded by the Chrome Trace Viewer
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from the same generated trace:
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::
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$ llvm-xray convert -symbolize -instr_map=./bin/llc \
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-output_format=trace_event xray-log.llc.5rqxkU \
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| gzip > llc-trace.txt.gz
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From a Chrome browser, navigating to ``chrome:///tracing`` allows us to load
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the ``sample-trace.txt.gz`` file to visualize the execution trace.
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Further Exploration
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-------------------
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The ``llvm-xray`` tool has a few other subcommands that are in various stages
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of being developed. One interesting subcommand that can highlight a few
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interesting things is the ``graph`` subcommand. Given for example the following
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toy program that we build with XRay instrumentation, we can see how the
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generated graph may be a helpful indicator of where time is being spent for the
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application.
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.. code-block:: c++
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// sample.cc
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#include <iostream>
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#include <thread>
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[[clang::xray_always_instrument]] void f() {
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std::cerr << '.';
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}
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[[clang::xray_always_instrument]] void g() {
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for (int i = 0; i < 1 << 10; ++i) {
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std::cerr << '-';
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}
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}
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int main(int argc, char* argv[]) {
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std::thread t1([] {
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for (int i = 0; i < 1 << 10; ++i)
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f();
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});
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std::thread t2([] {
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g();
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});
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t1.join();
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t2.join();
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std::cerr << '\n';
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}
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We then build the above with XRay instrumentation:
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::
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$ clang++ -o sample -O3 sample.cc -std=c++11 -fxray-instrument -fxray-instruction-threshold=1
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$ XRAY_OPTIONS="patch_premain=true xray_mode=xray-basic" ./sample
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We can then explore the graph rendering of the trace generated by this sample
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application. We assume you have the graphviz toosl available in your system,
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including both ``unflatten`` and ``dot``. If you prefer rendering or exploring
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the graph using another tool, then that should be feasible as well. ``llvm-xray
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graph`` will create DOT format graphs which should be usable in most graph
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rendering applications. One example invocation of the ``llvm-xray graph``
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command should yield some interesting insights to the workings of C++
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applications:
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::
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$ llvm-xray graph xray-log.sample.* -m sample -color-edges=sum -edge-label=sum \
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| unflatten -f -l10 | dot -Tsvg -o sample.svg
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Next Steps
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----------
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If you have some interesting analyses you'd like to implement as part of the
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llvm-xray tool, please feel free to propose them on the llvm-dev@ mailing list.
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The following are some ideas to inspire you in getting involved and potentially
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making things better.
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- Implement a query/filtering library that allows for finding patterns in the
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XRay traces.
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- Collecting function call stacks and how often they're encountered in the
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XRay trace.
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