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| <h1 id="debugging-bootstrap"><a class="header" href="#debugging-bootstrap">Debugging bootstrap</a></h1> |
| <p>There are two main ways of debugging (and profiling bootstrap). The first is through println logging, and the second is through the <code>tracing</code> feature.</p> |
| <h2 id="println-logging"><a class="header" href="#println-logging"><code>println</code> logging</a></h2> |
| <p>Bootstrap has extensive unstructured logging. Most of it is gated behind the <code>--verbose</code> flag (pass <code>-vv</code> for even more detail).</p> |
| <p>If you want to see verbose output of executed Cargo commands and other kinds of detailed logs, pass <code>-v</code> or <code>-vv</code> when invoking bootstrap. Note that the logs are unstructured and may be overwhelming.</p> |
| <pre><code>$ ./x dist rustc --dry-run -vv |
| learning about cargo |
| running: RUSTC_BOOTSTRAP="1" "/home/jyn/src/rust2/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "metadata" "--format-version" "1" "--no-deps" "--manifest-path" "/home/jyn/src/rust2/Cargo.toml" (failure_mode=Exit) (created at src/bootstrap/src/core/metadata.rs:81:25, executed at src/bootstrap/src/core/metadata.rs:92:50) |
| running: RUSTC_BOOTSTRAP="1" "/home/jyn/src/rust2/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "metadata" "--format-version" "1" "--no-deps" "--manifest-path" "/home/jyn/src/rust2/library/Cargo.toml" (failure_mode=Exit) (created at src/bootstrap/src/core/metadata.rs:81:25, executed at src/bootstrap/src/core/metadata.rs:92:50) |
| ... |
| </code></pre> |
| <h2 id="tracing-in-bootstrap"><a class="header" href="#tracing-in-bootstrap"><code>tracing</code> in bootstrap</a></h2> |
| <p>Bootstrap has a conditional <code>tracing</code> feature, which provides the following features:</p> |
| <ul> |
| <li>It enables structured logging using <a href="https://docs.rs/tracing/0.1.41/tracing/index.html"><code>tracing</code></a> events and spans.</li> |
| <li>It generates a <a href="https://www.chromium.org/developers/how-tos/trace-event-profiling-tool/">Chrome trace file</a> that can be used to visualize the hierarchy and durations of executed steps and commands. |
| <ul> |
| <li>You can open the generated <code>chrome-trace.json</code> file using Chrome, on the <code>chrome://tracing</code> tab, or e.g. using <a href="https://ui.perfetto.dev/">Perfetto</a>.</li> |
| </ul> |
| </li> |
| <li>It generates <a href="https://graphviz.org/doc/info/lang.html">GraphViz</a> graphs that visualize the dependencies between executed steps. |
| <ul> |
| <li>You can open the generated <code>step-graph-*.dot</code> file using e.g. <a href="https://github.com/jrfonseca/xdot.py">xdot</a> to visualize the step graph, or use e.g. <code>dot -Tsvg</code> to convert the GraphViz file to an SVG file.</li> |
| </ul> |
| </li> |
| <li>It generates a command execution summary, which shows which commands were executed, how many of their executions were cached, and what commands were the slowest to run. |
| <ul> |
| <li>The generated <code>command-stats.txt</code> file is in a simple human-readable format.</li> |
| </ul> |
| </li> |
| </ul> |
| <p>The structured logs will be written to standard error output (<code>stderr</code>), while the other outputs will be stored in files in the <code><build-dir>/bootstrap-trace/<pid></code> directory. For convenience, bootstrap will also create a symlink to the latest generated trace output directory at <code><build-dir>/bootstrap-trace/latest</code>.</p> |
| <blockquote> |
| <p>Note that if you execute bootstrap with <code>--dry-run</code>, the tracing output directory might change. Bootstrap will always print a path where the tracing output files were stored at the end of its execution.</p> |
| </blockquote> |
| <h3 id="enabling-tracing-output"><a class="header" href="#enabling-tracing-output">Enabling <code>tracing</code> output</a></h3> |
| <p>To enable the conditional <code>tracing</code> feature, run bootstrap with the <code>BOOTSTRAP_TRACING</code> environment variable.</p> |
| <pre><code class="language-bash">$ BOOTSTRAP_TRACING=trace ./x build library --stage 1 |
| </code></pre> |
| <p>Example output<sup class="footnote-reference" id="fr-unstable-1"><a href="#footnote-unstable">1</a></sup>:</p> |
| <pre><code>$ BOOTSTRAP_TRACING=trace ./x build library --stage 1 --dry-run |
| Building bootstrap |
| Finished `dev` profile [unoptimized] target(s) in 0.05s |
| 15:56:52.477 INFO > tool::LibcxxVersionTool {target: x86_64-unknown-linux-gnu} (builder/mod.rs:1715) |
| 15:56:52.575 INFO > compile::Assemble {target_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }} (builder/mod.rs:1715) |
| 15:56:52.575 INFO > tool::Compiletest {compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu} (builder/mod.rs:1715) |
| 15:56:52.576 INFO > tool::ToolBuild {build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "compiletest", path: "src/tools/compiletest", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "internal_output_capture", cargo_args: [], artifact_kind: Binary} (builder/mod.rs:1715) |
| 15:56:52.576 INFO > builder::Libdir {compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu} (builder/mod.rs:1715) |
| 15:56:52.576 INFO > compile::Sysroot {compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, force_recompile: false} (builder/mod.rs:1715) |
| 15:56:52.578 INFO > compile::Assemble {target_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }} (builder/mod.rs:1715) |
| 15:56:52.578 INFO > tool::Compiletest {compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu} (builder/mod.rs:1715) |
| 15:56:52.578 INFO > tool::ToolBuild {build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "compiletest", path: "src/tools/compiletest", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "internal_output_capture", cargo_args: [], artifact_kind: Binary} (builder/mod.rs:1715) |
| 15:56:52.578 INFO > builder::Libdir {compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu} (builder/mod.rs:1715) |
| 15:56:52.578 INFO > compile::Sysroot {compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, force_recompile: false} (builder/mod.rs:1715) |
| Finished `release` profile [optimized] target(s) in 0.11s |
| Tracing/profiling output has been written to <src-root>/build/bootstrap-trace/latest |
| Build completed successfully in 0:00:00 |
| </code></pre> |
| <h4 id="controlling-tracing-output"><a class="header" href="#controlling-tracing-output">Controlling tracing output</a></h4> |
| <p>The environment variable <code>BOOTSTRAP_TRACING</code> accepts a <a href="https://docs.rs/tracing-subscriber/0.3.19/tracing_subscriber/filter/struct.EnvFilter.html"><code>tracing_subscriber</code> filter</a>. If you set <code>BOOTSTRAP_TRACING=trace</code>, you will enable all logs, but that can be overwhelming. You can thus use the filter to reduce the amount of data logged.</p> |
| <p>There are two orthogonal ways to control which kind of tracing logs you want:</p> |
| <ol> |
| <li>You can specify the log <strong>level</strong>, e.g. <code>debug</code> or <code>trace</code>. |
| <ul> |
| <li>If you select a level, all events/spans with an equal or higher priority level will be shown.</li> |
| </ul> |
| </li> |
| <li>You can also control the log <strong>target</strong>, e.g. <code>bootstrap</code> or <code>bootstrap::core::config</code> or a custom target like <code>CONFIG_HANDLING</code> or <code>STEP</code>. |
| <ul> |
| <li>Custom targets are used to limit what kinds of spans you are interested in, as the <code>BOOTSTRAP_TRACING=trace</code> output can be quite verbose. Currently, you can use the following custom targets: |
| <ul> |
| <li><code>CONFIG_HANDLING</code>: show spans related to config handling.</li> |
| <li><code>STEP</code>: show all executed steps. Executed commands have <code>info</code> event level.</li> |
| <li><code>COMMAND</code>: show all executed commands. Executed commands have <code>trace</code> event level.</li> |
| <li><code>IO</code>: show performed I/O operations. Executed commands have <code>trace</code> event level. |
| <ul> |
| <li>Note that many I/O are currently not being traced.</li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ol> |
| <p>You can of course combine them (custom target logs are typically gated behind <code>TRACE</code> log level additionally):</p> |
| <pre><code class="language-bash">$ BOOTSTRAP_TRACING=CONFIG_HANDLING=trace,STEP=info,COMMAND=trace ./x build library --stage 1 |
| </code></pre> |
| <p>Note that the level that you specify using <code>BOOTSTRAP_TRACING</code> also has an effect on the spans that will be recorded in the Chrome trace file.</p> |
| <h5 id="fixme96176-specific-tracing-for-compiler-vs-compiler_for"><a class="header" href="#fixme96176-specific-tracing-for-compiler-vs-compiler_for">FIXME(#96176): specific tracing for <code>compiler()</code> vs <code>compiler_for()</code></a></h5> |
| <p>The additional targets <code>COMPILER</code> and <code>COMPILER_FOR</code> are used to help trace what |
| <code>builder.compiler()</code> and <code>builder.compiler_for()</code> does. They should be removed |
| if <a href="https://github.com/rust-lang/rust/issues/96176">#96176</a> is resolved.</p> |
| <h3 id="using-tracing-in-bootstrap"><a class="header" href="#using-tracing-in-bootstrap">Using <code>tracing</code> in bootstrap</a></h3> |
| <p>Both <code>tracing::*</code> macros and the <code>tracing::instrument</code> proc-macro attribute need to be gated behind <code>tracing</code> feature. Examples:</p> |
| <pre><code class="language-rs">#[cfg(feature = "tracing")] |
| use tracing::instrument; |
| |
| struct Foo; |
| |
| impl Step for Foo { |
| type Output = (); |
| |
| #[cfg_attr(feature = "tracing", instrument(level = "trace", name = "Foo::should_run", skip_all))] |
| fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> { |
| trace!(?run, "entered Foo::should_run"); |
| |
| todo!() |
| } |
| |
| fn run(self, builder: &Builder<'_>) -> Self::Output { |
| trace!(?run, "entered Foo::run"); |
| |
| todo!() |
| } |
| } |
| </code></pre> |
| <p>For <code>#[instrument]</code>, it's recommended to:</p> |
| <ul> |
| <li>Gate it behind <code>trace</code> level for fine-granularity, possibly <code>debug</code> level for core functions.</li> |
| <li>Explicitly pick an instrumentation name via <code>name = ".."</code> to distinguish between e.g. <code>run</code> of different steps.</li> |
| <li>Take care to not cause diverging behavior via tracing, e.g. building extra things only when tracing infra is enabled.</li> |
| </ul> |
| <h3 id="rust-analyzer-integration"><a class="header" href="#rust-analyzer-integration">rust-analyzer integration?</a></h3> |
| <p>Unfortunately, because bootstrap is a <code>rust-analyzer.linkedProjects</code>, you can't ask r-a to check/build bootstrap itself with <code>tracing</code> feature enabled to get relevant completions, due to lack of support as described in <a href="https://github.com/rust-lang/rust-analyzer/issues/8521">https://github.com/rust-lang/rust-analyzer/issues/8521</a>.</p> |
| <hr> |
| <ol class="footnote-definition"><li id="footnote-unstable"> |
| <p>This output is always subject to further changes. <a href="#fr-unstable-1">↩</a></p> |
| </li> |
| </ol> |
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