Debugging bootstrap

There are two main ways of debugging (and profiling bootstrap). The first is through println logging, and the second is through the tracing feature.

println logging

Bootstrap has extensive unstructured logging. Most of it is gated behind the --verbose flag (pass -vv for even more detail).

If you want to see verbose output of executed Cargo commands and other kinds of detailed logs, pass -v or -vv when invoking bootstrap. Note that the logs are unstructured and may be overwhelming.

$ ./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)
...

tracing in bootstrap

Bootstrap has a conditional tracing feature, which provides the following features:

  • It enables structured logging using tracing events and spans.
  • It generates a Chrome trace file that can be used to visualize the hierarchy and durations of executed steps and commands.
    • You can open the generated chrome-trace.json file using Chrome, on the chrome://tracing tab, or e.g. using Perfetto.
  • It generates GraphViz graphs that visualize the dependencies between executed steps.
    • You can open the generated step-graph-*.dot file using e.g. xdot to visualize the step graph, or use e.g. dot -Tsvg to convert the GraphViz file to an SVG file.
  • 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.
    • The generated command-stats.txt file is in a simple human-readable format.

The structured logs will be written to standard error output (stderr), while the other outputs will be stored in files in the <build-dir>/bootstrap-trace/<pid> directory. For convenience, bootstrap will also create a symlink to the latest generated trace output directory at <build-dir>/bootstrap-trace/latest.

Note that if you execute bootstrap with --dry-run, 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.

Enabling tracing output

To enable the conditional tracing feature, run bootstrap with the BOOTSTRAP_TRACING environment variable.

$ BOOTSTRAP_TRACING=trace ./x build library --stage 1

Example output[^unstable]:

$ 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

[^unstable]: This output is always subject to further changes.

Controlling tracing output

The environment variable BOOTSTRAP_TRACING accepts a tracing_subscriber filter. If you set BOOTSTRAP_TRACING=trace, you will enable all logs, but that can be overwhelming. You can thus use the filter to reduce the amount of data logged.

There are two orthogonal ways to control which kind of tracing logs you want:

  1. You can specify the log level, e.g. debug or trace.
    • If you select a level, all events/spans with an equal or higher priority level will be shown.
  2. You can also control the log target, e.g. bootstrap or bootstrap::core::config or a custom target like CONFIG_HANDLING or STEP.
    • Custom targets are used to limit what kinds of spans you are interested in, as the BOOTSTRAP_TRACING=trace output can be quite verbose. Currently, you can use the following custom targets:
      • CONFIG_HANDLING: show spans related to config handling.
      • STEP: show all executed steps. Executed commands have info event level.
      • COMMAND: show all executed commands. Executed commands have trace event level.
      • IO: show performed I/O operations. Executed commands have trace event level.
        • Note that many I/O are currently not being traced.

You can of course combine them (custom target logs are typically gated behind TRACE log level additionally):

$ BOOTSTRAP_TRACING=CONFIG_HANDLING=trace,STEP=info,COMMAND=trace ./x build library --stage 1

Note that the level that you specify using BOOTSTRAP_TRACING also has an effect on the spans that will be recorded in the Chrome trace file.

FIXME(#96176): specific tracing for compiler() vs compiler_for()

The additional targets COMPILER and COMPILER_FOR are used to help trace what builder.compiler() and builder.compiler_for() does. They should be removed if #96176 is resolved.

Using tracing in bootstrap

Both tracing::* macros and the tracing::instrument proc-macro attribute need to be gated behind tracing feature. Examples:

#[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!()
    }    
}

For #[instrument], it's recommended to:

  • Gate it behind trace level for fine-granularity, possibly debug level for core functions.
  • Explicitly pick an instrumentation name via name = ".." to distinguish between e.g. run of different steps.
  • Take care to not cause diverging behavior via tracing, e.g. building extra things only when tracing infra is enabled.

rust-analyzer integration?

Unfortunately, because bootstrap is a rust-analyzer.linkedProjects, you can't ask r-a to check/build bootstrap itself with tracing feature enabled to get relevant completions, due to lack of support as described in https://github.com/rust-lang/rust-analyzer/issues/8521.