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<main>
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<h1 id="tracing"><a class="header" href="#tracing">Tracing</a></h1>
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<p>Similar to other tools in the Rust ecosystem like rustc and cargo,
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rustup also provides observability/logging features via the <code>tracing</code> crate.</p>
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<p>The verbosity of logs is controlled via the <code>RUSTUP_LOG</code> environment
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variable using <code>tracing_subscriber</code>’s <a href="https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html#directives">directive syntax</a>.</p>
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<h2 id="console-based-tracing"><a class="header" href="#console-based-tracing">Console-based tracing</a></h2>
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<p>A <code>tracing_subscriber</code> that prints log lines directly to <code>stderr</code> is directly
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available in the prebuilt version of rustup since v1.28.0.</p>
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<p>For historical reasons, if <code>RUSTUP_LOG</code> is not set, this subscriber will print
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the log lines in a format that mimics the “legacy” <code>stderr</code> output in older
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versions of rustup:</p>
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<pre><code class="language-console">> rustup default stable
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info: using existing install for 'stable-aarch64-apple-darwin'
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info: default toolchain set to 'stable-aarch64-apple-darwin'
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stable-aarch64-apple-darwin unchanged - rustc 1.79.0 (129f3b996 2024-06-10)
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||
</code></pre>
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<p>However, once <code>RUSTUP_LOG</code> is set to any value, rustup’s “custom logging mode” will
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be activated, and <code>tracing_subscriber</code>’s builtin output format will be used instead:</p>
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<pre><code class="language-console">> RUSTUP_LOG=trace rustup default stable
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2024-06-16T12:08:48.732894Z INFO rustup::cli::common: using existing install for 'stable-aarch64-apple-darwin'
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2024-06-16T12:08:48.739232Z INFO rustup::cli::common: default toolchain set to 'stable-aarch64-apple-darwin'
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stable-aarch64-apple-darwin unchanged - rustc 1.79.0 (129f3b996 2024-06-10)
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</code></pre>
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<p>Please note that since <code>RUSTUP_LOG=trace</code> essentially accepts log lines from
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all possible sources, you might sometimes see log lines coming from rustup’s
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dependencies, such as <code>hyper_util</code> in the following example:</p>
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<pre><code class="language-console">> RUSTUP_LOG=trace rustup update
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[..]
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2024-06-16T12:12:45.569428Z TRACE hyper_util::client::legacy::client: http1 handshake complete, spawning background dispatcher task
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2024-06-16T12:12:45.648682Z TRACE hyper_util::client::legacy::pool: pool dropped, dropping pooled (("https", static.rust-lang.org))
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stable-aarch64-apple-darwin unchanged - rustc 1.79.0 (129f3b996 2024-06-10)
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nightly-aarch64-apple-darwin unchanged - rustc 1.81.0-nightly (3cf924b93 2024-06-15)
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2024-06-16T12:12:45.693350Z INFO rustup::cli::rustup_mode: cleaning up downloads & tmp directories
|
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</code></pre>
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<p>It is also possible to limit the sources of the log lines and the desired
|
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max level for each source. For example, set <code>RUSTUP_LOG=rustup=DEBUG</code> to
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receive log lines only from <code>rustup</code> itself with a max verbosity of <code>DEBUG</code>.</p>
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<h2 id="opentelemetry-tracing"><a class="header" href="#opentelemetry-tracing">Opentelemetry tracing</a></h2>
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<p>The feature <code>otel</code> can be used when building rustup to turn on Opentelemetry
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tracing with an OLTP GRPC exporter.</p>
|
||
<p>This can be very useful for diagnosing performance or correctness issues in more
|
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complicated scenarios.</p>
|
||
<p>The normal <a href="https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md">OTLP environment
|
||
variables</a>
|
||
can be used to customise its behaviour, but often the simplest thing is to just
|
||
run a Jaeger docker container on the same host:</p>
|
||
<pre><code class="language-sh">docker run -d --name jaeger -e COLLECTOR_ZIPKIN_HOST_PORT=:9411 -e COLLECTOR_OTLP_ENABLED=true -p 6831:6831/udp -p 6832:6832/udp -p 5778:5778 -p 16686:16686 -p 4317:4317 -p 4318:4318 -p 14250:14250 -p 14268:14268 -p 14269:14269 -p 9411:9411 jaegertracing/all-in-one:latest
|
||
</code></pre>
|
||
<p>Then build <code>rustup-init</code> with tracing:</p>
|
||
<pre><code class="language-sh">cargo build --features=otel
|
||
</code></pre>
|
||
<p>Run the operation you want to analyze. For example, we can now run <code>rustup show</code> with tracing:</p>
|
||
<pre><code class="language-sh">RUSTUP_FORCE_ARG0="rustup" ./target/debug/rustup-init show
|
||
</code></pre>
|
||
<p>And <a href="http://localhost:16686/search?service=rustup">look in Jaeger for a trace</a>.</p>
|
||
<p>Tracing can also be used in tests to get a trace of the operations taken during the test.
|
||
To use this feature, build the project with <code>--features=otel,test</code>.</p>
|
||
<h2 id="adding-instrumentation"><a class="header" href="#adding-instrumentation">Adding instrumentation</a></h2>
|
||
<p>Instrumenting a currently uninstrumented function is mostly simply done like so:</p>
|
||
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>#[tracing::instrument(level = "trace", err(level = "trace"), skip_all)]
|
||
<span class="boring">}</span></code></pre>
|
||
<p>Sometimes you might want to instrument a function only when the <code>otel</code> feature is enabled.
|
||
In this case, you will need to use conditional compilation with <code>cfg_attr</code>:</p>
|
||
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>#[cfg_attr(feature="otel", tracing::instrument(level = "trace", err(level = "trace"), skip_all))]
|
||
<span class="boring">}</span></code></pre>
|
||
<p><code>skip_all</code> is not required, but some core structs don’t implement <code>Debug</code> yet, and
|
||
others have a lot of output in <code>Debug</code>: tracing adds some overheads, so keeping
|
||
spans lightweight can help avoid frequency bias in the results - where
|
||
parameters with large debug in frequently called functions show up as much
|
||
slower than they are.</p>
|
||
<p>Some good general heuristics:</p>
|
||
<ul>
|
||
<li>Do instrument slow blocking functions</li>
|
||
<li>Do instrument functions with many callers or that call many different things,
|
||
as these tend to help figure the puzzle of what-is-happening</li>
|
||
<li>Default to not instrumenting thin shim functions (or at least, only instrument
|
||
them temporarily while figuring out the shape of a problem)</li>
|
||
<li>Be way of debug build timing - release optimisations make a huge difference,
|
||
though debug is a lot faster to iterate on. If something isn’t a problem in
|
||
release don’t pay it too much heed in debug.</li>
|
||
</ul>
|
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