| commit | b2dea4fd22b79fa27ef1ebd737401616095a7de6 | [log] [tgz] |
|---|---|---|
| author | Mircea Trofin <[email protected]> | Tue Apr 08 13:59:38 2025 |
| committer | GitHub <[email protected]> | Tue Apr 08 13:59:38 2025 |
| tree | f0d23614c227e1c144b676b397b1fd5b8771e400 | |
| parent | f19c6f23abefac56fde9f2b393c9ffa7595c86c6 [diff] |
[ctxprof] root autodetection mechanism (#133147) This is an optional mechanism that automatically detects roots. It's a best-effort mechanism, and its main goal is to *avoid* pointing at the message pump function as a root. This is the function that polls message queue(s) in an infinite loop, and is thus a bad root (it never exits). High-level, when collection is requested - which should happen when a server has already been set up and handing requests - we spend a bit of time sampling all the server's threads. Each sample is a stack which we insert in a `PerThreadCallsiteTrie`. After a while, we run for each `PerThreadCallsiteTrie` the root detection logic. We then traverse all the `FunctionData`, find the ones matching the detected roots, and allocate a `ContextRoot` for them. From here, we special case `FunctionData` objects, in `__llvm_ctx_profile_get_context, that have a `CtxRoot` and route them to `__llvm_ctx_profile_start_context`. For this to work, on the llvm side, we need to have all functions call `__llvm_ctx_profile_release_context` because they _might_ be roots. This comes at a slight (percentages) penalty during collection - which we can afford since the overall technique is ~5x faster than normal instrumentation. We can later explore conditionally enabling autoroot detection and avoiding this penalty, if desired. Note that functions that `musttail call` can't have their return instrumented this way, and a subsequent patch will harden the mechanism against this case. The mechanism could be used in combination with explicit root specification, too.
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