[EventPipe][UserEvents] Deduplicate EventPipeEvent Metadata emission #117686
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Pull Request Overview
This PR deduplicates EventPipe event metadata per session by tracking metadata emission with a mask, and adds an atomic compare-and-exchange for int64 across runtimes.
- Introduce
metadata_written_maskand its update function onEventPipeEvent - Use
ep_rt_atomic_compare_exchange_int64_tto atomically set/reset metadata bits - Modify session and provider logic to emit metadata only once per session and reset on config changes
Reviewed Changes
Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| src/native/eventpipe/ep-session.c | Wrap metadata header emit in per-session mask check |
| src/native/eventpipe/ep-rt.h | Declare ep_rt_atomic_compare_exchange_int64_t |
| src/native/eventpipe/ep-provider.c | Pass session_mask to refresh functions and clear metadata mask |
| src/native/eventpipe/ep-event.h | Add metadata_written_mask field and update function declaration |
| src/native/eventpipe/ep-event.c | Initialize and implement ep_event_update_metadata_written_mask |
| src/mono/mono/eventpipe/ep-rt-mono.h | Implement ep_rt_atomic_compare_exchange_int64_t for Mono branch |
| src/coreclr/vm/eventing/eventpipe/ep-rt-coreclr.h | Implement ep_rt_atomic_compare_exchange_int64_t for CoreCLR branch |
| src/coreclr/nativeaot/Runtime/eventpipe/ep-rt-aot.h/.cpp | Implement ep_rt_atomic_compare_exchange_int64_t for NativeAOT branch |
Comments suppressed due to low confidence (2)
src/native/eventpipe/ep-session.c:830
- The metadata payload is written unconditionally, even when
should_write_metadatais false. You should wrap both the header and payload writes (includingio[i].iov_base,iov_len, andextension_lenupdates) inside theif (should_write_metadata)block to avoid emitting metadata twice or without a header.
io[io_index].iov_len = metadata_len;
src/native/eventpipe/ep-session.c:821
- Declaring
metadata_leninside theifblock makes it inaccessible when writing the payload below. This will cause a compilation error. Consider declaringmetadata_lenbefore theifso it can be reused for both header and payload writes (or move the payload write inside theif).
uint32_t metadata_len = ep_event_get_metadata_len (ep_event);
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This was referenced Jul 16, 2025
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This was referenced Jul 16, 2025
lateralusX
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Jul 17, 2025
Move metadata_written_mask bit flip to after writing the event Fix bytes_written operator precedence bug
noahfalk
approved these changes
Jul 17, 2025
This was referenced Jul 18, 2025
|
/ba-g The windows-x64 NativeAOT lane failure should be #117795, the linux System.Net.HttpListener.Tests failure is asserting on the same assertion as #117795, the rest are test timeouts mostly seemingly due to machine inavailability, which shouldn't be impacted by this PRs changes as there are no user_events tests yet. |
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Updating docs in response to dotnet/runtime#117686 and dotnet/runtime#118294
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Follow-up to #115265
EventPipeEvent Metadata is typically emitted on the first time it's being written, a.k.a. the "Metadata Event". Typically, EventPipe Sessions would track the "Metadata Event" through this logic, leveraging single-threaded writers to track whether or not metadata has been written for that particular session.
For user_events-based EventPipe Sessions, events are written directly to the kernel's
user_events_datafile. As such, we need to keep track of whether or not Metadata has been transmitted for a particular EventPipeEvent for a particular EventPipe Session. To account for:This PR adds a
metadata_written_maskfield toEventPipeEvents to keep track of whether an Event has been written for a particular active session. In addition, for a lock-free and thread safe mechanism to update themetadata_written_mask,ep_rt_atomic_compare_exchange_int64_tis implemented.Testing
Manual testing with the same C program used in #117435 and #115265.
Metadata is only emitted once for each EventID 50 and 303, indicated by just 2 instances of
extension=01.Moreover, after disabling the session, the
EventPipeEventinstance'smetadata_written_maskbits had been reset accordingly.