[release/10.0] Configurable EventPipe CPU sampling rate

[steveisok]

Note

This pull request was prepared with assistance from GitHub Copilot.

Backport of the CoreCLR / NativeAOT portion of #127292 to release/10.0.

Customer Impact

• [] Customer reported
• Found internally

Cosmic has reported dramatic overhead when trying to enable the sample profiler due to our default sampling rate being too high. They saw this change land in main and requested we backport to 10 so that they can override to a lower sampling rate.

Regression

• Yes
• No

Testing

Added a functional test to change the default sampling rate and make sure it lines up with what was expected.

Risk

Low - this only activates with the DOTNET_EventPipeThreadSamplingRate environment variable.

[steveisok]

I need to change the PR description to the typical backport template

[Copilot]

Pull request overview

This PR backports the CoreCLR/NativeAOT portion of #127292 onto release/10.0, adding a DOTNET_EventPipeThreadSamplingRate configuration knob so the EventPipe CPU sample profiler interval can be overridden from the default (1ms).

Changes:

• Adds a new CoreCLR config value (INTERNAL_EventPipeThreadSamplingRate) and runtime-specific implementations to read the sampling interval (ms).
• Updates EventPipe initialization to apply the configured sampling rate (ms → ns), falling back to platform defaults when unset/0.
• Includes a small CoreCLR PROFILING_SUPPORTED guard adjustment for ep_rt_notify_profiler_provider_created, plus a Mono stub implementation to keep shared native EventPipe code building.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/coreclr/inc/clrconfigvalues.h	Registers INTERNAL_EventPipeThreadSamplingRate CoreCLR knob (ms, 0 = default).
src/coreclr/vm/eventing/eventpipe/ep-rt-coreclr.h	Implements CoreCLR getter via CLRConfig and adjusts profiler-provider notification guard.
src/coreclr/nativeaot/Runtime/eventpipe/ep-rt-aot.h	Implements NativeAOT getter via RhConfig::Environment::TryGetIntegerValue (decimal).
src/native/eventpipe/ep-rt.h	Declares ep_rt_config_value_get_sampling_rate() in the shared runtime abstraction.
src/native/eventpipe/ep.c	Reads the configured sampling rate in ep_init and applies it (ms → ns).
src/mono/mono/eventpipe/ep-rt-mono.h	Adds a Mono implementation reading DOTNET_/COMPlus_EventPipeThreadSamplingRate for build compatibility.

[pavelsavara]

It would be good to test it on Net11 on non-wasm OS before we backport it.

[steveisok]

Note

This comment was authored with assistance from GitHub Copilot.

Added a regression test in 8277aea covering the new DOTNET_EventPipeThreadSamplingRate knob.

Test: src/tests/tracing/eventpipe/samplingrate/

Design — two-process, same binary:

• Parent spawns a child via Process.Start with DOTNET_EventPipeThreadSamplingRate=50 set in StartInfo.Environment (the runtime reads the value in ep_init during startup, so it has to be set before the .NET process launches).
• Child runs IpcTraceTest.RunAndValidateEventCounts against itself, subscribes to Microsoft-DotNETCore-SampleProfiler, burns CPU for ~3 s, then inspects ThreadSample events (provider name + event ID 0, to avoid relying on the friendly event name).
• Validation on the busiest sampled thread:
  • count >= 30 — sanity: sampling actually happened
  • count <= 600 — rejects the default ~1 ms rate (which produces ~1500 samples/thread in 3 s)
  • median inter-sample interval >= 20 ms — also rejects the default ~1 ms rate

Local verification on osx-arm64:

• With env var set: 36 samples/thread, median interval 85 ms → PASS
• Without env var (i.e. knob ignored): 1573 samples/thread → FAIL with the targeted error message

This is intended to be upstreamed to main as a follow-up if the backport lands.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.