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[browser] Fix EventPipe CPU sampling stall under coarse browser timer resolution - #129848
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pavelsavara merged 1 commit intoJun 26, 2026
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…esolution update_sample_frequency divided by ms_since_last_sample which can round to 0 with coarse browser timer resolution, producing +Inf that saturates skips_per_period to INT_MAX and permanently disables sampling. Skip recompute when no wall-clock time elapsed and clamp the averaged skip count. Re-enables the EventPipe CPU sample tests previously disabled by dotnet#129584.
pavelsavara
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ilonatommy, lateralusX, maraf, steveisok and vitek-karas
as code ownersJune 25, 2026 13:17
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Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara |
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Pull request overview
This PR adjusts Mono’s browser EventPipe CPU sampling skip-calculation to avoid pathological behavior when the browser’s performance.now() resolution is coarse (e.g., repeated 0 deltas), and re-enables the previously-disabled Blazor EventPipe CPU sampling tests.
Changes:
- Guard
update_sample_frequency()to only recomputeskips_per_periodwhenms_since_last_sample > 0, and clamp the computed skip range. - Re-enable Blazor EventPipe CPU sampling tests by removing the
ActiveIssueskips.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| src/mono/mono/eventpipe/ep-rt-mono-runtime-provider.c | Adds elapsed-time guard and clamps adaptive skip computation for single-threaded browser sampling. |
| src/mono/wasm/Wasm.Build.Tests/Blazor/EventPipeDiagnosticsTests.cs | Removes ActiveIssue attributes to re-enable CPU sampling test coverage. |
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maraf
approved these changes
Jun 25, 2026
pavelsavara
enabled auto-merge (squash)
June 25, 2026 14:09
pavelsavara
commented
Jun 26, 2026
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pavelsavara
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Jun 26, 2026
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eiriktsarpalis pushed a commit
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Jul 15, 2026
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Fixes#129584.
Problem
On WebAssembly the EventPipe CPU sample profiler could permanently stop emitting samples.
update_sample_frequencyrecomputesskips_per_periodby dividingsample_skip_counterbyms_since_last_sample. With the coarse browser timer resolution,ms_since_last_samplecan round to0during a burst of instrumentation callbacks. Dividing by0yields+Inf, which — once cast tointunder WASM's saturating, non-trapping float-to-int conversion — pinsskips_per_periodatINT_MAX, permanently disabling sampling.Fix
ms_since_last_sample > 0); otherwise keep the previousskips_per_period.[1, 10000]) so sampling stays responsive, matching the CoreCLR single-threaded sampler behavior.This re-enables the EventPipe CPU sample Blazor tests (
BlazorEventPipeTestWithCpuSamplesAOTandBlazorEventPipeTestWithCpuSamples) that were disabled by the issue.Note
This PR description was generated with assistance from GitHub Copilot.