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[browser] Fix EventPipe CPU sampling stall under coarse browser timer resolution #129848

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[browser] Fix EventPipe CPU sampling stall under coarse browser timer resolution #129848

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18 changes: 15 additions & 3 deletions src/mono/mono/eventpipe/ep-rt-mono-runtime-provider.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1435,15 +1435,27 @@ static void update_sample_frequency ()
double now = profiler_now ();
double ms_since_last_sample = now - last_sample_time;

if (desired_sample_interval_ms > 0 && last_sample_time != 0) {
// Only recompute when wall-clock time actually advanced. With coarse browser
// timer resolution ms_since_last_sample can round to 0 during a burst of
// instrumentation callbacks; dividing by it produces +Inf which, once cast to
// int (saturating, nontrapping-float-to-int), pins skips_per_period at INT_MAX
// and permanently disables sampling. Also clamp the result to keep sampling
// responsive, matching the CoreCLR single-threaded sampler.
if (desired_sample_interval_ms > 0 && last_sample_time != 0 && ms_since_last_sample > 0) {
// recalculate ideal number of skips per period
double skips_per_ms = ((double)sample_skip_counter) / ms_since_last_sample;
double newskips_per_period = (skips_per_ms * ((double)desired_sample_interval_ms));
skips_per_period = (int)((newskips_per_period + ((double)sample_skip_counter) + ((double)prev_skips_per_period)) / 3.0);
double avg_skips = (newskips_per_period + ((double)sample_skip_counter) + ((double)prev_skips_per_period)) / 3.0;
if (avg_skips < 1.0)
avg_skips = 1.0;
else if (avg_skips > 10000.0)
avg_skips = 10000.0;
skips_per_period = (int)avg_skips;
prev_skips_per_period = sample_skip_counter;
} else {
} else if (last_sample_time == 0 || desired_sample_interval_ms <= 0) {
skips_per_period = 0;
}
// else (ms_since_last_sample == 0): keep the previous skips_per_period unchanged
last_sample_time = now;
sample_skip_counter = 0;
}
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2 changes: 0 additions & 2 deletions src/mono/wasm/Wasm.Build.Tests/Blazor/EventPipeDiagnosticsTests.cs
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Expand Up @@ -30,13 +30,11 @@ public EventPipeDiagnosticsTests(ITestOutputHelper output, SharedBuildPerTestCla

[Fact]
[TestCategory("native-mono")]
[ActiveIssue("https://github.com/dotnet/runtime/issues/129584", typeof(BuildTestBase), nameof(BuildTestBase.IsMonoRuntime))]
public Task BlazorEventPipeTestWithCpuSamplesAOT() => BlazorEventPipeTestWithCpuSamples(Configuration.Release, aot: true);

[Theory]
[InlineData(Configuration.Debug, false)]
[InlineData(Configuration.Release, false)]
[ActiveIssue("https://github.com/dotnet/runtime/issues/129584", typeof(BuildTestBase), nameof(BuildTestBase.IsMonoRuntime))]
public async Task BlazorEventPipeTestWithCpuSamples(Configuration config, bool aot)
{
string extraProperties = @"
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