Fix MCP trace loading hang by ivberg · Pull Request #45 · microsoft/profile-explorer

ivberg · 2026-04-07T21:17:46Z

Fix MCP trace loading hang: add PMC sample support and remove dangerous UI fallback

Add PerfInfoPMCSample handler to BuildProcessSummary so traces collected with hardware counter profiling (not just standard CPU sampling) correctly enumerate processes
Remove fallback from OpenTrace to OpenTraceAsync when process list extraction fails — the old code returned a clean error, but baa4f95 introduced a path that opened a ProfileLoadWindow which also couldn't load, causing a hang
Reopen trace source for merged ETL files ("[multiple files]") so the second Process() pass gets fresh events instead of an exhausted source
Close ProfileLoadWindow in finally blocks to prevent dialog accumulation across repeated MCP open_trace calls
Guard MessageBox.Show calls with SuppressDialogsForAutomation to prevent UI deadlocks during MCP automation
Cache FindTraceProcesses result to avoid double ETL parsing when GetAvailableProcessesAsync and OpenTraceAsync run back-to-back
Use bidirectional Contains matching for process names so compound inputs like "msedgewebview2.exe webview-exe-name=searchhost.exe" resolve correctly
Add diagnostic logging throughout MCP flow for future debugging

…us UI fallback - Add PerfInfoPMCSample handler to BuildProcessSummary so traces collected with hardware counter profiling (not just standard CPU sampling) correctly enumerate processes - Remove fallback from OpenTrace to OpenTraceAsync when process list extraction fails — the old code returned a clean error, but baa4f95 introduced a path that opened a ProfileLoadWindow which also couldn't load, causing a hang - Reopen trace source for merged ETL files ("[multiple files]") so the second Process() pass gets fresh events instead of an exhausted source - Close ProfileLoadWindow in finally blocks to prevent dialog accumulation across repeated MCP open_trace calls - Guard MessageBox.Show calls with SuppressDialogsForAutomation to prevent UI deadlocks during MCP automation - Cache FindTraceProcesses result to avoid double ETL parsing when GetAvailableProcessesAsync and OpenTraceAsync run back-to-back - Use bidirectional Contains matching for process names so compound inputs like "msedgewebview2.exe webview-exe-name=searchhost.exe" resolve correctly - Add diagnostic logging throughout MCP flow for future debugging

Copilot

Pull request overview

Fixes an MCP automation hang when opening traces by improving process enumeration for more trace types and removing UI-based fallback paths that could deadlock during automation.

Changes:

Add PMC (PerfInfoPMCSample) support and trace re-open logic in ETW process summary building to handle hardware-counter traces and merged ETLs.
Update MCP OpenTrace flow to avoid falling back to UI loading on process-list extraction failure, add diagnostics, and improve process-name matching.
Add automation hooks in ProfileLoadWindow to suppress modal dialogs and allow injecting a preloaded process list to avoid double ETL parsing.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/ProfileExplorerUI/Windows/ProfileLoadWindow.xaml.cs	Adds MCP automation hooks (skip TextChanged processing, inject preloaded process list) and guards MessageBox usage to avoid UI deadlocks.
src/ProfileExplorerUI/Mcp/McpActionExecutor.cs	Adds cached process list reuse between `GetAvailableProcessesAsync` and `OpenTraceAsync`, ensures ProfileLoadWindow cleanup, and adds diagnostic logging.
src/ProfileExplorerCore/Profile/ETW/ETWEventProcessor.cs	Extends process-summary building to handle PMC samples and reopens merged traces so the second processing pass can read events.
src/ProfileExplorer.Mcp/ProfileExplorerMcpServer.cs	Improves OpenTrace matching logic and removes the UI fallback when process enumeration fails; adds trace timing diagnostics.

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…eason - Rename CloseProfileLoadWindowOnFailureAsync to CloseProfileLoadWindowIfVisibleAsync since it's called unconditionally in finally blocks as a safety net - Change FailureReason from "ProcessListEmpty" to "GetAvailableProcessesFailed" to accurately describe the operation that failed regardless of underlying cause

When a trace has no CPU sampling, PMC, or CSwitch data (all process weights are zero), return a NoProfilingData error from OpenTrace instead of attempting to load an empty profile. Also fall back to process start/stop events for the process list when no sample events exist, so the trace contents are still discoverable via GetAvailableProcesses

trgibeau · 2026-04-09T02:48:03Z

+
            // First, check if this might be an ambiguous query by getting available processes
            GetAvailableProcessesResult processesResult = await _executor.GetAvailableProcessesAsync(profileFilePath);
+            Trace.TraceInformation($"[MCP] OpenTrace: GetAvailableProcesses completed in {stopwatch.ElapsedMilliseconds}ms, success={processesResult.Success}, count={processesResult.Processes?.Length ?? 0}");


call Stop on the stopwatch right before this line

trgibeau · 2026-04-09T02:50:55Z

+                        Description = "The trace file does not contain CPU sampling or performance counter data. It may have been collected without profiling enabled.",
+                        Timestamp = DateTime.UtcNow
+                    };
+                    return System.Text.Json.JsonSerializer.Serialize(noSamplesResult, new System.Text.Json.JsonSerializerOptions { WriteIndented = true });


We should update SerializeOpenTraceResult for these new errors, or create a new method to handle this.

trgibeau · 2026-04-09T02:53:41Z

+      if (!traceReopened) {
+        kernel = ReopenTrace();
+        DiagnosticLogger.LogInfo("[BuildProcessSummary] Reopened trace for merged ETL second pass");
+      }


What bug is this one fixing?

trgibeau · 2026-04-09T02:55:04Z

      }
    };

+    // Also handle PMC (Performance Monitor Counter) sample events, which are used


Do we need to flag that the event comes from PMC in the sample event data we are collecting?

trgibeau · 2026-04-09T02:56:24Z

+    if (result.Count == 0 && profile.Processes != null && profile.Processes.Count > 0) {
+      DiagnosticLogger.LogInfo($"[BuildProcessSummary] No samples found, building process list from {profile.Processes.Count} discovered processes");
+      foreach (var proc in profile.Processes) {
+        result.Add(new ProcessSummary(proc, TimeSpan.Zero));


What about having a negative weight? Or null weight?

0 weight could cause some issues if we have some sampled data?

trgibeau · 2026-04-09T02:57:51Z

+    /// Prevents dialog accumulation across multiple open_trace calls.
+    /// On success the dialog closes itself via LoadButton_Click; this is a safety net.
+    /// </summary>
+    private async Task CloseProfileLoadWindowIfVisibleAsync(ProfileExplorer.UI.ProfileLoadWindow window)


More reason to move this to a library we can consume...

trgibeau · 2026-04-09T02:59:30Z

-                textChangedMethod.Invoke(profileLoadWindow, new object[] { profileLoadWindow, new RoutedEventArgs() });
-            }
-        });
+                var textChangedMethod = profileLoadWindow.GetType().GetMethod("ProfileAutocompleteBox_TextChanged",


This could break very easily

trgibeau · 2026-04-09T03:00:16Z

+            var sw = Stopwatch.StartNew();
            var processSummaries = await ETWProfileDataProvider.FindTraceProcesses(
                profileFilePath, options, _ => { }, cancelableTask);
+            DiagnosticLogger.LogInfo($"[MCP] GetAvailableProcessesAsync: FindTraceProcesses completed in {sw.ElapsedMilliseconds}ms, count={processSummaries?.Count ?? 0}");


Are we still needing these debug stop watches?

ivberg assigned trgibeau Apr 7, 2026

Copilot AI review requested due to automatic review settings April 7, 2026 21:17

Copilot started reviewing on behalf of ivberg April 7, 2026 21:21 View session

Copilot AI reviewed Apr 7, 2026

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Comment thread src/ProfileExplorerUI/Mcp/McpActionExecutor.cs

Comment thread src/ProfileExplorerUI/Mcp/McpActionExecutor.cs

Comment thread src/ProfileExplorerCore/Profile/ETW/ETWEventProcessor.cs

Comment thread src/ProfileExplorer.Mcp/ProfileExplorerMcpServer.cs

ivberg added 2 commits April 7, 2026 15:35

trgibeau reviewed Apr 9, 2026

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Fix MCP trace loading hang#45

Fix MCP trace loading hang#45
ivberg wants to merge 3 commits intomainfrom
user/ivberg/fix-mcp-trace-loading-regression

ivberg commented Apr 7, 2026 •

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ivberg commented Apr 7, 2026 •

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