Question about long UI “stall” reports with stacks in Chromium message loop

[hewro]

Hi Chromium folks,

I’m working on responsiveness monitoring for a Chromium-based application. We report a UI stall when the browser main thread has no heartbeat for more than 6 seconds. The heartbeat is application-defined: the UI thread periodically posts to a watchdog thread.

A noticeable portion of these reports capture a stack that is only in the normal message-loop wait path, with no product/business logic above it.

On Windows, stacks often look like:

- `ZwUserMsgWaitForMultipleObjectsEx`
- `user32!RealMsgWaitForMultipleObjectsEx`
- `base::MessagePumpForUI::WaitForWork`
- `base::MessagePumpForUI::DoRunLoop`

On macOS, they often look like:

- `_mach_msg`
- `CFRunLoopRunSpecific`
- `-[NSApplication run]`
- `base::MessagePumpNSApplication::DoRun`

Our current interpretation is mainly:

1. the UI thread/process was not scheduled for a long time due to system-level reasons (high load, suspension, etc.), or
2. the sampled stack is not the actual hang site, and the thread had already returned to the message loop by the time we captured it.

We also account for possible watchdog-thread delay on our side, with separate telemetry to detect that.

For this kind of pure wait-path stack, is that the right way to think about it from the Chromium team’s perspective? Are there other common explanations we should consider?

Also, are there any recommended diagnostics, heuristics, or practical mitigation tricks for investigating and reducing this kind of report?

Thanks!

[Joe Mason]

+scheduler-dev

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[Joe Mason]

Is your app registering a HangWatcher? (See https://source.chromium.org/chromium/chromium/src/+/main:base/threading/hang_watcher.h;l=144;drc=70e6ba389f518889549997d14ab0659de18a8b1d). Your heartbeat sounds like it would detect the same situations that the hang watcher already covers, so I'm curious if there's a discrepancy.