Tracing with Perfetto {#tracing-perfetto}#

Requirements#

Monado uses the Perfetto/Percetto framework for tracing support. You need to first build and install Percetto in a place where CMake can find it. Build Perfetto (you will have gotten the source at least as part of build Percetto). It is a good idea to familiarise yourself with Perfetto before proceeding. You then need to build Monado with CMake and make sure XRT_FEATURE_TRACING is enabled.

• Build and install Percetto - note Depending on the version of Percetto you are using you might need to have a release version of Perfetto available or use the one that is included in percetto. A release version of Perfetto is needed due to the sdk/ directory and amalgamated files is only made there.
• Build and get Perfetto running.
• Build Monado with CMake and with XRT_FEATURE_TRACING being ON.

Running#

Save the following file to data_events.cfg, next to your perfetto folder. Please refer to Perfetto documentation about the format and options of this config file, but the most important bits is the tracker_event section.

flush_period_ms: 30000

incremental_state_config {
	clear_period_ms: 500
}

buffers: {
	size_kb: 63488
	fill_policy: DISCARD
}

# This is the important bit, this enables all data events from Monado.
data_sources: {
	config: {
		name: "track_event"
		target_buffer: 0
	}
}

Then run the following commands before launching Monado.

# Start the daemon.
# Only needs to be run once and keeps running.
./perfetto/out/linux_clang_release/traced &

# Start the daemon ftrace probes daemon.
# Only needs to be run once and keeps running.
# Not needed with the preceding config.
./perfetto/out/linux_clang_release/traced_probes &

# When you want to run a capture do and then run Monado.
./perfetto/out/linux_clang_release/perfetto --txt -c data_events.cfg -o /tmp/trace.protobuf

Finally run the App and Monado with XRT_TRACING=true exported.

XRT_TRACING=true monado-serivce

Gotchas#

Here's where we write down bugs or other sharp corners that we found while running Monado with Perfetto/Percetto and tracing enabled.

OpenXR CTS#

Running multiple CTS tests in one run causes Perfetto to crash, this is because the CTS loads and unloads the OpenXR runtime multiple times, and there seems to be a race on destruction.

"Value doesn't exist" in web viewer#

This is probably because you don't have read permissions on your tracefile, probably because you ran traced/tracebox as root. Don't do that, instead do sudo chown -R $USER /sys/kernel/tracing and run traced/tracebox as your normal user.

(If you really have to run it as root, then before you open the tracefile do sudo chown $USER <tracefile>).