Tracing with 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>).