Tracing across reboot

This data source is supported only on the linux-based systems.

The "linux.frozen_ftrace" data source is used for reading the ftrace trace data recorded in the previous boot on the persistent ring buffer.

This data source allows you to dump the last seconds of the ftrace trace data in the previous boot time, for analyzing the system crash reason from the ftrace trace log.

Therefore, this is expected that the user ran the another perfetto trace session in background on the special persistent ring buffer.

Creating a persistent ring buffer

You have to set up a ftrace persistent ring buffer via the kernel cmdline. If you need a 20MiB persistent ring buffer, you need to add following kernel options to the kernel cmdline when boot.

This creates a boot_mapped ftrace instance on a reserved memory area, which will preseve the data and be attatched again in the next boot. (Note: this is not 100% sure if the kernel configuration has been changed or kernel address mapping is changed by KASLR.)

Use the persistent ring buffer

Normally, perfetto will record the ftrace data in the top level instance instead of the sub-instances. Thus you need to specify instance_name: option to your trace config. Also, you need to run the trace session as long-time backend session. What you need are;

• Specify RING_BUFFER fill_policy to all buffers which receives ftrace data source.
• Specify instance_name: "boot_mapped" to the ftrace data source. (NOTE: Split the atrace data source from this data source, since atrace related events can not be used for this instance.)
• Do not specify duration_ms:.

And run the perfetto command with --background option.

Once you have done it, prepare for a crash.

Read out the data after crash

After the system crash, you will see the boot_mapped instance, which should keep the trace data recorded in the last seconds.

Run the perfetto with "linux.frozen_ftrace" data source like;