Perfetto <-> Ftrace interoperability

This doc is WIP, stay tuned.

This doc should:

  • Describe the ftrace trace_pipe_raw -> protobuf translation.
  • Describe how we deal with kernel ABI (in)stability and ftrace fields changing over kernel versions (we process event/**/format files on-device).
  • Describe how to generate ftrace protos (tools/, tools/
  • Describe how session multiplexing works.
  • Describe the page-by-page scheduling algorithm that uses vmsplice()

Code lives in /src/traced/probes/ftrace.


main thread [drain] [unblock] /: | post .-----' : | / : v worker #0 [splice ...] [wakeup] [block ............] [splice] : worker #1 [splice ...] [wakeup] [block ........] [splice] : worker #2 [splice ..........................................] : : drain period (100ms) In other words, the splice(2) system call is used to move data from the raw kernel ftrace pipe into an intermediate pipe at a page granularity. This call allows every per-cpu worker to sleep until there is at least one page of data available. When a worker wakes up, it will attempt to move as many pages as possible to its staging pipe (up to 64K, depending on the system's pipe buffer size) in a non-blocking way. After this, it will notify the main thread that data is available. This notification will block the calling worker until the main thread has drained the data. When at least one worker has woken up, we schedule a drain operation on the main thread for the next drain period (every 100ms by default). The drain operation parses ftrace data from the staging pipes of every worker having pending data. After this, each waiting worker is allowed to issue another call to splice(), restarting the cycle.