perf record [unknown] frames

README.md

If you see [unknown], the following might be useful assuming you're using --call-stack dwarf:

• Note that --call-graph dwarf,1024 (where 1024 is the stack size) dumps first 1024 bytes of the stack to the record file, then use DWARF debug information to deduce the frames later.

  This is quite inefficient (because it dumps the whole stack instead of just the addresses for each sample), but more importantly, if the stack is too deep (1024 bytes is insufficient) then the result is [unknown] frames.

  So, either try increasing it to dwarf,65528 (which is the maximum on my machine), or if it still doesn't work, --call-graph lbr or --call-graph fp (the last one may need recompile).

Other things to try:

• echo 0 |sudo tee /proc/sys/kernel/kptr_restrict (suggested in https://users.rust-lang.org/t/flamegraph-shows-every-caller-is-unknown/52408)

• perf map file must be owned by the correct user (https://stackoverflow.com/a/39662781/5267751)

• sudo sysctl kernel.perf_event_paranoid=-1 (perf script would print this out if needed)

• Flame graph can be generated by

  perf script flamegraph -F 100 ./program args

  but you can also explicitly separate the recording and the reporting step (to aid with debugging) by

  perf record --call-graph lbr -F 100 ./program args
  perf script report flamegraph

Questions that needs to be cleaned up:

• https://stackoverflow.com/questions/27842281/unknown-events-in-nodejs-v8-flamegraph-using-perf-events
• https://stackoverflow.com/questions/10933408/how-can-i-get-perf-to-find-symbols-in-my-program
• https://stackoverflow.com/questions/68259699/how-can-you-get-frame-pointer-perf-call-stacks-flamegraphs-involving-the-c-sta
• https://unix.stackexchange.com/questions/276179/missing-stack-symbols-with-perf-events-perf-report-despite-fno-omit-frame-poi

References:

• https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/monitoring_and_managing_system_status_and_performance/recording-and-analyzing-performance-profiles-with-perf_monitoring-and-managing-system-status-and-performance

Notes:

• https://brendangregg.com/flamegraphs.html --- explains what is a flame chart (similar to flamegraph, but order the blocks by time instead of by size. I think perf doesn't support this...? Not sure if there's a way)

Other unrelated issues:

• https://stackoverflow.com/questions/51157938/perf-doesnt-add-up-to-100