How to load symbol files to BCC profiler

Question

With bcc tools' profile, I am getting mostly "[unknown]" in the profile output on my C program. This is, of course, expected because the program symbol are not loaded. However, I am not sure how to properly load the symbols so that "profile" program can pick it up. I have built my program with debug enabled "-g", but how do I load the debug symbols to "profile"?

Answer 1

Please see the DEBUGGING section in bcc profile's manpage:

See "[unknown]" frames with bogus addresses? This can happen for different reasons. Your best approach is to get Linux perf to work first, and then to try this tool. Eg, "perf record -F 49 -a -g -- sleep 1; perf script", and to check for unknown frames there.

The most common reason for "[unknown]" frames is that the target software has not been compiled with frame pointers, and so we can't use that simple method for walking the stack. The fix in that case is to use software that does have frame pointers, eg, gcc -fno-omit-frame-pointer, or Java's -XX:+PreserveFramePointer.

Another reason for "[unknown]" frames is JIT compilers, which don't use a traditional symbol table. The fix in that case is to populate a /tmp/perf-PID.map file with the symbols, which this tool should read. How you do this depends on the runtime (Java, Node.js).

If you seem to have unrelated samples in the output, check for other sampling or tracing tools that may be running. The current version of this tool can include their events if profiling happened concurrently. Those samples may be filtered in a future version.

In your case, since it's a C program, I would recommend compiling with -fno-omit-frame-pointer.