Set Function.start_line when symbolizing

[prattmic]

Please answer these questions before submitting your issue. Thanks!

What version of pprof are you using?

If you are using pprof via go tool pprof, what's your go env output?
If you run pprof from GitHub, what's the Git revision?

a5a03c7

What operating system and processor architecture are you using?

linux-amd64

What did you do?

If possible, provide a recipe for reproducing the error.
Attaching a profile you are trying to analyze is good.

Analyze a perf.data profile, automatically converted via perf_to_profile.

$ pprof -raw perf.data                                                                                                            
Converting perf.data to a profile.proto... (May take a few minutes)                                                                           
[WARNING:src/quipper/perf_reader.cc:1323] Skipping 120 bytes of metadata: HEADER_CPU_TOPOLOGY                                                 
[WARNING:src/quipper/perf_reader.cc:1070] Skipping unsupported event PERF_RECORD_ID_INDEX                                                     
[WARNING:src/quipper/perf_reader.cc:1070] Skipping unsupported event PERF_RECORD_CPU_MAP                                                      
[WARNING:src/quipper/perf_reader.cc:1070] Skipping unsupported event UNKNOWN_EVENT_82                                                         
[INFO:src/quipper/perf_reader.cc:1061] Number of events stored: 2461                                                                          
[INFO:src/quipper/perf_parser.cc:272] Parser processed: 2 MMAP/MMAP2 events, 2 COMM events, 4 FORK events, 5 EXIT events, 2445 SAMPLE events, 2364 of these were mapped, 0 SAMPLE events with a data address, 0 of these were mapped
[INFO:src/perf_data_handler.cc:103] Using the build id found for the file name: [kernel.kallsyms], build id: ca67921314de38c21c74839716f10a65eba6f6ba.
[INFO:src/perf_data_handler.cc:103] Using the build id found for the file name: [kernel.kallsyms]_text, build id: ca67921314de38c21c74839716f10a65eba6f6ba.
[INFO:src/perf_data_handler.cc:103] Using the build id found for the file name: [kernel.kallsyms]_stext, build id: ca67921314de38c21c74839716f10a65eba6f6ba.
[WARNING:src/perf_data_handler.cc:577] stat: missing_callchain_mmap 2647/11713             
Comment: perf-version:6.5.6                                                                                                                   
Comment: perf-command:/usr/bin/perf record -g ./expensive                                                                                     
PeriodType:                                                                                                                                   
Period: 0                                                                                                                                     
Samples:                                                                                                                                      
cycles:Pu_sample/count cycles:Pu_event/count[dflt]
...
Locations                                                                                                                                     
     1: 0x0                                                                                                                                   
     2: 0x455180 M=1 _rt0_amd64 /usr/lib/google-golang/src/runtime/asm_amd64.s:16 s=0
     3: 0x40250c M=1 memeqbody /usr/lib/google-golang/src/internal/bytealg/equal_amd64.s:123 s=0
     4: 0x452e55 M=1 runtime.vdsoauxv /usr/lib/google-golang/src/runtime/vdso_linux.go:280 s=0
     5: 0x42a8cc M=1 runtime.sysauxv /usr/lib/google-golang/src/runtime/os_linux.go:315 s=0
     6: 0x42a624 M=1 runtime.sysargs /usr/lib/google-golang/src/runtime/os_linux.go:246 s=0
     7: 0x43dd5e M=1 runtime.args /usr/lib/google-golang/src/runtime/runtime1.go:69 s=0
     8: 0x457e04 M=1 runtime.args.abi0 ./<autogenerated>:1 s=0
     9: 0x4552b1 M=1 runtime.rt0_go.abi0 /usr/lib/google-golang/src/runtime/asm_amd64.s:347 s=0
    10: 0x41219c M=1 runtime.sysAllocOS /usr/lib/google-golang/src/runtime/mem_linux.go:34 s=0
...

What did you expect to see?

Function start lines are determined during symbolization.

What did you see instead?

No function start lines (s=0) in any of the locations.

None of the symbolizers attempt to look up function start lines, so this isn't too surprising.

However, since LLVM 13, llvm-symbolizer has a JSON output (first mentioned in #606) which provides function start line. I prototyped this in prattmic@e80491e (without the necessary fallback for old versions), and it is pretty straightforward. I am happy to finish this up and turn into a real PR.

My motivation here is using perf.data profiles for Go PGO. perf_to_profile doesn't symbolize profiles, but pprof does, so pprof -proto perf.data > perf.pprof is a convenient way to convert + symbolize. But Go PGO requires function start line, so today this isn't quite sufficient. We could of course write a separate symbolization tool, but IMO it seems reasonable for pprof to try to symbolize all of applicable proto fields.

[aalexand]

This seems reasonable to support and the change does look simple enough. I was going to ask whether the switch to hardcode llvm-symbolizer-14 means older version support would break but then I saw you mention explicitly that the code for that would be added.

The added benefit is that reading the JSON output is cleaner since this is clearly a machine-readable format. My only worry is whether this is going to be slower on large binaries - e.g. maybe LLVM is for any reason slower to produce JSON and parsing it would be added CPU time and RAM cost on the pprof side as well. It would be good to measure that at least in a couple of manual experiments.

[prattmic]

Sounds good, thanks! I'll be sure to measure the performance.

I was going to ask whether the switch to hardcode llvm-symbolizer-14 means older version support would break but then I saw you mention explicitly that the code for that would be added.

That change is just a hack. My machine didn't have llvm-symbolizer in PATH (only variants with a version suffix), so I just hacked pprof's lookup code. That change won't be in a real PR.

[aalexand]

Ack, thanks! Will review the PR when you have it (no rush, the pace is up to you).