RFR: 8254723: add diagnostic command to write Linux perf map file [v4]

[Nick Gasson]

On Tue, 27 Oct 2020 01:07:04 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> Nick Gasson has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:
>> 
>>  - Merge master
>>  - Add -XX:+DumpPerfMapAtExit option
>>  - Update for review comments
>>  - 8254723: add diagnostic command to write Linux perf map file
>>    
>>    When using the Linux "perf" tool to do system profiling, symbol names of
>>    running Java methods cannot be decoded, resulting in unhelpful output
>>    such as:
>>    
>>      10.52% [JIT] tid 236748 [.] 0x00007f6fdb75d223
>>    
>>    Perf can read a simple text file format describing the mapping between
>>    address ranges and symbol names for a particular process [1].
>>    
>>    It's possible to generate this already for Java processes using a JVMTI
>>    plugin such as perf-map-agent [2]. However this requires compiling
>>    third-party code and then loading the agent into your Java process. It
>>    would be more convenient if Hotspot could write this file directly using
>>    a diagnostic command. The information required is almost identical to
>>    that of the existing Compiler.codelist command.
>>    
>>    This patch adds a Compiler.perfmap diagnostic command on Linux only. To
>>    use, first run "jcmd <pid> Compiler.perfmap" and then "perf top" or
>>    "perf record" and the report should show decoded Java symbol names for
>>    that process.
>>    
>>    As this just writes a snapshot of the code cache when the command is
>>    run, it will become stale if methods are compiled later or unloaded.
>>    However this shouldn't be a big problem in practice if the map file is
>>    generated after the application has warmed up.
>>    
>>    [1] https://github.com/torvalds/linux/blob/master/tools/perf/Documentation/jit-interface.txt
>>    [2] https://github.com/jvm-profiling-tools/perf-map-agent
>
> Hi Nick,
> 
> This looks good.
> Thank you for the update.
> 
> Thanks,
> Serguei

> I don't see any reason for this to be a product flag, rather than diagnostic.

OK sure, I've made it a diagnostic flag.

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PR: https://git.openjdk.java.net/jdk/pull/760