RFR: 8254723: add diagnostic command to write Linux perf map file [v2]

[Nick Gasson]

> 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

Nick Gasson has updated the pull request incrementally with one additional commit since the last revision:

  Update for review comments

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/760/files
  - new: https://git.openjdk.java.net/jdk/pull/760/files/5b0c2695..a3cb0ed4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=760&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=760&range=00-01

  Stats: 16 lines in 3 files changed: 1 ins; 2 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/760.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/760/head:pull/760

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