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

[Yasumasa Suenaga]

On Tue, 20 Oct 2020 09:27:45 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

> 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

Changes requested by ysuenaga (Reviewer).

src/hotspot/share/services/diagnosticCommand.hpp line 589:

> 587:     return "Write map file for Linux perf tool.";
> 588:   }
> 589:   static const char* impact() {

This operation does not seems to happen in Safepoint. It can be defined as "Low" IMHO.

src/hotspot/share/code/codeCache.cpp line 1562:

> 1560: }
> 1561:
> 1562: void CodeCache::write_perf_map(outputStream* st) {

Should this code (and changes in header files) be included for Linux only? (`#ifdef LINUX` like the change in
diagnosticCommand.cpp)

src/hotspot/share/code/codeCache.cpp line 1570:

> 1568:   fileStream fs(fname, "w");
> 1569:   if (!fs.is_open()) {
> 1570:     st->print_cr("Failed to create %s", fname);

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