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

Yasumasa Suenaga ysuenaga at openjdk.java.net
Thu Oct 22 02:09:14 UTC 2020

On Wed, 21 Oct 2020 09:13:08 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
> Nick Gasson has updated the pull request incrementally with one additional commit since the last revision:
>   Update for review comments

src/hotspot/share/code/codeCache.cpp line 1571:

> 1569:   if (!fs.is_open()) {
> 1570:     log_warning(codecache)("Failed to create %s for perf map", fname);
> 1571:     st->print_cr("Failed to create %s", fname);

`st->print_cr()` is still needed? and also I've intended to replace all `print_cr()` to UL (L1587)


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

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