RFR: 8320304: Refactor and simplify monitor deflation functions [v2]

[Daniel D. Daugherty]

On Mon, 20 Nov 2023 09:31:09 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> src/hotspot/share/runtime/synchronizer.cpp line 1731:
>> 
>>> 1729:     ThreadBlockInVM tbivm(current);
>>> 1730: 
>>> 1731:     log.after_blocked();
>> 
>> This sequence is pre-existent but the check for safepoint/handshakes happens in ~ThreadBlockInVM so there is no point in logging before and after ThreadBlockInVM since we are not going to block. Also with this sequence the timer will still run if we do block in ~ThreadBlockInVM. So I would remove them and move log.end() at the end of this block before the ~ThreadBlockInVM runs since we are done by then. If we want to log when we the deflater actually returns then we can add a log there too.
>
> Good point. This code is going to be rewritten for https://bugs.openjdk.org/browse/JDK-8319137 and https://bugs.openjdk.org/browse/JDK-8319048. The comment here https://github.com/openjdk/jdk/pull/16412#issuecomment-1818358260 is a suggestion for how to solve both issues, and I think it should include the fixes to the logging.

The call to `log.before_blocked()` on L1725 will stop the timer so it will not
be running during the ThreadBlockInVM.
The call to `log.after_blocked()` on L1731 will restart the timer.
The advantage of having these two calls is that you know how long we spent
in ThreadBlockInVM based on the timestamps in the two log mesgs.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16706#discussion_r1399718342