RFR: 8351142: Add JFR monitor deflation and statistics events

[David Holmes]

On Tue, 4 Mar 2025 14:47:09 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> We already have JFR JavaMonitorInflate event, which tells when the monitor is inflated. We are missing JavaMonitorDeflate event, which would tell us when the monitor is deflated. This makes it hard to see the monitor lifecycle, and/or estimate the population of currently inflated monitors. I believe we should add JavaMonitorDeflate event. It would also be useful to have the statistics for the number of currently used/deflating monitors. Deflation event alone would require post-processing to investigate this, so it would be good to have the statistics event as well.
> 
> This would also replace two of the RT counters that are going away in [JDK-8348829](https://bugs.openjdk.org/browse/JDK-8348829).
> 
> Monitor deflation is done asynchronously in `MonitorDeflationThread`, so the additional overhead of recording the deflation events would likely be performance neutral. We still only enable the statistics event by default to be on a safer side.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `jdk_jfr`

src/hotspot/share/runtime/objectMonitor.cpp line 663:

> 661:                                        const oop obj) {
> 662:   assert(event != nullptr, "invariant");
> 663:   event->set_monitorClass(obj->klass());

Now that I have seen the "hidden wait" logic in the other PR, should inflation/deflation events not also check `is_excluded`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/23900#discussion_r1980805176