RFR: 8259808: Add JFR event to detect GC locker stall [v4]

[Stefan Johansson]

On Sun, 24 Jan 2021 01:48:57 GMT, Denghui Dong <ddong at openjdk.org> wrote:

>> GC locker will stall the operation of GC, resulting in some Java threads can not continue to run until GC locker is released, thus affecting the response time of the application. Add a JFR event to report this information is helpful to detect this issue.
>> 
>> For the test purpose, I add two Whitebox methods to lock/unlock critical.
>
> Denghui Dong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   add GCLockerTracer::is_started() that makes the logic more clear

I think this looks good now but please await a second reviewer.

I took if for a spin in out internal testing and tier1-2 looks ok as well as the JFR tests.

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

Marked as reviewed by sjohanss (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2088