RFR: 8259808: Add JFR event to detect GC locker stall [v5]

[Erik Gahlin]

On Tue, 26 Jan 2021 11:06:57 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 descriptions and fix the format problem

Marked as reviewed by egahlin (Reviewer).

src/hotspot/share/jfr/metadata/metadata.xml line 1094:

> 1092: 
> 1093:   <Event name="GCLocker" category="Java Virtual Machine, GC, Detailed" description="GC Locker Information" label="GC Locker" startTime="true" thread="true" stackTrace="true">
> 1094:     <Field type="uint" name="lockCount" label="Lock Count" description="the number of java threads in a critical section when the GC locker is started" />

I would suggest changing this to:
 
"The number of Java threads in a critical section when the GC locker is started"
"The number of Java threads stalled by the GC locker"

src/hotspot/share/jfr/metadata/metadata.xml line 1093:

> 1091:   </Event>
> 1092: 
> 1093:   <Event name="GCLocker" category="Java Virtual Machine, GC, Detailed" description="GC Locker Information" label="GC Locker" startTime="true" thread="true" stackTrace="true">

"GC Locker Information" is not very useful. Remove the description completely or provide more information.

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