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

[Thomas Schatzl]

On Tue, 26 Jan 2021 11:03:15 GMT, Denghui Dong <ddong at openjdk.org> wrote:

>> test/jdk/jdk/jfr/event/gc/detailed/TestGCLockerEvent.java line 116:
>> 
>>> 114:             ts[i] = new Thread(() -> {
>>> 115:                 STALL_COUNT_SIGNAL.countDown();
>>> 116:                 for (int j = 0; j < LOOP; j++) {
>> 
>> Since the test already uses WhiteBox, please use whitebox to trigger a gc instead of this dodgy method.
>
> Triggering a GC is not enough, I hope these threads could be stalled by the GC locker(call GCLocker::stall_until_clear) so that a correct assertion of the number of stall count could be added.
> I think it could not be done by WhiteBox::youngGC/fullGC, please correct me if I'm wrong.

I agree, although this is more a theoretical concern since it's not checked. Let's keep this for now as is though.

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