RFR: JDK-8297796 GenShen: instrument the remembered set scan [v15]

[Y. Srinivas Ramakrishna]

On Tue, 27 Dec 2022 21:54:24 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahScanRemembered.cpp line 83:
>> 
>>> 81:   _rp->set_mark_closure(worker_id, &cl);
>>> 82:   struct ShenandoahRegionChunk assignment;
>>> 83:   while (_work_list->next(&assignment)) {
>> 
>> No need for a code change here, but just want to make clear that we may want to enable cancellation of rem-set scanning at some future time.  The primary benefit would be to allow quicker transition to Full GC in the case that we have to degenerate.
>
> This would also accelerate the transition to degenerated mode, so we can get ourselves out of the STW pause more quickly.

When a work item is picked up, it should be completed since it can't currently be placed back on the work list. Hence the code change, moving the check to the end of the loop. My expectation had been that this would fix the issue with prompt cancellation causing crashes previously. However, testing revealed that this was still causing crashes, so I left the cancellation commented out with the intention of following up on this in the fullness of time. 

I'll leave a comment to that effect as you suggest, and mark it with a TODO so it's easily flagged/found.

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PR: https://git.openjdk.org/shenandoah/pull/176