RFR: 8368159: Significant performance overhead when started with jdwp agent and unattached debugger [v2]

[Serguei Spitsyn]

On Tue, 23 Sep 2025 23:21:30 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>>> Can you please explain why this change is required? Doesn't 'invalidate_cur_stack_depth' make sense only when interp_only mode is enabled for the threads only?
>> 
>> This is a right question to ask, thanks. I agree this confusing. The issue is with the pure continuations which are executed not in a context of a virtual thread. Without this check the following test is stably failed:
>> 
>>   test/hotspot/jtreg/serviceability/jvmti/vthread/ContStackDepthTest
>> 
>> I'm currently kind of puzzled on how to make this check better.
>
>> It seems odd to me that a method called invalidate_jvmti_stack() sometimes doesn't invalidate the stack. Even before this change it was not invalidating unless it was in interp_only mode, which also seems odd. If the cached value is not used for compiled frames, why bother with the interp_only check?
> 
> I can rename this function to `cond_ invalidate_jvmti_stack()` if you want. The `interp_only` check is needed for optimization to avoid a performance overhead of current stack depth invalidation.

I was thinking a lot on how to get rid of this current stack depth recalculation mechanism used in `interp_only` mode but have not come with a good approach yet. We have a constant trouble from this mechanism needed for debugger optimization purposes.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27403#discussion_r2373667768