RFR: 8349094: GenShen: Race between control and regulator threads may violate assertions [v12]

[Kelvin Nilsen]

On Fri, 14 Feb 2025 17:43:48 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> There are several changes to the operation of Shenandoah's control threads here.
>> * The reason for cancellation is now recorded in `ShenandoahHeap::_cancelled_gc` as a `GCCause`, instead of various member variables in the control thread.
>>   *  The cancellation handling is driven entirely by the cancellation cause
>>   * The graceful shutdown, alloc failure, humongous alloc failure and preemption requested flags are all removed
>>   * The shutdown sequence is simpler
>> * The generational control thread uses a lock to coordinate updates to the requested cause and generation
>>   * APIs have been simplified to avoid converting between the generation `type` and the actual generation instance
>>   * The old heuristic, rather than the control thread itself, is now responsible for resuming old generation cycles
>>   *  The control thread doesn't loop on its own (unless the pacer is enabled).
>> 
>> ## Testing
>> * jtreg hotspot_gc_shenandoah
>> * dacapo, extremem, diluvian, specjbb2015, specjvm2018, heapothesys
>
> William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 30 commits:
> 
>  - Merge remote-tracking branch 'jdk/master' into fix-control-regulator-threads
>  - Old gen bootstrap cycle must make it to init mark
>  - Merge remote-tracking branch 'jdk/master' into fix-control-regulator-threads
>  - Improve message for assertion
>  - Make shutdown safer for threads requesting (or expecting) gc
>  - Do not accept requests if control thread is terminating
>  - Notify waiters when control thread terminates
>  - Add event for control thread state changes
>  - Fix shutdown livelock error
>  - Fix includes
>  - ... and 20 more: https://git.openjdk.org/jdk/compare/ba6c9659...915ffbda

Thank you.  This looks very clean to me.

src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp line 98:

> 96:   }
> 97: 
> 98:   // In case any threads are waiting for a cycle to happen, let them know it isn't.

maybe "it isn't happening", or "it won't happen".

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Marked as reviewed by kdnilsen (Author).

PR Review: https://git.openjdk.org/jdk/pull/23475#pullrequestreview-2618641262
PR Review Comment: https://git.openjdk.org/jdk/pull/23475#discussion_r1956626274