RFR: 8334147: Shenandoah: Avoid taking lock for disabled free set logging [v9]

[Y. Srinivas Ramakrishna]

On Thu, 27 Jun 2024 20:52:11 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> Hi all, 
>> This pull request propose a fix for the issue https://bugs.openjdk.org/browse/JDK-8334147
>> 
>>> There are multiple places in Shenandoah where we take heap lock for potential free set diagnostics. There is no point in taking that lock if we do not report anything. We should at very least take the lock only when logging is actually needed.
>> 
>> Basically it adds a public method ```ShenandoahFreeSet::log_status_with_heap_lock``` which acquire lock when there is need to print logs, also make the ```log_status``` private since not expect it to be called out of ShenandoahFreeSet class.
>> 
>> <s>The change for ShenandoahFreeSet::rebuild is probably debatable, ShenandoahFreeSet::log_status will acquire lock again after ShenandoahFreeSet::rebuild is executed, hence the metrics/information in log may not be always consistent, but it might be fine. </s>
>> 
>> Additional test:
>> - [x] `make test TEST=hotspot_gc_shenandoah`
>> 
>> Test summary
>> ==============================
>>    TEST                                              TOTAL  PASS  FAIL ERROR   
>>    jtreg:test/hotspot/jtreg:hotspot_gc_shenandoah      259   259     0     0   
>> ==============================
>> TEST SUCCESS
>> 
>> 
>> Best,
>> Xiaolong.
>
> Xiaolong Peng has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - typo
>  - Fix indenting
>  - Add one line comments for log_status and log_status_under_lock

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp line 343:

> 341: 
> 342:   void recycle_trash();
> 343:   // log status, assuming lock has alredy been acquired by the caller.

"already". Also see previous comment about removing enablement check in `log_status` and asserting. Will then need to check other callers and retest.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19915#discussion_r1657801256