RFR: 8350285: Regression caused by ShenandoahLock under extreme contention [v2]

[Xiaolong Peng]

> We have noticed there is significant regression in at-safepoint time with recent changes made to ShenandoahLock, more specifically this [PR](https://github.com/openjdk/jdk/pull/19570), a local reproducer was written to reproduce the issue, here is the top N at-safepoint time in `ns` comparison:
> 
> Tip:
> 
> 94069776
> 50993550
> 49321667
> 33903446
> 32291313
> 30587810
> 27759958
> 25080997
> 24657404
> 23874338
> 
> Tip + reverting [PR](https://github.com/openjdk/jdk/pull/19570)
> 
> 58428998
> 44410618
> 30788370
> 20636942
> 15986465
> 15307468
> 9686426
> 9432094
> 7473938
> 6854014
> 
> Note: command line for the test:
> 
> java -Xms256m -Xmx256m -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:-ShenandoahPacing  -XX:-UseTLAB -Xlog:gc -Xlog:safepoint ~/Alloc.java | grep -Po "At safepoint: \d+ ns" | grep -Po "\d+" | sort -nr
> 
> 
> With further digging, we found the real problem is more runnable threads after the [PR](https://github.com/openjdk/jdk/pull/19570) causes longer time for VM_Thread to call `futex(FUTEX_WAKE_PRIVATE)` to disarm wait barrier when leaving safepoint. Fixing in the issue in VM_Thread benefits other GCs as well but it is more complicated(see the details here https://bugs.openjdk.org/browse/JDK-8350324). 
> With some tweaks in ShenandoahLock, we could mitigate the regression caused by [PR](https://github.com/openjdk/jdk/pull/19570), also improve the long tails of at-saftpoint time by more than 10x, here is the result from the same test with this changes of this PR:
> 
> 
> 1890706
> 1222180
> 1042758
> 853157
> 792057
> 785697
> 780627
> 757817
> 740607
> 736646
> 725727
> 725596
> 724106
> 
> 
> ### Other test
> - [x] `make test TEST=hotspot_gc_shenandoah`
> - [x] Tier 2

Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23701/files
  - new: https://git.openjdk.org/jdk/pull/23701/files/756f7820..68e1b985

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23701&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23701&range=00-01

  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23701.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23701/head:pull/23701

PR: https://git.openjdk.org/jdk/pull/23701