[jdk17u-dev] RFR: 8227060: Optimize safepoint cleanup subtask order [v2]

[Aleksey Shipilev]

> Unclean backport to improve the safepoint cleanup times. Provides the grounds for the ultimate improvement in safepoint cleanup ([JDK-8306738](https://bugs.openjdk.org/browse/JDK-8306738)). The backport is not clean, because there are copyright years conflicts, the rename of `AbstractGangTask` to `WorkerTask` is missing, `WorkGang` -> `WorkerThreads` rename is missing, `JavaThread::cast` cleanup is missing.
> 
> Additional testing:
>  - [x] Ad-hoc benchmarks
>  - [x] Linux x86_64 fastdebug `tier1 tier2 tier3`
> 
> The safepoint cleanup times are slightly better on small tests, and there seem to be no regressions.
> ![plot](https://github.com/openjdk/jdk17u-dev/assets/1858943/7e012482-71f8-459f-aca0-4c043b791e92)

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Upstream hunk uses java_threads_do without is_JavaThread check
 - Merge branch 'master' into JDK-8227060-optimize-safepoint-cleanup-order
 - Backport 96a542feb2064dba155ebf05311752995d164038

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

Changes:
  - all: https://git.openjdk.org/jdk17u-dev/pull/1377/files
  - new: https://git.openjdk.org/jdk17u-dev/pull/1377/files/be2841ae..1dbff5ed

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1377&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1377&range=00-01

  Stats: 1056 lines in 31 files changed: 947 ins; 13 del; 96 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1377.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1377/head:pull/1377

PR: https://git.openjdk.org/jdk17u-dev/pull/1377