RFR: 8366865: G1: Allocation GC Pauses Triggered after JVM has started shutdown [v2]

[Ivan Walulya]

> Please review this patch to skip VM_GC_Collect_Operations  if initiated after the VM shutdown process has begun. We add a _is_shutting_down flag to CollectedHeap, which is set while holding the Heap_lock. This ensures mutual exclusion with VM_GC_Collect_Operations, which also require the Heap_lock.
> 
> Skipping VM_GC_Collect_Operation would otherwise cause allocation requests to fail (resulting in OutOfMemoryError) if requesting daemon threads were allowed to continue, we instead block these threads on a monitor. They remain stalled until they are terminated as part of the VM shutdown sequence.
> 
> Testing: Tier 1-7

Ivan Walulya 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 nine additional commits since the last revision:

 - return on timeout
 - Merge remote-tracking branch 'upstream/master' into shutting_down_gcs
 - timed wait
 - Merge remote-tracking branch 'upstream/master' into shutting_down_gcs
 - space
 - remove debug logs
 - remove debug logs
 - log_cpu_time
 - init

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27190/files
  - new: https://git.openjdk.org/jdk/pull/27190/files/8a05ec45..187a463b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27190&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27190&range=00-01

  Stats: 11286 lines in 575 files changed: 5300 ins; 2963 del; 3023 mod
  Patch: https://git.openjdk.org/jdk/pull/27190.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27190/head:pull/27190

PR: https://git.openjdk.org/jdk/pull/27190