RFR: 8328507: Move StackWatermark code from safepoint cleanup

[Axel Boldt-Christmas]

On Tue, 19 Mar 2024 13:30:39 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

> There is some stack watermark code in safepoint cleanup that flushes out concurrent stack processing, as a defensive measure to guard safepoint operations that assume the stacks have been fixed up. However, we wish to get rid of safepoint cleanup so this operation should move out from the safepoint. This proposal moves it into CollectedHeap::safepoint_synchronize_begin instead which runs before we start shutting down Java threads. This moves the code into more obvious GC places and reduces time spent inside of non-GC safepoints.

lgtm.

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Marked as reviewed by aboldtch (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18378#pullrequestreview-1946285372