RFR: 8357218: G1: Remove loop in G1CollectedHeap::try_collect_fullgc

[Thomas Schatzl]

On Mon, 19 May 2025 08:25:02 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/g1CollectedHeap.cpp line 1875:
>> 
>>> 1873:     // Request is trivially finished.
>>> 1874:     if (!GCCause::is_explicit_full_gc(cause) || op.gc_succeeded()) {
>>> 1875:       return op.gc_succeeded();
>> 
>> Complete removal of this part isn't correct.  The premise of this change, that "full-gc always
>> run-to-completion" is not correct.  A `_g1_periodic_collection` may be cancelled:
>> https://github.com/openjdk/jdk/blob/50a7c61d28b9885ff48f4fcd8bfd460b507bbcef/src/hotspot/share/gc/g1/g1VMOperations.cpp#L39-L48
>> Such a collection is not an explicit full-gc, so that part of the test may be true, when this function
>> returns false because the gc did not succeed.
>
> Updated the description. The overall result of `!GCCause::is_explicit_full_gc(cause) || op.gc_succeeded()` is always true.

The pre-existing code seems to be buggy: if the operation had been skipped, it should have returned false given the one caller that reads the boolean result (periodic collection invocation) to print a log line. I.e. maybe something like:

`return op.prologue_succeeded() && op.gc_succeeded();`

Similar to `G1CollectedHeap::do_collection_pause()`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25296#discussion_r2095521025