RFR: 8370520: GenShen: Track and report on promotion failures [v2]

[Kelvin Nilsen]

On Fri, 24 Oct 2025 22:27:26 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> We only take the heap lock there conditionally when we haven't yet "squelched" the log message.  We could change this to a `log_debug(gc, plab)` level message. The message is still useful when trying to understand the history and context for a how a thread became unable to promote. I'm not completely convinced there aren't still cases where a thread _should_ be able to promote but can't for some (unknown) reason.
>
> If we made this a `log_debug(gc, plab)` message, we could make the whole block of code conditional on the log level being enabled.

I'd be comfortable with that.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27962#discussion_r2467171498