RFR: 8315149: Add hsperf counters for CPU time of internal GC threads [v48]

[Jonathan Joo]

On Thu, 30 Nov 2023 09:30:14 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Both `publish_gc_total_cpu_time` and `~ThreadTotalCPUTimeClosure` are called by the vm-thread inside a safepoint, so there shouldn't be any other threads running simultaneously, I believe.
>
> Me and Albert just spoke and we do see the problem that two concurrent threads could be executing the closure at the same time. So if having a total counter we need to sync the updates. But when talking we started to question how useful it is to have the gc_total counter. It is just an aggregate of the other gc-counters, but it is out of sync between safepoints. So you will always get a more accurate value by looking at the individual gc-counters.
> 
> We came to the conclusion that it would probably be easier to drop `gc_total` right now, rather than trying to keep it in sync for all updates to the individual counters. Because having them out of sync doesn't feel like a great option. 
> 
> Are we missing anything or do you agree?

@simonis was the original suggester of this counter, so I will defer to his expertise. I do agree that dropping the counter would simplify things, but it also might not hurt to just leave it in. I'm okay with either option!

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PR Review Comment: https://git.openjdk.org/jdk/pull/15082#discussion_r1411270546