RFR: 8331572: Allow using OopMapCache outside of STW GC phases

[Aleksey Shipilev]

On Wed, 15 May 2024 20:06:31 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> As the reproducer in the issue shows, we would also like to use the `OopMapCache` during the concurrent GC phases. Zhengyu mentions there is also a production problem for stack walking that would benefit from letting `OopMapCache` be used without looking at GC at all.
>> 
>> This PR unblocks `OopMapCache` uses for everything. Cleanups are nominally done by service thread. But, still appreciating that majority of use cases would be from GCs, we leave the proactive cleanups from the GC ops here as well. It requires the synchronization between readers that might be copying out the entries out of the hashmap and the concurrent reclamation. Handily, `GlobalCounter` can be used for that purpose. 
>> 
>> After this lands, I think we can go over `OopMapCache::compute_one_oop_map` uses and see if they would instead like to use the cached `lookup` to benefit from this cache too. I think those paths are for OSR and deopts, so their performance is unlikely to be critical. This PR already covers the concurrent GC paths well.
>> 
>> Additional testing:
>>  - [x] Performance test reproducer from the bug improves significantly
>>  - [x] Linux AArch64 server fastdebug, `hotspot_gc_shenandoah` (10x)
>>  - [x] Linux AArch64 server fastdebug, `all`
>>  - [x] Linux x86_64 server fastdebug, `all`
>
> src/hotspot/share/interpreter/oopMapCache.cpp line 545:
> 
>> 543: 
>> 544:   // First search for an empty slot
>> 545:   for (int i = 0; i < _probe_depth; i++) {
> 
> Does the GlobalCounter read barrier belong around this too?

I don't think so: GlobalCounter guards against the reclamation of `OopMapCacheEntry`-es, so we only need to protect the paths that access their contents. We don't need it for anything else, like just poking into the array slots here. I used to have the critical section that spans this entire method, but reasoned it was excessive.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19229#discussion_r1602752873