RFR: Make weakref processing always use Executor

[Aleksey Shipilev]

On 08/09/2018 04:38 PM, Roman Kennke wrote:
> This is a solution to the problem that Zhengyu reported recently, where
> the evac-oom-scope for VMThread would block evac-oom-scope for worker
> threads in weak-ref-processing.
> 
> The idea is to always go through the executor, no matter if it's 1- or
> N-threaded. The executor in turn calls the proxy, which then sets up the
> correct scopes. In the 1-threaded case, the executor doesn't need to
> fire up worker threads, but can call straight to the proxy in its
> current thread (the VMThread). In other words, it preserves the current
> behaviour, but moves setting up the scopes into the right places.
> 
> As a nice side-effect, this unclutters the code a little bit, and will
> unclutter the code *significantly* as soon as WeakProcessor becomes
> MT-capable (it's currently , in which case we can move it to the proxy
> too and get rid of the mess that we currently need to drive it. I
> suspect this might be welcome upstream.
> 
> http://cr.openjdk.java.net/~rkennke/weakrefs-1thread/webrev.00/

I do not understand all the changes in the patch. It seems to mess up our refproc handling, and it
increases our upstream exposure.

*) It claims to only move the scopes up into the right places, but what it does *on top* of that is
moving the calls to process_discovered_references, and changing what closures it is using. It is
entirely not clear why that change is correct:

 728     rp->process_discovered_references(NULL, NULL, NULL,
 729                                       &executor, &pt);

*) The pt.print_all_references() is moved closer to process_discovered_references in traversal GC,
but not in concurrent GC.

> Testing: tier1_gc_shenandoah

This is not enough. Run at least tier3, and specjbb for completeness?

-Aleksey