RFR: 8210713: Let CollectedHeap::ensure_parsability() take care of TLAB statistics gathering

[Per Liden]

I should probably mention that this patch is just an intermediate step 
towards the end goal, which is to allow a GC to retire TLABs and collect 
statistics with a single pass over the thread list and in parallel if so 
desired. I'm just breaking up the steps to get there in more easy to 
review pieces, which also have some value on their own.

cheers,
Per

On 09/13/2018 01:46 PM, Per Liden wrote:
> A call to CollectedHeap::ensure_parsability(retire_tlabs), where 
> retire_tlabs is true must always be preceded by a call to 
> CollectedHeap::accumulate_statistics_all_tlabs(), otherwise we will 
> loose TLAB statistics as it will be reset when the TLABs are retired. 
> Instead of having all collectors do this dance and risk getting it 
> wrong, I propose that we move this responsibility into 
> ensure_parsability(), which will gather the TLAB statistics if it's told 
> to retire all TLABs.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8210713
> Webrev: http://cr.openjdk.java.net/~pliden/8210713/webrev.0
> 
> /Per