RFR: 8228657: ZGC: ZObjectAllocator::used() should take undone allocations into account

[Per Liden]

Thanks Erik!

/Per

On 7/29/19 2:28 PM, Erik Österlund wrote:
> Hi Per,
> 
> Looks good.
> 
> Thanks,
> /Erik
> 
> On 2019-07-26 15:11, Per Liden wrote:
>> In ZObjectAllocator, when we allocate a page we increment _used. 
>> However, we might later undo that page allocation, but we then leave 
>> _used as is. This results in a sometimes slightly incorrect value 
>> being reported by ZCollectedHeap::tlab_used(), which is fed into the 
>> TLAB heuristics. This incorrectness is not that big of a deal, but we 
>> should still fix this.
>>
>> We can't safely decrement _used (because we might be executing on a 
>> different CPU now), but we can track the amount of undos we've done, 
>> and later subtract that from _used.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8228657
>> Webrev: http://cr.openjdk.java.net/~pliden/8228657/webrev.0
>>
>> /Per
>