RFR(XS): 8175900: Assertion too strict in G1CollectedHeap::new_mutator_alloc_region

[Thomas Schatzl]

Hi,

On Mon, 2017-02-27 at 18:10 +0100, Volker Simonis wrote:
> Hi,
> 
> can I please have a review and sponsor for the following tiny change:
> 
> http://cr.openjdk.java.net/~simonis/webrevs/2017/8175900/
> https://bugs.openjdk.java.net/browse/JDK-8175900
> 
> Gunter Haug (gunter.haug at sap.com) provided the following bug report
> and fix:
> 
> The assertion (!force || g1_policy()->can_expand_young_list()) in
> G1CollectedHeap::new_mutator_alloc_region appears to be too strict.
> In
> fact, new_mutator_alloc_region with force=true is called from
> attempt_allocation_slow only under the condition of
> g1_policy()->can_expand_young_list() indirectly, via this hierarchy:
> 
> G1CollectedHeap::new_mutator_alloc_region(unsigned long, bool)
>   MutatorAllocRegion::allocate_new_region(unsigned long, bool)
>     G1AllocRegion::new_alloc_region_and_allocate(unsigned long, bool)
>       G1AllocRegion::attempt_allocation_force(unsigned long, bool)
>         G1CollectedHeap::attempt_allocation_slow(unsigned long,
> unsigned char, unsigned int*, int*)
> 
> However, this happens in a mutator thread while the
> G1YoungRemSetSamplingThread is running concurrently and may call
> revise_young_list_target_length_if_necessary(), which in turn calls
> update_max_gc_locker_expansion() where _young_list_max_length may be
> decreased and therefor g1_policy()->can_expand_young_list() is not
> true anymore.
> 
> This behavior is rare, we only observed it a few times in our nightly
> tests over several years. However, by suspending the mutator thread
> in attempt_allocation_slow, for a few seconds before calling
> attempt_allocation_force, we were able to reproduce the behavior.
> 
> We'd like to propose to remove the assertion.

Okay, looks good.

> 
> Also, _young_list_max_length and the other counters in
> G1DefaultPolicy which are accessed concurrently should be declared
> volatile. But that should be handled in an separate issue.

Okay.


I assume that you also need a sponsor for that change, with you
(simonis) as second reviewer?

Thanks,
  Thomas