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

[Volker Simonis]

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.

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.

Thank you and best regards,
Volker