RFR: 8369013: Shenandoah: passive mode should support enabling ShenandoahCardBarrier

[Rui Li]

On Fri, 24 Oct 2025 20:44:01 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Add card barriers to passive mode to test out the price of card barriers. 
>> 
>> How this change is implemented is to instantiate the old region in passive mode - old region owns the card table so this would minimize the code change with a bit price of native memory. It does sound weird to have old gen in passive mode, but since passive mode is a just diagnostic mode, we'll go with it for the cleanliness of the change.
>
> src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.hpp line 47:
> 
>> 45: 
>> 46:   static ShenandoahGenerationalHeap* heap() {
>> 47:     shenandoah_assert_generational();
> 
> Can we put these assertions back? I don't see any code in the PR that would invalidate these assertions. I also don't see any changes that would instantiate the generational heap for non generational mode, so if there _is_ code trying to use the generational heap, its behavior will be undefined.

It would crash at here with the assertion: [code](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shenandoah/shenandoahReferenceProcessor.cpp#L65). Provided some of the stacktrace below. It's mostly because we enabled card barrier flag and the code just fell through and assumes it's generational.

Would it be more appropriate to change [this line](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shenandoah/shenandoahReferenceProcessor.cpp#L65) to be `ShenandoahHeap* heap = ShenandoahHeap::heap();`?




Stack: [0x00007f1082bd2000,0x00007f1082cd2000],  sp=0x00007f1082cce370,  free space=1008k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V  [libjvm.so+0x13e0e90]  NativeStackPrinter::print_stack(outputStream*, char*, int, unsigned char*&, bool
, int)+0x70  (shenandoahGenerationalHeap.hpp:47)
V  [libjvm.so+0x18fd2e9]  VMError::report(outputStream*, bool)+0x1bff  (vmError.cpp:979)
V  [libjvm.so+0x1900d01]  VMError::report_and_die(int, char const*, char const*, __va_list_tag*, Thread*, 
unsigned char*, void const*, void const*, char const*, int, unsigned long)+0x8b1  (vmError.cpp:1887)
V  [libjvm.so+0xacf5ca]  report_vm_status_error(char const*, int, char const*, int, char const*)+0x0  (deb
ug.cpp:196)
V  [libjvm.so+0xacf33b]  print_error_for_unit_test(char const*, char const*, __va_list_tag*)+0x0  (debug.c
pp:149)
V  [libjvm.so+0x15bbec1]  ShenandoahAsserts::assert_generational(char const*, int)+0x8b  (shenandoahAssert
s.cpp:532)
V  [libjvm.so+0x15be196]  ShenandoahGenerationalHeap::heap()+0x19  (shenandoahGenerationalHeap.hpp:47)
V  [libjvm.so+0x16993bc]  void card_mark_barrier<narrowOop>(narrowOop*, oopDesc*)+0x73  (shenandoahReferenceProcessor.cpp:65)
V  [libjvm.so+0x169a2ac]  bool ShenandoahReferenceProcessor::discover<narrowOop>(oopDesc*, ReferenceType, unsigned int)+0x1da  (shenandoahReferenceProcessor.cpp:409)
V  [libjvm.so+0x1698a5d]  ShenandoahReferenceProcessor::discover_reference(oopDesc*, ReferenceType)+0xed  (shenandoahReferenceProcessor.cpp:433)
V  [libjvm.so+0x1684aeb]  bool InstanceRefKlass::try_discover<narrowOop, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0> >(oopDesc*, ReferenceType, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0>*)+0x77  (instanceRefKlass.inline.hpp:74)
V  [libjvm.so+0x1682bfc]  void InstanceRefKlass::oop_oop_iterate_discovery<narrowOop, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0>, AlwaysContains>(oopDesc*, ReferenceType, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0>*, AlwaysContains&)+0x2c  (instanceRefKlass.inline.hpp:84)
V  [libjvm.so+0x1680acc]  void InstanceRefKlass::oop_oop_iterate_ref_processing<narrowOop, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0>, AlwaysContains>(oopDesc*, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0>*, AlwaysContains&)+0x80  (instanceRefKlass.inline.hpp:111)
V  [libjvm.so+0x167ec2e]  void InstanceRefKlass::oop_oop_iterate_ref_processing<narrowOop, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0> >(oopDesc*, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0>*)+0x2c  (instanceRefKlass.inline.hpp:134)
V  [libjvm.so+0x167ba88]  void InstanceRefKlass::oop_oop_iterate<narrowOop, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0> >(oopDesc*, ShenandoahMarkRefsClosure<(ShenandoahGenerationType)0>*)+0x16c  (instanceRefKlass.inline.hpp:154)

> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 255:
> 
>> 253:   // for the purpose of having card table.
>> 254:   if (ShenandoahCardBarrier && !(mode()->is_generational())) {
>> 255:     _generation_sizer.heap_size_changed(max_capacity());
> 
> I think we could simplify here by not using the `_generation_sizer`. It should be fine to pass `max_capacity()` instead of `max_capacity_old` here.

Yeah that'd be simpler. Updating. Thanks.

> src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp line 533:
> 
>> 531: 
>> 532:   ShenandoahOldGeneration*   old_generation()    const {
>> 533:     assert(mode()->is_generational(), "Old generation requires generational mode");
> 
> Can we assert that `ShenandoahCardBarrier` is on instead of removing this assertion?

Sure

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/27966#discussion_r2462100322
PR Review Comment: https://git.openjdk.org/jdk/pull/27966#discussion_r2462131735
PR Review Comment: https://git.openjdk.org/jdk/pull/27966#discussion_r2462100239