RFR: 8296875: Generational ZGC: Refactor loom code

[Erik Österlund]

On Sat, 12 Nov 2022 08:08:15 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> The current loom code makes some assumptions about GC that will not work with generational ZGC. We should make this code more GC agnostic, and provide a better interface for talking to the GC.
>> 
>> In particular,
>> 1) All GCs have a way of encoding oops inside of the heap differently to oops outside of the heap. For non-ZGC collectors, that is compressed oops. For ZGC, that is colored pointers. With generational ZGC, pointers on-heap will be colored and pointers off-heap will be "colorless". So we need to generalize encoding and decoding of oops in the heap, for loom.
>> 
>> 2) The cont_oop is located on a stack. In order to access it we need to start_processing on that thread, if it isn't the current thread. This happened to work so far for ZGC, because the stale pointers had enough colors. But with generational ZGC, these on-stack oops will be colorless, so we have to be more accurate here and ensure processing really has started on any thread that cont_oop is used on. To make life a bit easier, I'm moving the oop processing responsibility for these oops to the thread instead. Currently there is no more than one of these, so doing it lazily per frame seems a bit overkill.
>> 
>> 3) Refactoring the stack chunk allocation code
>> 
>> Tested with tier1-5 and manually running Skynet. No regressions detected. We have also been running with this (yet a slightly different backend) in the generational ZGC repo for a while now.
>
> PS: I see JVM crashes when running Skynet with extra VM option: -XX:+VerifyContinuations on linux-aarch64 platform.
> 
> $java  --enable-preview -XX:+VerifyContinuations Skynet
> 
> 
> # A fatal error has been detected by the Java Runtime Environment:
> 
> # after -XX: or in .hotspotrc:  SuppressErrorAt=#
> #  Internal Error/stackChunkOop.cpp (/home/realfyang/openjdk-jdk/src/hotspot/share/oops/stackChunkOop.cpp:433), pid=1904185:433, tid=1904206
> 
> [thread 1904216 also had an error]#  assert(_chunk->bitmap().at(index)) failed: Bit not set at index 208 corresponding to 0x0000000637c512d0
> 
> #
> # JRE version: OpenJDK Runtime Environment (20.0) (fastdebug build 20-internal-adhoc.realfyang.openjdk-jdk)
> # Java VM: OpenJDK 64-Bit Server VM (fastdebug 20-internal-adhoc.realfyang.openjdk-jdk, mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, linux-aarch64)

@RealFYang did you have a chance to see if my RISC-V changes worked out for you?

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PR: https://git.openjdk.org/jdk/pull/11111