RFR: 8279016: JFR Leak Profiler is broken with Shenandoah

[Erik Gahlin]

On Thu, 25 Jul 2024 11:26:45 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> While testing unrelated Shenandoah patch, I caught a GC assert when Leak Profiler was running ([JDK-8337194](https://bugs.openjdk.org/browse/JDK-8337194)). 
> 
> Leak Profiler is notorious in using the mark words for its own needs. We have been trying to mitigate its impact on GCs by moving to separate bitsets for tracking marked objects, or by treating "marked without fwdptr" as "JFR marked" and handling it. But this is not reliable, since things like [putting indexes in mark word](https://github.com/openjdk/jdk/blob/3baff2af6a30cc6cb2e0d4391db1cf7e61c33f64/src/hotspot/share/jfr/leakprofiler/chains/edgeStore.cpp#L275-L280) sneak in. This is okay for Leak Profiler alone, since it restores the mark words after the operation completes, but that is still not enough when GC is already running.
> 
> I say we side-step this whack-a-mole by cleanly bailing from JFR op, when we know it is unsafe to do. I thought to use `VM_Operation::doit_prologue`, but I think GC start may sneak in between checking in prologue and op start.
> 
> This realistically only affects Shenandoah. All other STW collectors would never see what Leak Profiler did with mark words. ZGC would not see it, since it does not care about mark words for its own operation.
> 
> Additional testing:
>  - [x] `jdk/jfr/event/oldobject/` pass by default (100x times)
>  - [x] `jdk/jfr/event/oldobject/` pass with `-XX:+UseShenandoah` (1000x)

test/jdk/jdk/jfr/event/oldobject/TestFieldInformation.java line 60:

> 58:             addToTestField();
> 59: 
> 60:             // Let GC catch up before we stop the recording and do the old object sample

We have worked hard to remove Thread.sleep, in the JFR tests, so we can run them in parallel, i.e. -conc:64, without introducing failures. I think the test was written before event streaming existed. The test could perhaps be rewritten to use an event stream, i.e:

    try (RecordingStream rs = new RecordingStream()) {
      rs.enable(EventNames.OldObjectSample).withoutStackTrace().with("cutoff", "infinity");
      rs.onEvent(e -> {
        if (hasValidField(e)) {
          rs.close();
        }
      });
      rs.startAsync();
      addToTestField();
      rs.awaitTermination();
    }

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

PR Review Comment: https://git.openjdk.org/jdk/pull/20328#discussion_r1692125586