RFR: 8318026: jcmd should provide access to detailed JVM object information [v9]

[Chris Plummer]

On Tue, 26 Mar 2024 21:55:38 GMT, Kevin Walls <kevinw at openjdk.org> wrote:

>> Introduce the jcmd "VM.inspect" to implement access to detailed JVM object information.
>> 
>> Not recommended for live production use.  Requires UnlockDiagnosticVMOptions and not included in jcmd help output, to remind us this is not a general-purpose customer-facing tool.
>
> Kevin Walls has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Undo include

It looks like the test only inspects a thread and a java object. Perhaps you could add tests for additional VM objects. Maybe grab a frame PC from a thread stack trace.

src/hotspot/share/services/diagnosticCommand.hpp line 590:

> 588:   }
> 589:   static const char* description() {
> 590:     return "Inspect VM object at address.";

Maybe distinguishing between java heap objects and VM objects would be useful to the reader/user.

src/hotspot/share/utilities/debug.cpp line 679:

> 677: }
> 678: 
> 679: // Additional "good oop" checks, separate method to not disturb existing asserts.

Suggestion:

// Additional "good oop" checks. Separate method to not disturb existing asserts.

src/hotspot/share/utilities/debug.cpp line 680:

> 678: 
> 679: // Additional "good oop" checks, separate method to not disturb existing asserts.
> 680: extern "C" bool dbg_is_good_oop_detailed(oopDesc* o) {

What was the motivation for adding this? Is this copied from other HS code? How do we know it's doing a good enough job of verification? What happens if verification is not thorough enough?

test/hotspot/jtreg/serviceability/dcmd/vm/VMInspectTest.java line 83:

> 81:         }
> 82: 
> 83:         // Tests where enabled:

Suggestion:

        // Tests run with VM.inspect support enabled:

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PR Review: https://git.openjdk.org/jdk/pull/17655#pullrequestreview-1962319603
PR Review Comment: https://git.openjdk.org/jdk/pull/17655#discussion_r1540483489
PR Review Comment: https://git.openjdk.org/jdk/pull/17655#discussion_r1540485389
PR Review Comment: https://git.openjdk.org/jdk/pull/17655#discussion_r1540486518
PR Review Comment: https://git.openjdk.org/jdk/pull/17655#discussion_r1540492385