RFR: 8316180: Thread-local backoff for secondary_super_cache updates [v4]

[Andrew Haley]

On Wed, 20 Sep 2023 16:17:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Work in progress, submitting for broader attention.
>> 
>> See more details in the bug and related issues.
>> 
>> This is the attempt to mitigate [JDK-8180450](https://bugs.openjdk.org/browse/JDK-8180450), while the more complex fix that would obviate the need for `secondary_super_cache` is being worked out. The goal for this fix is to improve performance in pathological cases, while keeping non-pathological cases out of extra risk, *and* staying simple enough and reliable for backports to currently supported JDK releases.
>> 
>> This implements the mitigation for AArch64 and x86_64. More platforms can be implemented in this PR, or deferred to later PRs. Port maintainers, feel free to suggest the patches for your arches, I'll be happy to fold them in.
>> 
>> Note that the code is supposed to be rather compact, because it is inlined in generated code. That is why, for example, we cannot easily do x86_32 version: we need a thread, so the easiest way would be to call into VM. But we cannot that easily: the code blowout would make some forward branches in external code non-short. I think we we cannot implement this mitigation on some architectures, so be it, it would be a sensible tradeoff for simplicity.
>> 
>> Setting backoff at `0` effectively disables the mitigation, and gives us safety hatch if something goes wrong.
>> 
>> Current PR deliberately sets backoff at `1000` to simplify testing. The actual value should be chosen by broader experiments.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 fastdebug, `tier1 tier2 tier3`
>>  - [x] Linux AArch64 fastdebug, `tier1 tier2 tier3`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Cleaner AArch64 code

OK for AArch64 with minor changes.

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 1542:

> 1540:     ldrw(rscratch1, Address(rthread, JavaThread::backoff_secondary_super_miss_offset()));
> 1541:     subsw(rscratch1, rscratch1, 1);
> 1542:     br(Assembler::GT, L_skip);

Suggestion:

    subw(rscratch1, rscratch1, 1);
    cbzw(rscratch1, 31, L_skip);


I know this is >= 0 rather than > 0, but that doesn't matter, and we should make this code small.

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 1552:

> 1550:     // The operations above destroy condition codes set by scan.
> 1551:     // This is the success path, restore them ourselves.
> 1552:     cmp(zr, zr); // Set Z flag

Suggestion:

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

Marked as reviewed by aph (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15718#pullrequestreview-1640804554
PR Review Comment: https://git.openjdk.org/jdk/pull/15718#discussion_r1334944173
PR Review Comment: https://git.openjdk.org/jdk/pull/15718#discussion_r1334943973