8290700: Optimize AArch64 nmethod entry barriers #9574

fisk · 2022-07-20T16:54:53Z

The original nmethod entry barrier supported only concurrent patching of data and was used by ZGC to solve concurrent class unloading problems. Now it is starting to see more uses. Notably, loom uses nmethod entry barriers to figure out what nmethods have been seen on-stack, needed to remove nmethods safely. However, the concurrent data patching variation was too slow for loom (showed a few regressions), so I brought over a faster nmethod entry barrier that we use in the generational ZGC repo, which additionally handles concurrent patching of data and instructions, which is needed there.
However, I still see some small regressions. So for the uses in loom, the classic GCs don't really patch anything interesting concurrently. This leads to the following possible enhancements to improve the situation:

Make a dedicated nmethod entry barrier for GCs that don't patch data nor code concurrently, consisting of basically only a conditional branch. There is no need to penalize STW GCs with seat belts protecting against concurrent races that simply do not exist.
Move the "guard" word and call into the VM trampoline, to an out-of-line stub towards the end of the nmethod, ensuring instruction caches are not polluted by non-hot instructions at the nmethod entry. Some machines also better optimize the branch-not-taken path of conditional branches.

With these optimizations, the small regressions did go away.

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Issue

JDK-8290700: Optimize AArch64 nmethod entry barriers

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Vladimir Kozlov (@vnkozlov - Reviewer)
Dean Long (@dean-long - Reviewer)

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bridgekeeper · 2022-07-20T16:55:55Z

👋 Welcome back eosterlund! A progress list of the required criteria for merging this PR into pr/9569 will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2022-07-20T16:58:53Z

@fisk The following labels will be automatically applied to this pull request:

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mlbridge · 2022-07-20T17:01:30Z

Webrevs

fisk · 2022-07-20T17:05:19Z

Note that this PR is dependent on #9569

openjdk-notifier · 2022-07-22T14:49:50Z

The dependent pull request has now been integrated, and the target branch of this pull request has been updated. This means that changes from the dependent pull request can start to show up as belonging to this pull request, which may be confusing for reviewers. To remedy this situation, simply merge the latest changes from the new target branch into this pull request by running commands similar to these in the local repository for your personal fork:

git checkout 8290700_faster_aarch64_entry_barriers
git fetch https://git.openjdk.org/jdk master
git merge FETCH_HEAD
# if there are conflicts, follow the instructions given by git merge
git commit -m "Merge master"
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openjdk · 2022-07-22T14:52:05Z

@fisk this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout 8290700_faster_aarch64_entry_barriers
git fetch https://git.openjdk.org/jdk master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push

vnkozlov

Looks reasonable to me.
Someone familiar with aarch64 code have to review it.

openjdk · 2022-07-22T19:13:23Z

@fisk This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8290700: Optimize AArch64 nmethod entry barriers

Reviewed-by: kvn, dlong

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At the time when this comment was updated there had been 2 new commits pushed to the master branch:

ef8486a: 8286270: [java.desktop] Replace color search in XColors with a switch statement
62f2263: 8290861: Remove unused field URLJarFile.BUF_SIZE

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fisk · 2022-07-22T19:48:16Z

Thanks for the review, @vnkozlov!

dean-long · 2022-07-22T20:04:16Z

src/hotspot/cpu/aarch64/gc/shared/barrierSetNMethod_aarch64.cpp

+        }
+      }
+      ShouldNotReachHere();
+    }


I'm guessing this function is not performance-critical. If it was, then we could consider adding a new field to the nmethod to keep track of the offset for the guard.

Yeah I thought about adding a new entry to CodeOffsets, but it seemed like this C2-only AArch64-only detail seemed a bit noisy in the shared code then. And it didn't seem warranted, given how cold this path is by design.

fisk · 2022-07-23T03:01:32Z

Thanks for the review @dean-long!

fisk · 2022-07-25T07:07:14Z

/integrate

openjdk · 2022-07-25T07:08:55Z

Going to push as commit 228e8e9.
Since your change was applied there have been 6 commits pushed to the master branch:

852e71d: 8290870: NMT: Increase MallocSiteTable size and allocate it only when needed
0599a05: 8290900: Build failure with Clang 14+ due to function warning attribute
987656d: 8289471: Issue in Initialization of keys in ErrorMsg.java and XPATHErrorResources.java
2660a92: 8290893: ProblemList java/lang/ClassLoader/loadLibraryUnload/LoadLibraryUnload
ef8486a: 8286270: [java.desktop] Replace color search in XColors with a switch statement
62f2263: 8290861: Remove unused field URLJarFile.BUF_SIZE

Your commit was automatically rebased without conflicts.

openjdk · 2022-07-25T07:09:14Z

@fisk Pushed as commit 228e8e9.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

fisk added 5 commits July 20, 2022 11:25

Optimize x86 nmethod entry barriers

2e8dd85

32 bit build fix

a71e4ae

fix 32 bit build again

eeb5f82

fixing 32 bit build again

c98b075

8290700: Optimize AArch64 nmethod entry barriers

58c98ac

openjdk bot added the rfr Pull request is ready for review label Jul 20, 2022

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openjdk-notifier bot changed the base branch from pr/9569 to master July 22, 2022 14:49

openjdk bot added the merge-conflict Pull request has merge conflict with target branch label Jul 22, 2022

Merge branch 'master' into 8290700_faster_aarch64_entry_barriers

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openjdk bot removed the merge-conflict Pull request has merge conflict with target branch label Jul 22, 2022

vnkozlov approved these changes Jul 22, 2022

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dean-long reviewed Jul 22, 2022

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8290700: Optimize AArch64 nmethod entry barriers #9574

8290700: Optimize AArch64 nmethod entry barriers #9574

fisk commented Jul 20, 2022 •

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8290700: Optimize AArch64 nmethod entry barriers #9574

8290700: Optimize AArch64 nmethod entry barriers #9574

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fisk commented Jul 20, 2022 • edited by openjdk bot

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