8358621: Reduce busy waiting in worse case at the synchronization point returning from native in CPU Time Profiler

[parttimenerd]

Simple optimization to reduce busy waiting in safepoint handlers.

---

Progress

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Issue

• JDK-8358621: Reduce busy waiting in worse case at the synchronization point returning from native in CPU Time Profiler (Bug - P3)

Reviewers

• Aleksey Shipilev (@shipilev - Reviewer)
• Jaroslav Bachorik (@jbachorik - Reviewer)
• Erik Gahlin (@egahlin - Reviewer)

Reviewing

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[bridgekeeper]

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[openjdk]

@parttimenerd 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:

8358621: Reduce busy waiting in worse case at the synchronization point returning from native in CPU Time Profiler

Reviewed-by: shade, jbachorik, egahlin

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 18 new commits pushed to the master branch:

• 563e876: 8358540: Enhance MathUtils in view of FloatingDecimal enhancements
• c70258c: 8358619: Fix interval recomputation in CPU Time Profiler
• 9697e5b: 8362151: Remove unnecessary ClassLoaderDataGraph friend classes
• ... and 15 more: https://git.openjdk.org/jdk/compare/99c299f0985c8be63b9b60e589db520d83fd8033...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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[openjdk]

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[mlbridge]

Webrevs

• 01: Full - Incremental (ad67f51c)
• 00: Full (7fe92854)

[shipilev]

There is an utility for this, look in utilities/spinYield.hpp. Would be something like:

SpinYield sy;
while (...) {
  sy.wait();
}

Also, well, this does not avoid the spinlock, it only makes it less aggressive. I read the original improvement suggested using the actual / blocking Mutex/Monitor. But that looks not great for signal-safety.

I propose we just rename the issue to something more related to the change. Also, maybe set Fix Version: 26, and then backport, to avoid complicating things unnecessarily.

[parttimenerd]

Mutex / Monitor cannot be used as the lock is also used in signal handlers. I'll update to use SpinYield.

[shipilev]

Looks OK to me, as long as JFR folks are happy with this solution.

/reviewers 2

[openjdk]

@shipilev
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 1 Reviewer, 1 Author).

[shipilev]

Also this: "I propose we just rename the issue to something more related to the change. Also, maybe set Fix Version: 26, and then backport, to avoid complicating things unnecessarily."

[parttimenerd]

Thanks. As a background: This lock should almost never be locked (confirmed with renaissance benchmark), when the safepoint handler comes in. So it is more a defence against a rare outlier.

[parttimenerd]

/integrate

[openjdk]

Going to push as commit d2082c5.
Since your change was applied there have been 18 commits pushed to the master branch:

• 563e876: 8358540: Enhance MathUtils in view of FloatingDecimal enhancements
• c70258c: 8358619: Fix interval recomputation in CPU Time Profiler
• 9697e5b: 8362151: Remove unnecessary ClassLoaderDataGraph friend classes
• ... and 15 more: https://git.openjdk.org/jdk/compare/99c299f0985c8be63b9b60e589db520d83fd8033...master

Your commit was automatically rebased without conflicts.

[openjdk]

@parttimenerd Pushed as commit d2082c5.

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

[parttimenerd]

/backport :jdk25

[openjdk]

@parttimenerd the backport was successfully created on the branch backport-parttimenerd-d2082c58-jdk25 in my personal fork of openjdk/jdk. To create a pull request with this backport targeting openjdk/jdk:jdk25, just click the following link:

➡️ Create pull request

The title of the pull request is automatically filled in correctly and below you find a suggestion for the pull request body:

Hi all,

This pull request contains a backport of commit d2082c58 from the openjdk/jdk repository.

The commit being backported was authored by Johannes Bechberger on 15 Jul 2025 and was reviewed by Aleksey Shipilev, Jaroslav Bachorik and Erik Gahlin.

Thanks!

If you need to update the source branch of the pull then run the following commands in a local clone of your personal fork of openjdk/jdk:

$ git fetch https://github.com/openjdk-bots/jdk.git backport-parttimenerd-d2082c58-jdk25:backport-parttimenerd-d2082c58-jdk25
$ git checkout backport-parttimenerd-d2082c58-jdk25
# make changes
$ git add paths/to/changed/files
$ git commit --message 'Describe additional changes made'
$ git push https://github.com/openjdk-bots/jdk.git backport-parttimenerd-d2082c58-jdk25