8358429: JFR: minimize the time the Threads_lock is held for sampling

[mgronlun]

Greetings,

Before JDK-8352251, the JFR Sampler Thread held onto the Threads_lock during the entire sampling period when iterating threads.

This was for two purposes:

1. Having a non-Java thread sampler thread race through an ongoing safepoint, touching oops is generally not supported by most GCs.
2. Since the sampler thread is a non-Java thread, and a JavaThread to be sampled can be in thread state _thread_in_native, it means both threads are outside the safepoint protocol. JFR evolves its global epoch during a safepoint.

With JDK-8352251, things have improved much. The sampling parts performed by the non-Java sampler thread do not touch any oops for threads sampled in state _thread_in_Java.
For sampling threads in native, the original issue persists (the sampler thread needs to capture the stack trace in the correct JFR epoch, and will also, in the case of virtual threads, need to touch oops).

However, we can significantly reduce the time the Threads_lock is held for sampling.

Testing: jdk_jfr, stress testing

Thanks
Markus

---

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Issue

• JDK-8358429: JFR: minimize the time the Threads_lock is held for sampling (Enhancement - P3)

Reviewers

• Erik Gahlin (@egahlin - Reviewer)

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8358429: JFR: minimize the time the Threads_lock is held for sampling

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At the time when this comment was updated there had been 37 new commits pushed to the master branch:

• 704b599: 8358534: Bailout in Conv2B::Ideal when type of cmp input is not supported
• e235b61: 8357987: [JVMCI] Add support for retrieving all methods of a ResolvedJavaType
• a44a470: 8358515: make cmp-baseline is broken after JDK-8349665
• ... and 34 more: https://git.openjdk.org/jdk/compare/e490b4f04dd094db996cdc4f664c2ed1a7ea2ade...master

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/integrate

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Going to push as commit b6d6028.
Since your change was applied there have been 49 commits pushed to the master branch:

• 955bfcd: 8357671: JFR: Remove JfrTraceIdEpoch synchronizing
• edf9272: 8356977: UTF-8 cleanups
• b5cfd76: 8358456: ZipFile.getInputStream(ZipEntry) throws unspecified IllegalArgumentException
• ... and 46 more: https://git.openjdk.org/jdk/compare/e490b4f04dd094db996cdc4f664c2ed1a7ea2ade...master

Your commit was automatically rebased without conflicts.

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@mgronlun Pushed as commit b6d6028.

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