[PROF-15268] POC: per-thread JFR-inflight counter (follow-up to #614)

[r1viollet]

What does this PR do?

POC follow-up to #614. Moves the SignalInflight counter from a single global atomic to per-thread storage on ProfiledThread, to see if the alternative discussed in PROF-15268 holds up.

How

• ProfiledThread gets an _jfr_inflight field (owner-thread atomic RMW, cross-thread ACQUIRE read from drain) and an intrusive _registry_next pointer.
• A spinlock-protected registry head lives on ProfiledThread. initCurrentThread / freeKey / current() insert and remove.
• SignalInflight::enter/exit use the current ProfiledThread when available and fall back to the existing global counter otherwise.
• SignalInflight::drain() iterates the registry summing per-thread counters plus the fallback.

What this fixes vs #614

• Cache-line contention on the counter is gone on the fast path — each thread writes only its own memory.
• Sets up the machinery to close the documented J9 longjmp leak: a follow-up commit can reset the current thread's counter in segvHandler/busHandler before chaining. Not done in this POC.

What this does not do

• No segvHandler/busHandler hooks yet — the J9 longjmp leak is still open until that follow-up.
• Fallback global counter kept for threads that fire signals before initCurrentThread runs.

Local testing

buildDebug, compileRelease --rerun-tasks, ShutdownTest, JavaProfilerTest, CollapsingSleepTest, SmokeWallTest (all cstack modes). All green.

For Datadog employees

• This PR doesn't touch any of that.
• JIRA: PROF-15268

[dd-octo-sts]

CI Test Results

Run:#28648085482 | Commit:d74b5e7 | Duration: 13m 42s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

---

Updated: 2026-07-03 08:39:21 UTC

[dd-octo-sts]

Benchmark Results (commit c27d0bd)

Pipeline: https://gitlab.ddbuild.io/DataDog/apm-reliability/benchmarking-platform/-/pipelines/122534027 Commit: c27d0bdc7e863d2710ce3999d8e48d8ae372428f

⚠️ Significant outliers

• 🟢 future-genetic (JDK 21): runtime -4.9% (2191→2084 ms)

Runtime details (per benchmark × JDK)

Benchmark	JDK	Latest	Dev	Δ (dev vs latest)	Issues L/D
akka-uct	21	✅ 10314 ms (7 iters)	✅ 10135 ms (7 iters)	≈ -1.7% (±19.2%)	— / —
akka-uct	25	✅ 8752 ms (8 iters)	✅ 8932 ms (8 iters)	≈ +2.1% (±20.4%)	— / —
finagle-chirper	21	✅ 5973 ms (11 iters)	✅ 5940 ms (11 iters)	≈ -0.6% (±45.5%)	⚠️ W:2 / ⚠️ W:1
finagle-chirper	25	✅ 5526 ms (12 iters)	✅ 5411 ms (12 iters)	≈ -2.1% (±42.4%)	⚠️ W:1 / ⚠️ W:1
fj-kmeans	21	✅ 2834 ms (22 iters)	✅ 2747 ms (23 iters)	≈ -3.1% (±4.3%)	— / —
fj-kmeans	25	✅ 2803 ms (22 iters)	✅ 2833 ms (22 iters)	≈ +1.1% (±4.5%)	— / —
future-genetic	21	✅ 2191 ms (28 iters)	✅ 2084 ms (30 iters)	🟢 -4.9%	— / —
future-genetic	25	✅ 2076 ms (30 iters)	✅ 2140 ms (29 iters)	≈ +3.1% (±4.4%)	— / —
naive-bayes	21	✅ 1215 ms (47 iters)	✅ 1298 ms (44 iters)	≈ +6.8% (±58%)	— / —
naive-bayes	25	✅ 1016 ms (56 iters)	✅ 1016 ms (56 iters)	≈ 0% (±53.9%)	— / —
reactors	21	✅ 16142 ms (5 iters)	✅ 16550 ms (5 iters)	≈ +2.5% (±14.6%)	— / —
reactors	25	✅ 19105 ms (5 iters)	✅ 18537 ms (5 iters)	≈ -3% (±7%)	— / —

Internal counter details (ddprof)

ddprof internal counters, latest / dev (✅ = 0, · = unavailable):

Benchmark	JDK	Dropped rec	Dropped jvmti	Dropped trace	Skipped WC	AGCT fail	Unwind fail
akka-uct	21	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅
akka-uct	25	✅ / ✅	✅ / ✅	7 / 4	2297 / 2130	✅ / ✅	✅ / ✅
finagle-chirper	21	✅ / ✅	✅ / ✅	1 / ✅	8310 / ✅	✅ / ✅	✅ / ✅
finagle-chirper	25	✅ / ✅	✅ / ✅	3 / 2	8690 / 8477	✅ / ✅	✅ / ✅
fj-kmeans	21	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅
fj-kmeans	25	✅ / ✅	✅ / ✅	2 / 8	1257 / 1274	✅ / ✅	✅ / ✅
future-genetic	21	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅
future-genetic	25	✅ / ✅	✅ / ✅	3 / 2	2879 / 2909	✅ / ✅	✅ / ✅
naive-bayes	21	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅
naive-bayes	25	✅ / ✅	✅ / ✅	4 / 3	3486 / 3453	✅ / ✅	✅ / ✅
reactors	21	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅	✅ / ✅
reactors	25	✅ / ✅	✅ / ✅	✅ / ✅	1937 / 1886	✅ / ✅	✅ / ✅

[datadog-prod-us1-6]

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 3 Pipeline jobs failed

DataDog/java-profiler | reliability-chaos-aarch64: [profiler+tracer, tcmalloc, 25.0.3-tem]

DataDog/java-profiler | reliability-chaos-aarch64: [profiler, tcmalloc, 21.0.3-tem]

DataDog/java-profiler | reliability-chaos-amd64: [profiler, tcmalloc, 21.0.3-tem]

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[dd-octo-sts]

Reliability & Chaos Results

❌ 1 failure(s) detected Pipeline: https://gitlab.ddbuild.io/DataDog/java-profiler/-/pipelines/123834391

❌ chaos: profiler tcmalloc amd64 21 0 3 temXchaos

Chaos harness crashed (RC=124)

[jbachorik]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved. The conflict arose because PR #615 (landed in origin/main) renamed thread.cpp → threadLocalData.cpp and deleted thread.h, absorbing ProfiledThread into threadLocalData.h. Our branch had modified thread.h to add the per-thread JFR-inflight tracking machinery.

Resolution (commit fix: resolve merge conflict):

• Deleted thread.h (accepting main's deletion)
• Ported _registry_head/_registry_lock static members, _jfr_inflight/_registry_next instance members, and all the enterJfrInflight/exitJfrInflight/jfrInflight/forEachRegistered/registryInsert/registryRemove declarations into threadLocalData.h
• Updated signalInflight.cpp to #include "threadLocalData.h" instead of "thread.h"
• threadLocalData.cpp had already been correctly auto-merged with the registry implementation