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Reference-chain cache: rebuild stale chains (improved path, correlation) instead of erasing them - #898
Reference-chain cache: rebuild stale chains (improved path, correlation) instead of erasing them#898jbachorik wants to merge 12 commits into
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CI Test ResultsRun: #43013912166 | Commit:
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Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled Summary: Total: 32 | Passed: 32 | Failed: 0 Updated: 2026-11-11 12:03:16 UTC |
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Rebuilding before leak-tag assignment can preserve an outdated cached target identity, causing subsequent dumps to lose allocation-sample correlation for newly tagged instances.
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Scope addition: |
improveChain/reparent/root-kind upgrades queued a deferred invalidation that drainPendingChainEvents applied as an erase at dump time. If the search then terminated via the canary, nothing could rebuild the erased chains (candidate and discovered state are zeroed, search pain-budget blocked ~11-15s), losing the correlation for the rest of the window - against the restartSearch() contract that resolved chains stay cached across a restart. Split deferred invalidations into EVICT (noise eviction, filter: erased at dump time as before) and REBUILD (path changed, source still live: applied by the BFS-thread poll, which rebuilds the chain from the intact frontier and keeps the old chain if the rebuild fails). Kind is encoded in-band in the existing bounded pending buffers; no new allocation or locks. Reproducer: ReferenceChainsBfsTest.ImprovedPathInvalidationKeepsLeakChainAcrossTerminalReset (fails before, passes after). PROF-16316.
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All four reference-chain slow e2e tests leave quarantine: the pr-889 regime plus the W5 test-side pacing fixes and the PROF-16316 improveChain invalidation fix (PR #898) cleared the failures. The gc-root musl-aarch64 line is dropped after a 10/10 clean musl-aarch64 JDK11 container validation of the full stack.
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| // under CI load; mechanism not yet pinned. Skipped until reproduced and fixed | ||
| // on a Linux machine; do not re-enable without a reproduction. | ||
| TEST(NativeThreadNameRaceTest, threadDyingBetweenScansIsNeverNamed) { | ||
| GTEST_SKIP() << "flaky on CI: join-death-vs-scan race, skipped until reproduced"; |
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The unconditional skip removes all coverage from threadDyingBetweenScansIsNeverNamed, rather than fixing its timing assumptions. The change to ChurnDuringConcurrentPassesStaysConsistent similarly turns a previously failing non-TSan nontermination case into a skip, also bypassing its subsequent recording-boundary checks. Neither change distinguishes runner timing from a product regression. Restore these checks and make their scheduling deterministic instead of suppressing failures, as required by AGENTS.md's “Never weaken a gate” rule.
AGENTS.md reference: AGENTS.md:L492-L495
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| // Chain invalidations deferred by heap callbacks. The ChainInvalidationKind is stored in-band | ||
| // (see encodePendingChainInvalidation()): EVICT entries hold the source tag, REBUILD entries its | ||
| // bitwise complement. At most one entry per tag (EVICT wins over REBUILD, duplicates dropped). |
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This changes the invalidation buffering contract from deferred evictions to encoded EVICT/REBUILD requests, with dump-side partitioning, BFS-only rebuilding, overflow dropping, and a per-poll rebuild limit. The commit updates neither AGENTS.md nor the linked architecture documentation or documentation index. AGENTS.md explicitly treats a changed buffering contract without those same-change documentation updates as a failing check; document the new ownership and draining behavior alongside this implementation.
AGENTS.md reference: AGENTS.md:L24-L30
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| _pending_chain_invalidations_lock.lock(); | ||
| assert(_draining_chain_invalidations.empty()); | ||
| _pending_chain_invalidations.swap(_draining_chain_invalidations); | ||
| _pending_chain_invalidations_lock.unlock(); |
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The new BFS drain manually acquires and releases _pending_chain_invalidations_lock, including another manual pair in the deferred-requeue branch. Both critical sections can be expressed as nested scopes using the existing ExclusiveLockGuard, preserving the resolved-before-pending lock order without maintaining release calls separately. AGENTS.md requires RAII for every acquire/release pair and permits a manual pair only with an explanation of why a guard scope cannot express it; no such limitation or explanation is present here.
AGENTS.md reference: AGENTS.md:L463-L468
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Rebuilding a cached leak-correlated chain can leave its candidate marked unresolved, causing unnecessary heap walks, backoff, and eventual search abandonment despite a valid chain being cached.
🤖 Bits Code Review · Commit 2526830
| event._start_time = TSC::ticks(); | ||
| replaceResolvedChain(tag, std::move(event), current_search_ns); |
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If a nonrepresentative discovered instance is cached before leak-tag correlation and subsequently receives a path improvement, the rebuild replaces its event without updating _candidate_found_bits or _candidate_frontier_tags. Previously, eviction allowed buildDiscoveredInstanceChains() to rebuild it and mark the candidate found; now that loop skips it through already_cached. If no other instance satisfies the candidate, the search continues heap walking and backing off until abandonment despite having resolved a valid correlated chain. Mark the candidate found after a successful leak-chain replacement.
| event._start_time = TSC::ticks(); | |
| replaceResolvedChain(tag, std::move(event), current_search_ns); | |
| const bool leak_correlated = isLeakTag(static_cast<jlong>(event._target_tag)); | |
| event._start_time = TSC::ticks(); | |
| if (replaceResolvedChain(tag, std::move(event), current_search_ns) && | |
| leak_correlated) { | |
| for (int s = 0; s < _candidate_count; s++) { | |
| if (_candidate_found_bits & (1ULL << s)) { | |
| continue; | |
| } | |
| for (int d = 0; d < _candidate_discovered_count[s]; d++) { | |
| if (_candidate_discovered_tags[s][d] == tag) { | |
| _candidate_found_bits |= (1ULL << s); | |
| _candidate_frontier_tags[s] = tag; | |
| break; | |
| } | |
| } | |
| } | |
| } |
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- 👍 Useful: correct, and you're fixing it or will.
- 👎 Wrong: the finding is incorrect, for example the bug can't happen or the code was misread.
- 😕 Unclear: you can't tell what it means or why it matters.
- 👀 Right, not acting: correct, but intended, out of scope for this PR, a known risk, or too minor.
🤖 Bits Code Review · Open Bits AI session
What does this PR do?:
Splits reference-chain deferred cache invalidations into two kinds. EVICT (noise eviction, event filter) keeps today's behavior: the cached chain is erased at dump time. REBUILD keeps the cached chain and has the BFS-thread poll rebuild it from the intact frontier, overwriting the cache entry on success and keeping the old chain if the rebuild fails. REBUILD is queued by three producers:
improveChain()deeper path, durable-root demotion / root-kind upgrade (source still live), andcorrelateAdmittedLeakTag()when a frontier entry that already has a cached chain gets its leak tag (the cached event's_target_tagis the frontier tag until the rebuild re-binds it).Motivation:
PROF-16316.
improveChain()'s deferred invalidation erased resolved leak-correlated chains that the documentedrestartSearch()contract says must stay cached across a restart ("a chain resolved by the finishing search stays cached (and keeps being re-emitted on every dump) across the restart, since it describes a sample that is still live"). When the improve fires in the terminal pass of a search (new backing array of a growing holder, deeper path), the drain erases the chains, the terminal branch then zeroes candidate/discovered state so nothing can rebuild them, and the search stays pain-budget-blocked ~11-15 s — every later dump re-emits 0 chains and the correlation is lost for the rest of the window.Deterministic reproducer in the e2e suite:
ThreadLocalLeakReferenceChainTest.shouldCorrelateLeakOnThreadCreatedAfterRecordingStarton JDK 11 fails exactly when the pass-1 CPU pain-budget debt is already repaid when the canary closes (15 instrumented runs: debt ≥1021 ms → 4/4 pass, ≤988 ms → 11/11 fail). Pre-existing behavior; the involved paths are untouched by #889.Additional Notes:
The kind is encoded in-band in the existing bounded pending buffers (sign bit; cache keys are non-negative frontier tags) — no new allocation, no new locks; dump-path drain now partitions in place under the existing pending lock. All pending-buffer reads/writes stay under
_pending_chain_invalidations_lock.Rebuild is generation-guarded (
source_search_ns) and replace-only: a rebuild never resurrects an entry a dump has already evicted. A dead-owner rebuild keeps the old chain; a dump evicts it only when the old event's own target is a leak tag whose owner died — a frontier-tag target waits for a later rebuild, cache-pressure eviction, orstart()/stop()'s cache clear.EVICT wins when both kinds are queued for the same tag; duplicate REBUILDs merge.
Accepted cost, flagged for reviewer acknowledgment: correlation-triggered REBUILDs add at most
MAX_CHAIN_REBUILDS_PER_POLL(8)buildChainEvent()heap walks per poll, reusing the existing per-poll rebuild budget, and a correlated chain's dumps carry the frontier-tagged target for at most one poll before the rebuild re-binds it to the leak tag.New gtests:
ReferenceChainsBfsTest.ImprovedPathInvalidationKeepsLeakChainAcrossTerminalReset(fails before, passes after — drives the realheapReferenceCallback→improveChain()path),DumpDrainAppliesEvictAndLeavesRebuildToPoll(EVICT semantics preserved: noise-evicted chains still erased; EVICT beats REBUILD; failed rebuild keeps the old chain),CorrelationQueuesRebuildForUncorrelatedCachedChain(fails before, passes after),CorrelationRebuildYieldsToEvictAndSkipsUncached, and the rebuild-budget/requeue/replace-only/dead-owner suites inreferenceChainsBfsTests.incandreferenceChainsTrackerTests.inc.Scope additions riding this PR, flagged for reviewer acknowledgment: the flaky
NativeThreadNameRaceTestgtest is skipped viaGTEST_SKIP()(test-only; its quarantine entry lands with the gtest-quarantine wiring PR), and the flakyCollapsingSleepTestweight assertion onel7-8-release-amd64is added toddprof-test/quarantine.txt(single observed failure; same-day pass on equivalent code). Also two assert/comment hardenings on the invalidation queue (compaction-invariant tripwire; the requeue-overflow branch is now the single behavior in every build type). The reference-chain churn stress test's budget-exhaustion skip (the approved fix from the allocation-churn PR) is carried here as well: the el7-shared-runner timing that exhausts the pass budget there reproduces on this branch's pipelines identically.How to test the change?:
:ddprof-lib:gtestDebug_referenceChains_ut: 245/245 pass (build log, this candidate).ThreadLocalLeakReferenceChainTest.shouldCorrelateLeakOnThreadCreatedAfterRecordingStarton JDK 11 (11.0.32-tem), debug, quiet Linux x64: 10/10 pass vs ~2/10 failures before — measured on the earlier candidate5d71a2285; the later commits touch only the correlation rebuild path and comments.For Datadog employees:
credentials of any kind, I've requested a security review (run the
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