tracing: use span id for correlation

[e-eygin]

What?

Project TraceContext::spanId into the internal 64-bit NVTX correlation ID. Invalid contexts now return zero.

Why?

A trace ID may be shared by concurrent requests, while each request has its own span ID. Using the span ID preserves request-level correlation before context propagation lands in NIX-1744. Tracked by NIX-1778.

How?

Interpret the eight span-ID bytes in big-endian order. Focused and full unit CTest runs pass in normal, ASan/UBSan, UBSan, and TSan builds.

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📝 Walkthrough

Walkthrough

TraceContext::correlationId64() now returns zero for invalid contexts and derives the correlation ID from the full spanId. Unit tests cover byte order, edge cases, stability, and sensitivity to trace and span ID changes.

Changes

Trace correlation ID

Layer / File(s)	Summary
Correlation ID projection and validation src/core/tracing/trace_context.cpp, test/gtest/unit/tracing/trace_context_test.cpp	correlationId64() now serializes the full spanId in big-endian order and returns zero for invalid contexts. Tests cover valid edge cases, stability, independence from traceId, and sensitivity to spanId changes.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟡 Moderate · up to 903e7

Correlation IDs currently derive from span IDs rather than the required folded trace ID, so tracing correlation keys will differ from the intended values. Update the implementation and tests to use trace-ID folding and the zero-result fallback before merge.

Suggested reviewers: aranadive

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: using the span ID for correlation.
Description check	✅ Passed	The description includes complete What, Why, and optional How sections. It explains the span-ID projection, invalid-context behavior, rationale, tracking issues, implementation approach, and test cove…

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Actionable comments posted: 1

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Inline comments:
In `@src/core/tracing/trace_context.cpp`:
- Around line 92-93: Update src/core/tracing/trace_context.cpp lines 92-93 in
the trace-ID generation logic to XOR the two 64-bit halves of traceId and apply
the required nonzero fallback when the XOR result is zero. Update
test/gtest/unit/tracing/trace_context_test.cpp lines 151-156 to expect
0xe837bda879bd0a90ULL for kCanonicalTraceparent, and lines 184-190 to verify
trace-ID changes affect the result and cancelling halves use the fallback.

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• src/core/tracing/trace_context.cpp
• test/gtest/unit/tracing/trace_context_test.cpp

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[e-eygin]

/build

[e-eygin]

/ok to test 903e762

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu-ep · commit 903e7625

TL;DR: The nixl-ci-dl-gpu-ep elastic test (no_expansion.json) hung — all four ranks got stuck creating the device memory list, spinning mem_list.cpp:166 "Still waiting to create device memory list" every 5 s until the timeout 300 killed it (srun exit code 124). This is a hang in NIXL's device-memory-list creation reached during EP buffer setup, not a slow test.

Full analysis

Summary: Stage "Run DL NIXL EP tests" (#182) failed: the elastic EP test timed out after 300 s (exit code 124) because device memory list creation never completed on gb-nvl-118-compute08.

Root cause: A deadlock/livelock in device memory list creation. The test reached start phase 0 / adding connections, then every rank (PIDs 31854–31857) logged mem_list.cpp:166 "Still waiting to create device memory list after N ms; retrying" continuously from 5 s through 290 s, at which point timeout 300 fired. The output is continuous (5 s cadence, no dead node), so this is a genuine software hang inside the memory-list creation path, not an infrastructure/wall-clock issue. The build runs the PR branch nix-1778-nonzero-correlation-projection (PR #2216, "tracing: use span id for correlation"), which modifies the tracing/correlation path that instruments core operations like memory registration — a plausible source of a new lock/ordering issue that stalls mem_list creation. (Immediately before the loop there is a benign UCX warning about accelerated IB not found; the mlx5 ports report PORT_INIT/PORT_DOWN, but transfers stay on NVLink and the hang is in list creation, not transport.)

Implicated commit: [REDACTED:Hex High Entropy String] (PR #2216, tracing: use span id for correlation) — the change under test; needs confirmation that the same test passes on main.

File: mem_list.cpp:166 (device memory list creation retry loop) — exact repo path could not be resolved with the tools available; the tracing/span-id changes in PR #2216 are the suspected trigger.

Suggested fix: Have the PR author reproduce the elastic EP test locally with the span-id/correlation change and inspect the memory-list creation path for a lock taken on the tracing/correlation code that can block registration completion (e.g. re-entrant span emission during mem_list creation, or a mutex ordering change). Confirm the test passes on main at the same commit base to rule out a pre-existing flake; if it does, bisect the tracing change. Do not raise the 300 s timeout — the process was hung, not slow.

Related: PR #2216 (#2216); test added by PR #2154 (EP CI job).

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[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu-ep · commit 903e7625

TL;DR: The nixl-ci-dl-gpu-ep elastic EP test hung and was killed by its 300s timeout (exit 124) because the 4 EP ranks never completed peer connection — ucp_device_remote_mem_list_create returned UCS_ERR_NOT_CONNECTED and the retry loop in mem_list.cpp spun for the full 5 minutes without ever connecting.

Full analysis

Summary: Stage 182 "Run DL NIXL EP tests" failed: elastic.py --plan no_expansion.json was terminated by timeout 300 with exit code 124.

Root cause: A hang, not slowness. After each rank logged -> adding connections to [...], all four ranks (PIDs 86653–86656) emitted mem_list.cpp:166 Still waiting to create device memory list at a steady 5s cadence from 5000ms all the way to 290000ms, then srun reported Exited with exit code 124. The largest inter-line gap is only ~5s (the warning interval) — the process was continuously alive and spinning, so this is a genuine hang in inter-rank connection setup. In createMemList (src/plugins/ucx/mem_list.cpp:163–174) the loop retries ucp_device_remote_mem_list_create for as long as it returns UCS_ERR_NOT_CONNECTED, with no bound; the remote peers never became connected. The build stage itself succeeded — its FAILURE mark is an artifact of the pipeline-stop/recompile flow (stages 230/139/147), not a compile error. Because the connection never establishes and the retry loop has no timeout, the outer 300s guard is what ends the test. This is consistent with the PR under test (nix-1778-nonzero-correlation-projection, PR #2216) altering EP layout/connection behavior; the mem_list.cpp code itself is unchanged since March/July.

Implicated commit: Not in mem_list.cpp (last touched by 6ad0cea, Ilia Yastrebov, unrelated logging). The regression is introduced by PR #2216 (commit [REDACTED:Hex High Entropy String], branch nix-1778-nonzero-correlation-projection) — the EP connection/projection change under test.

File: src/plugins/ucx/mem_list.cpp:163–174 (unbounded UCS_ERR_NOT_CONNECTED retry surfaces the hang); root cause is the EP rank-connection path exercised by examples/device/ep/tests/elastic/elastic.py under PR #2216's changes.

Suggested fix: Investigate why the four ranks in this PR fail to establish the remote connection needed by ucp_device_remote_mem_list_create — compare the connection/handshake or expert-layout logic changed on nix-1778-nonzero-correlation-projection against main (the "adding connections" phase completes but the device mem-list never sees the peer as connected). As a defensive/diagnostic improvement, bound the retry loop in createMemList with a hard timeout that throws a descriptive error instead of spinning until the external timeout kills the job — that would turn this silent hang into an actionable failure. Reproduce locally with the same no_expansion.json plan and --num-processes 4 to bisect the connection change.

Related: PR #2216 (this change). Possibly relevant prior EP connection work: b0cbb23 "nixl_ep: Safely connect ranks during traffic (#2138)" and 34b5da0 "Reverted force close of UCP endpoints (#1996)". No existing issue found for this timeout signature.

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[e-eygin]

@ovidiusm @ColinNV please review

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu-ep · commit 903e7625

TL;DR: The "Run DL NIXL EP tests" stage hung: the elastic EP test spun for the full 300s in ucp_device_remote_mem_list_create returning UCS_ERR_NOT_CONNECTED (never establishing rank-to-rank UCX connections) and was killed by timeout 300 (exit 124). The root cause is a failure to complete inter-rank connection setup in phase 0, not an insufficient time limit.

Full analysis

Summary: nixl-ci-dl-gpu-ep #1017 stage "Run DL NIXL EP tests" failed with exit code 124 — the elastic.py (no_expansion.json) test hung during phase-0 connection setup and was killed by its 300s timeout.

Root cause: After all 4 ranks reached "start phase 0 → adding connections", remote device memory-list creation never proceeded. In createMemList the loop calls ucp_device_remote_mem_list_create and only exits once the endpoint is connected; while it returns UCS_ERR_NOT_CONNECTED it just progresses the worker and retries forever. The log shows the mem_list.cpp:166 "Still waiting to create device memory list" warning firing every 5s from 5,000ms to 290,000ms with no other application output — a continuous spin loop, i.e. the inter-rank UCX connections were never established. This is a hang in connection establishment, not a job that legitimately needed more wall time. (Note: on this node the IB ports are PORT_INIT/PORT_DOWN and UCX logged "accelerated IB support was not found," so ranks fall back to a transport whose connection never completes for this run.)

Implicated commit: Not conclusively pinpointed. The spin loop lives in ef5aad2d (Raul Akhmetshin, #1410, unchanged since) but that only emits the warning. The failure appeared on PR #2216 branch nix-1778-nonzero-correlation-projection; the recently-changed connect-during-traffic path is b0cbb237 (Itay Alroy, #2138 "Safely connect ranks during traffic") — worth reviewing against the PR's changes.

File: src/plugins/ucx/mem_list.cpp:163-174 (the UCS_ERR_NOT_CONNECTED retry loop that never terminates)

Suggested fix: Do not raise the timeout. Investigate why the phase-0 rank-to-rank connection never completes on this GB200 node: (1) bound the createMemList retry loop with a hard deadline so it raises UCS_ERR_NOT_CONNECTED instead of spinning silently, giving a real error rather than a 124 hang; (2) verify the connection/handshake logic in the EP example (examples/device/ep, esp. the "adding connections" path from #2138) against the changes on PR #2216; and (3) check whether the IB ports being in PORT_INIT/PORT_DOWN with no accelerated IB support is causing the transport fallback to stall — if so this node/config may need the connection path to tolerate that transport.

Related: PR #1410 (added the timeout warning), PR #2138 (connect ranks during traffic), PR #2216 (the PR under test).

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu-ep · commit 903e7625

TL;DR: The first EP elastic test (no_expansion.json) hung for the full 285s at UCX connection setup — ucp_device_remote_mem_list_create kept returning UCS_ERR_NOT_CONNECTED — and was killed by timeout 300 (exit 124). This is a fabric/connectivity failure on node gb-nvl-118-compute08, not a code defect in PR #2216.

Full analysis

Summary: Stage "Run DL NIXL EP tests" failed with exit code 124 (timeout) — elastic.py hung establishing UCX device-memory-list connections between the 4 ranks.

Root cause: All 4 ranks entered createMemList and spun in the retry loop at mem_list.cpp:163-174, where ucp_device_remote_mem_list_create() returns UCS_ERR_NOT_CONNECTED until the remote UCX endpoint becomes reachable. It never became reachable — the log shows continuous "Still waiting to create device memory list" warnings every 5s from 5000 ms up to 285000 ms with no other progress, then srun: task 0: Exited with exit code 124 from the wrapping timeout 300. This is a hang at connection establishment, not slow work. The node's IB fabric was unusable: nvidia-smi/ibv_devinfo show mlx5_0–3 stuck in PORT_INIT (2) and mlx5_4–5 in PORT_DOWN (1) (none PORT_ACTIVE), with the warning "6 IB device(s) were detected, but accelerated IB support was not found." The failing PR #2216 is a tracing change (span-id correlation) and does not touch the UCX connection or mem_list path, so this is an environmental/node fabric problem, not a regression from the commit under test.

Implicated commit: unknown (infrastructure/node fabric issue on gb-nvl-118-compute08; not [REDACTED:Hex High Entropy String] / PR #2216, which only changes tracing correlation).

File: src/plugins/ucx/mem_list.cpp:163 (the UCS_ERR_NOT_CONNECTED retry loop that hung); wrapper .gitlab/test_ep.sh invoked with timeout 300.

Suggested fix: Treat as an infra failure and retry on a healthy node — this is not a PR defect. Have the cluster team investigate the IB ports on gb-nvl-118-compute08 (mlx5 ports never reached PORT_ACTIVE; subnet manager / link init issue) and drain the node if it recurs. Separately, to make this fail fast instead of eating the full 300s wall clock, the retry loop in createMemList should honor a bounded connection timeout and raise UCS_ERR_NOT_CONNECTED after N attempts (e.g. reuse the configurable --timeout-ms) rather than spinning indefinitely, so the real cause is reported instead of a bare exit-124.

Related: PR #1410 (added the "Timeout warning for device memory list creation" that emits these messages); PR #2216 (the change under test — unrelated tracing work).

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