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backend: pass the request trace context to prepXfer and postXfer - #2324
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e-eygin merged 4 commits intoOct 2, 2026
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nixl_opt_b_args_t gains a pointer to the TraceContext the request handle owns. makeXferReq, createXferReq and postXferReq set it, so a plugin can read the context of the request it is executing and copy it onto its own request handle. It points at the zeroed invalid context when tracing is off, and stays null for prepMemView, which has no request. Plugins get the value rather than the encoded 26-byte record, so one that only stamps phases does not pay for encoding; a carrier calls encodeTraceContext itself. The type is only forward-declared, as nixlBackendTraceSink is, because backend_aux.h is installed and tracing/trace_context.h is not. Appending the member moves no existing field and every object is constructed in core, so NIXL_PLUGIN_API_VERSION stays at 2. NIX-1877 Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
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Included review availability: This review used your included allowance. Your plan provides up to 12 included reviews per hour; 11 remain after this review. 📝 WalkthroughWalkthroughTransfer requests expose their trace context. The agent passes that context to backend preparation and posting operations. Mock backends capture the context, and tests check propagation with tracing enabled and disabled. ChangesTransfer trace-context propagation
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant nixlXferReqH
participant nixlAgent
participant BackendEngine
nixlAgent->>nixlXferReqH: Read traceContext()
nixlAgent->>BackendEngine: Pass context in optional arguments
Suggested reviewers: Merge Risk: ⚪ Minimal · up to Trace-context access remains limited to in-tree plugins, and no concrete user-facing or operational merge risk is identified. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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Review comments at @src/api/cpp/backend/backend_aux.h:
- Line 45: Update the NIXL_PLUGIN_API_VERSION contract for the added
traceContext field in nixl_opt_b_args_t so plugins using the expanded options
layout cannot load with an older core; increment the API version consistently
with the loader’s version check.
Review comments at @src/core/transfer_request.h:
- Around line 77-78: Add a Doxygen comment to the public traceContext() accessor
identifying the returned context as owned by the request and documenting that
the reference remains valid only while the request exists.
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Review round on ai-dynamo#2324. - backend_aux.h: the comment now says prep and every post of a request hand over the same context, so a plugin can copy it once at prep. - docs/tracing.md: document traceContext next to the phase sink, and note it as the first step of cross-rank correlation. - TraceContext gets a defaulted operator==, which replaces the field-by-field matcher in the agent tests; the accessor check is folded into Tracing.RequestStoresFixedCorrelationContext. - The mock records the sink and the context behind one mutex, and holds GMockBackendEngine directly instead of casting. NIX-1877 Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
Review round on ai-dynamo#2324. - backend_aux.h: the comment now says prep and every post of a request hand over the same context, so a plugin can copy it once at prep. - docs/tracing.md: document traceContext next to the phase sink, and note it as the first step of cross-rank correlation. - TraceContext gets a defaulted operator==, which replaces the field-by-field matcher in the agent tests; the accessor check is folded into Tracing.RequestStoresFixedCorrelationContext. - The mock records the sink and the context behind one mutex, and holds GMockBackendEngine directly instead of casting. NIX-1877 Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
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Inline comments:
Review comments at @src/api/cpp/backend/backend_aux.h:
- Around line 43-45: Replace the ordinary comment above
nixlBackendOptionalArgs::traceContext with Doxygen documentation describing its
call-only lifetime, that it is copied during prepXfer for use through the
request’s postXfer calls, and that it is null when no request is involved.
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Review round 2 on ai-dynamo#2324. The paragraph on what core spans cover sat under "Backend phase spans" since ai-dynamo#2295 inserted that section; it describes the instrumented-operations table, so it moves back directly below it. NIX-1877 Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
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only still-valid issues, skip the rest with a brief reason, keep changes
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Review comments at @docs/tracing.md:
- Around line 173-174: Update the Chakra references in the tracing documentation
so they consistently describe its status: either mark Chakra as planned in both
places or remove it from the available-backend example. Locate the references in
the offline-backend example and the backend status list.
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Oct 2, 2026
Review round 3 on ai-dynamo#2324: match the other TraceContext members. The defaulted comparison is not noexcept on its own, because comparing the std::array members is potentially-throwing. NIX-1877 Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
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What?
Backends can now see the trace context of the request they are executing.
nixl_opt_b_args_tgainstraceContext, a pointer to the context the request handle owns, set bymakeXferReq,createXferReqandpostXferReq. It is the same context at prep and at every post of that request, so a plugin copies it onto its own request handle once at prep. It points at the zeroed, invalid context when tracing is off, and is null inprepMemView, which has no request. No plugin reads it yet.Why?
NIX-1877, part of distributed tracing (NIX-1537). #2295 let a plugin record phases; this lets it tell which request a phase, or a message it puts on the wire, belongs to. The UCX carrier (NIX-1879) is the first consumer.
How?
Plugins get the
TraceContextvalue rather than the encoded 26-byte record, so a plugin that only records phases does not pay for encoding; a carrier callsencodeTraceContextitself. As with the sink, the type is only forward-declared in the installedbackend_aux.h, so only in-tree plugins can dereference it.No
NIXL_PLUGIN_API_VERSIONbump, deliberately. An older plugin with this core is safe: the member is appended, so no existing offset moves, and everynixl_opt_b_args_tis constructed in core while plugins only receive aconstpointer. The reverse, a plugin that reads the field loaded by a core from before this PR, is the case #2295 bumped for. Here it is covered by sequencing instead: version 2 is in no release yet (v1.5.0 predates #2295), the first reader is the UCX carrier (NIX-1879), and NIX-1878 lands before it with the bump to 3 that its newnixlBackendEnginevirtual requires. Every plugin that readstraceContextwill therefore report 3, and a core without the field refuses it.A reposted request hands over the same ids, by design: the context belongs to the request handle, not to a post (
Tracing.RequestContextsAreDistinctAndStable), so a carrier will send the same ids for every post of a reused handle.Tests
prepXfer/postXfer. They check that each request's own context reaches prep and post with tracing on (including a repost and the preppedmakeXferReqpath), that tracing off hands over the zeroed context, and that the request exposes the context it owns.Summary by CodeRabbit
New Features
Documentation