[https://nvbugs/5863443][fix] Fix message truncation in Helix CP cache transmission

[brb-nv]

Description

This MR fixes https://nvbugs/5863443.

Issue

Cache transmission fails with certain disaggregated serving configurations, specifically when using high context PP (e.g., ctx_PP=4) with generation-side CP (e.g., gen_TP=2, gen_CP=2). The failure manifests as:

• Message truncated errors in DataRequester .
• std::bad_cast exception during KV cache receive.

Root Cause

• The cacheIdx calculation in MLACacheFormatter::format() incorrectly assumed connection ordering. Connections from targetIRanks() are ordered CP-major (all connections for CP=0, then CP=1, etc.), but cacheIdx was computed as processIdx % (pPDomainSize * cPDomainSize), which doesn't account for this ordering.

Fix

// Before (incorrect):
auto cacheIdx = processIdx % (pPDomainSize * cPDomainSize);

// After (correct):
auto connectionsPerCPDomain = connections.size() / cPDomainSize;
auto cpDomainIdx = processIdx / connectionsPerCPDomain;
auto ppDomainIdx = (processIdx % connectionsPerCPDomain) % pPDomainSize;
auto cacheIdx = cpDomainIdx * pPDomainSize + ppDomainIdx;

Test Coverage

$ TRTLLM_USE_UCX_KVCACHE=1 mpirun -n 8 ./tests/unit_tests/multi_gpu/cacheTransceiverTest --gtest_filter="AsymmetricCaseTest*WithCPForMLAUnevenLayer*"

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Summary by CodeRabbit

• Bug Fixes

  • Improved cache index computation with CP-aware ordering for multi-GPU environments.
  • Enhanced rank calculation logic in distributed multi-dimensional MPI setups.

• Tests

  • Expanded test coverage for asymmetric cache configurations and uneven layer distributions.
  • Updated test diagnostics with improved debugging context.

[coderabbitai]

📝 Walkthrough

Walkthrough

Changes refactor cache indexing in the MLA cache formatter to use CP-major ordering instead of modulo-based indexing. Related test updates adjust rank calculation logic and add multiple new parameterized test instantiations for asymmetric cache scenarios with uneven layer distributions.

Changes

Cohort / File(s)	Summary
Cache Indexing Logic cpp/tensorrt_llm/batch_manager/mlaCacheFormatter.cpp	Replaces cache index calculation with CP-major ordering aware computation. Derives cpDomainIdx from processIdx, derives ppDomainIdx from within-CP-domain position, and computes cacheIdx as cpDomainIdx \* pPDomainSize + ppDomainIdx. Adds validation for connectionsPerCPDomain > 0 and documents CP-major and PP-domain ordering intent.
Test Framework Updates cpp/tests/unit_tests/multi_gpu/cacheTransceiverTest.cpp	Reorders multi-dimensional MPI rank calculations: mCpRank now computed first as mRankInInstance % mCpSize, mTpRank as (mRankInInstance / mCpSize) % mTpSize. Updates assertion messages to include WorldRank and CpRank. Adds four new asymmetric test instantiations with uneven layer distributions (AsymmetricCaseTest0/1/2WithCPForMLAUnevenLayer and related variants).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 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%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the fix: correcting message truncation in Helix CP cache transmission, directly matching the main change in the changeset.
Description check	✅ Passed	The description comprehensively covers the issue, root cause, and fix with code examples, test coverage command, and completed PR checklist items as required by the template.

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@cpp/tensorrt_llm/batch_manager/mlaCacheFormatter.cpp`:
- Around line 252-270: The CP-major mapping can produce out-of-range cacheIdx
when connections.size() isn't divisible by cPDomainSize or when
connectionsPerCPDomain isn't divisible by pPDomainSize; in sendBufferFun compute
connectionsPerCPDomain as const, then add explicit checks that
connections.size() % cPDomainSize == 0 and connectionsPerCPDomain % pPDomainSize
== 0 (or otherwise validate cpDomainIdx and cacheIdx against
outputSplitCaches.size()) and bail/log if they fail, and mark cpDomainIdx,
ppDomainIdx and cacheIdx as const to follow immutability guidelines.

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[brb-nv]

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[tensorrt-cicd]

PR_Github #34831 [ run ] triggered by Bot. Commit: ed6919d

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PR_Github #34831 [ run ] completed with state SUCCESS. Commit: ed6919d
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[tensorrt-cicd]

PR_Github #34855 [ run ] triggered by Bot. Commit: ed6919d

[tensorrt-cicd]

PR_Github #34855 [ run ] completed with state SUCCESS. Commit: ed6919d
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PR_Github #34914 [ run ] triggered by Bot. Commit: ed6919d

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