[Refactor][EC Connector] Add backend extension points to ECCPUWorker

[Akine-Ko]

Purpose

ECCPUWorker provides the shared execution model for CPU-backed encoder-cache offloading. It owns the shared mmap layout, descriptor management, transfer batching, Event-based completion tracking, save reconstruction, load reconstruction, and worker metadata reporting.

Device backends use the same lifecycle while supplying their platform-specific Host-memory registration and transfer operations.

This implementation supports the Ascend backend introduced by:

vllm-project/vllm-ascend#13894

Backend Extension Points

ECCPUWorker exposes three backend extension points.

_pin_shared_region

Prepares the shared mmap region for device transfers.

The base implementation uses vLLM's standard Host-memory pinning path. AscendECCPUWorker uses this point to register the mmap as pinned Host memory through CANN.

_submit_transfer

Submits a batch of transfer descriptors for H2D or D2H execution.

The base implementation dispatches through swap_blocks_batch. AscendECCPUWorker dispatches through the corresponding CANN batched-copy operation.

ECCPUTransferDirection identifies the H2D or D2H direction passed to the backend implementation.

_shutdown_transfer_backend

Performs backend-specific shutdown after the common worker lifecycle has waited for tracked in-flight transfer Events.

Device backends use this point for platform synchronization, transfer-resource release, and pinned Host-memory unregistration. The common shutdown path then releases worker state and cleans up the shared region.

With these extension points, device workers inherit the upstream save_caches, flush_saves, start_load_caches, completion reporting, buffer recycling, and shutdown lifecycle.

Model Runner Support

ECCPUConnector supports Model Runner V2 only.

Test Plan

CPU and mock regression

python -m pytest -q \
  tests/v1/ec_connector/unit/test_worker_ec_connector.py \
  tests/v1/ec_connector/unit/test_metadata.py \
  tests/v1/ec_connector/unit/cpu/scheduler \
  tests/v1/ec_connector/unit/cpu/test_connector.py \
  tests/v1/ec_connector/unit/cpu/test_ec_shared_region.py

This coverage includes connector construction and role handling, the Model Runner V2 requirement, scheduler behavior, metadata handling, and shared-region lifecycle.

NVIDIA CUDA worker validation

python -m pytest -q -rs -s \
  tests/v1/ec_connector/unit/cpu/worker/test_worker.py

The worker coverage verifies that both save and load paths delegate transfer submission through _submit_transfer() with the correct direction, and that common shutdown invokes _shutdown_transfer_backend() before shared-region cleanup.

CUDA VLM end-to-end validation

python -m pytest -q -rs -s \
  tests/v1/ec_connector/unit/test_ec_cpu_connector.py

[claude]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Akine-Ko.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[gty111]

Thanks for the work. Can you also check whether #53190 is simlar to this PR?

[Akine-Ko]

Thanks for the work. Can you also check whether #53190 is simlar to this PR?

Yes, part of the fix is similar. I can remove the duplicate part from my PR. Is that bugfix PR expected to be merged soon?

[gty111]

Thanks for the work. Can you also check whether #53190 is simlar to this PR?

Yes, part of the fix is similar. I can remove the duplicate part from my PR. Is that bugfix PR expected to be merged soon?

Yes. It will be merged soon.

[coderabbitai]

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Configuration used: Repository UI

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Run ID: 5281c080-52c0-416a-b9b3-e9e769739a25

📥 Commits

Reviewing files that changed from the base of the PR and between 199cb9b and 892e7d3.

📒 Files selected for processing (7)

• tests/v1/ec_connector/unit/cpu/test_connector.py
• tests/v1/ec_connector/unit/cpu/worker/test_worker.py
• tests/v1/ec_connector/unit/test_worker_ec_connector.py
• vllm/distributed/ec_transfer/ec_connector/cpu/connector.py
• vllm/distributed/ec_transfer/ec_connector/cpu/worker/__init__.py
• vllm/v1/worker/ec_connector_model_runner_mixin.py
• vllm/v1/worker/gpu_model_runner.py

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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Walkthrough

The change makes NIXL validation conditional, centralizes directional CPU transfer submission, adds a transfer-backend shutdown hook, and routes zero-token model steps through EC connector no-forward processing.

Changes

EC connector flow

Layer / File(s)	Summary
Conditional NIXL configuration vllm/distributed/ec_transfer/ec_connector/cpu/connector.py, tests/v1/ec_connector/unit/cpu/test_connector.py	The connector parses boolean NIXL settings and rejects the V1 model runner only when NIXL is enabled. Tests cover supported truthy and falsy values.
Directional CPU transfer lifecycle vllm/distributed/ec_transfer/ec_connector/cpu/worker/__init__.py, tests/v1/ec_connector/unit/cpu/worker/test_worker.py	The worker centralizes save and load submission with ECCPUTransferDirection, preserves direction-specific transfer arguments, and calls _shutdown_transfer_backend() during shutdown.
No-forward connector processing vllm/v1/worker/ec_connector_model_runner_mixin.py, vllm/v1/worker/gpu_model_runner.py, tests/v1/ec_connector/unit/test_worker_ec_connector.py	The model runner captures connector metadata and processes EC transfers without model execution. Zero-token execution now uses this path. Tests verify metadata reporting and cache loading.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to 892e7

No concrete merge-blocking risk remains in the reviewed changes.

Sequence Diagram(s)

sequenceDiagram
  participant GPUModelRunner
  participant ECConnectorModelRunnerMixin
  participant ECTransfer
  GPUModelRunner->>ECConnectorModelRunnerMixin: process zero-token output
  ECConnectorModelRunnerMixin->>ECTransfer: poll transfers and start load caches
  ECTransfer-->>ECConnectorModelRunnerMixin: return connector output and metadata
  ECConnectorModelRunnerMixin-->>GPUModelRunner: return ModelRunnerOutput

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Suggested reviewers: omerpaz95

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 44.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 25 functions across 7 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 identifies the main change: adding backend extension points to ECCPUWorker.
Description check	✅ Passed	The description directly explains the ECCPUWorker extension points, Model Runner support, backend use cases, and test coverage.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

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[gty111]

/ci run

[github-actions]

✅ @Akine-Ko, CI is now available for this PR.

• /ci run starts upstream CI; /amd-ci run starts AMD CI only.
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[github-actions]

✅ Triggered Buildkite CI #87803 for commit 93b12b50c190.