[feat] Add RLHF rollout integration support (verl)

[sijyang]

Overview

This PR enables ATOM to serve as a rollout backend for verl, a distributed RLHF training framework. In RLHF training, the system alternates between two phases:

1. Training phase — the trainer updates model weights via gradient descent (handled by verl)
2. Rollout phase — the inference engine generates responses using the latest weights (handled by ATOM)

This requires ATOM to support a new lifecycle that traditional serving doesn't need: receiving weight updates from an external trainer, dynamically releasing/reclaiming GPU memory between phases, and coordinating these operations across multiple DP ranks and TP ranks.

The integration is designed as a plugin layer (atom/rollout/) that extends ATOM's existing engine without modifying its core inference path. All changes to existing files are purely incremental additions (new methods, new fields, new message types), no existing behavior is altered.

Architecture

verl trainer (PyTorch DDP)
│
ATOMHttpServer (verl side, per-node)
│ ZMQ RPC
AsyncLLMEngine (atom/rollout/async_engine.py)
├── sleep() → release KV cache, free GPU memory for training
├── wake_up() → reallocate KV cache, ready for generation
├── load_weights() → receive updated weights via CUDA IPC
└── generate() → standard ATOM inference with logprobs

Weight Synchronization

Weight transfer uses CUDA IPC (weight_sync.py → weight_updater.py) for zero-copy GPU-to-GPU transfer via cudaIpcGetMemHandle/cudaIpcOpenMemHandle. Weights are packed into a GPU buffer, and IPC handles are sent to ModelRunner subprocesses. On multi-GPU setups (DP>1), per-GPU buffers ensure same-device IPC.

Weights are accumulated into fixed-size buckets and flushed incrementally, keeping peak memory overhead bounded regardless of model size.

Weight Update Pipeline

weight_updater.py handles the ModelRunner side of weight loading:

• Maps incoming parameter names to ATOM's internal weight names (handling TP sharding, column/row parallel splits)
• Supports packed weights (e.g., QKV fused) by slicing incoming tensors to correct offsets
• Handles FP8 requantization — when the model uses FP8, incoming FP16/BF16 weights are quantized in-place with updated scales
• Clears KV cache after weight update to prevent stale cache from previous weights

GPU Memory Lifecycle

memory_manager.py manages the sleep/wake cycle:

• Sleep: deallocate KV cache blocks → torch.cuda.empty_cache() → memory returned to PyTorch/ROCm for trainer
• Wake: empty_cache() → recalculate available blocks → reallocate KV cache → ready for inference
• Each DP rank manages its own KV cache independently

DP Isolation

model_runner_ext.py (RLHFModelRunner) extends ATOM's ModelRunner for DP-isolated execution. Each DP rank's ModelRunners form an independent NCCL world scoped to TP only, with correct physical-to-logical device mapping and NCCL binding patches for ROCm multi-GPU setups.

Changes

New files (atom/rollout/)

File	Purpose
__init__.py	Package exports
async_engine.py	AsyncLLMEngine wrapper (sleep/wake/load_weights API)
engine_utility.py	Utility command handlers (update_weights, release/resume_memory)
memory_manager.py	GPU memory lifecycle (KV cache alloc/release, weight discard/resume)
model_runner_ext.py	RLHFModelRunner with DP isolation, NCCL device binding patch
weight_sync.py	Weight transfer via CUDA IPC (per-GPU buffers)
weight_updater.py	Weight update logic (packed weights, FP8 requantize, TP sharding)

Incremental changes to existing files

• engine_core.py: utility queue, sleep mode, UTILITY_RESPONSE message type, DP sleep state sync
• engine_core_mgr.py: utility_response_queue, broadcast_utility_command, broadcast_utility_command_sync
• llm_engine.py: request_ids and logprobs support in add_request/generate/postprocess
• async_proc.py: TP-rank barrier for safe weight update buffer reuse
• scheduler.py: logprobs tracking in ScheduledBatch/ScheduledBatchOutput
• sequence.py: request_id, return_logprobs, logprobs fields
• sampling_params.py: logprobs parameter
• config.py: runner_qualname, compilation_config dict→object conversion

[ZhangLirong-amd]

recover it, we need support it

[sijyang]

Recovered.

[ZhangLirong-amd]

do we have to change it?

[sijyang]

Yes, this is needed for veRL rollout. AsyncLLMEngine uses RLHFModelRunner instead of the default ModelRunner.