[EPLB] Refactor Async EPLB synchronization logic (#37601)

Signed-off-by: Sage Moore <sage@neuralmagic.com>
Co-authored-by: Tyler Michael Smith <tyler@neuralmagic.com>

tests/distributed/test_eplb_events.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import threading
+import time
+
+import torch
+
+from vllm.distributed.eplb.eplb_utils import CpuGpuEvent
+
+
+def test_wait_blocks_until_record():
+    event = CpuGpuEvent()
+    record_stream = torch.cuda.Stream()
+    wait_stream = torch.cuda.Stream()
+    wait_returned = threading.Event()
+
+    def waiter():
+        event.wait(stream=wait_stream)
+        wait_returned.set()
+
+    t = threading.Thread(target=waiter)
+    t.start()
+
+    time.sleep(0.05)
+    assert not wait_returned.is_set(), "wait() returned before record() was called"
+
+    event.record(stream=record_stream)
+    t.join(timeout=5.0)
+
+    assert not event._recorded.is_set()
+
+
+def test_reuse_across_multiple_cycles():
+    wrapper = CpuGpuEvent()
+    record_stream = torch.cuda.Stream()
+    wait_stream = torch.cuda.Stream()
+    NUM_CYCLES = 8
+    completed_cycles = []
+    barriers = [threading.Barrier(2) for _ in range(NUM_CYCLES)]
+
+    def waiter():
+        for i in range(NUM_CYCLES):
+            wrapper.wait(stream=wait_stream)
+            completed_cycles.append(True)
+            barriers[i].wait()
+
+    t = threading.Thread(target=waiter)
+    t.start()
+
+    for i in range(NUM_CYCLES):
+        wrapper.record(stream=record_stream)
+        barriers[i].wait()
+
+    t.join(timeout=10.0)
+    assert len(completed_cycles) == NUM_CYCLES
+
+
+def test_producer_consumer():
+    """
+    This test uses the CpuGpuEvent to synchronize reads and writes to/from a shared GPU
+    tensor on multiple CPU threads.
+    """
+    worker_stream = torch.cuda.Stream()
+    # Create a single element counter that will be shared between two threads
+    buf = torch.zeros(1, device="cuda")
+    NUM_ROUNDS = 5
+
+    ready_cpu = [threading.Event() for _ in range(NUM_ROUNDS)]
+    events = [CpuGpuEvent() for _ in range(NUM_ROUNDS)]
+    errors: list[str] = []
+
+    # For each round, the worker thread (writer) sets the counter in buf and waits for
+    # the main thread to read it.
+    def worker():
+        for i in range(NUM_ROUNDS):
+            if i > 0:
+                events[i - 1].wait(stream=worker_stream)
+
+            with torch.cuda.stream(worker_stream):
+                buf.fill_(float(i + 1))
+
+            worker_stream.synchronize()
+            ready_cpu[i].set()
+
+    t = threading.Thread(target=worker)
+    t.start()
+
+    for i in range(NUM_ROUNDS):
+        ready_cpu[i].wait()
+        snapshot = buf.clone()
+        events[i].record()
+        val = snapshot.item()
+        if val != float(i + 1):
+            errors.append(f"round {i}: expected {i + 1:.1f}, got {val:.1f}")
+
+    t.join(timeout=10.0)
+    assert not errors, f"Buffer ordering errors: {errors}"

tests/distributed/test_eplb_utils.py

@@ -80,7 +80,7 @@ def test_commit_eplb_maps_for_layer_logical_padding():
         .contiguous()
     )
     layer = 0
-    _commit_eplb_maps_for_layer(model_state, new_phy2log, layer)
+    _commit_eplb_maps_for_layer(model_state, new_phy2log[layer], layer)
 
     assert torch.all(model_state.logical_to_physical_map[layer, :, 2] == -1)
 
@@ -143,7 +143,7 @@ def test_commit_eplb_maps_for_layer():
     )
     new_logcnt = torch.tensor([[2, 1, 1], [1, 2, 1]], dtype=torch.long)
 
-    _commit_eplb_maps_for_layer(model_state, new_phy2log, layer=0)
+    _commit_eplb_maps_for_layer(model_state, new_phy2log[0], layer=0)
 
     # Layer 0 updated
     assert torch.equal(model_state.physical_to_logical_map[0], new_phy2log[0])

vllm/distributed/eplb/async_worker.py

@@ -14,7 +14,8 @@ from torch.distributed import ProcessGroup
 from vllm.distributed.parallel_state import get_eplb_group
 from vllm.logger import init_logger
 
-from .rebalance_execute import transfer_layer
+from .eplb_utils import CpuGpuEvent
+from .rebalance_execute import AsyncEplbLayerResult, transfer_layer
 
 if TYPE_CHECKING:
     from .eplb_state import EplbModelState, EplbState
@@ -60,17 +61,13 @@ def run_rebalance_experts(
     model_state: "EplbModelState",
     eplb_state: "EplbState",
     physical_to_logical_map_cpu: torch.Tensor,
-) -> None:
+    cuda_stream: torch.cuda.Stream,
+) -> torch.Tensor:
     assert model_state.eplb_stats is not None
     eplb_stats = model_state.eplb_stats
 
-    # Wait for the main thread's all-reduce and clone to complete before
-    # accessing the global_expert_load_window tensor.
-    assert model_state.window_ready_event is not None
-    model_state.window_ready_event.wait()
-    model_state.window_ready_event = None
-
     # Move the global expert load window to CPU for computation.
+    with torch.cuda.stream(cuda_stream):
         global_expert_load_window = eplb_stats.global_expert_load_window.cpu()
     # Compute new expert mappings for the model
     new_physical_to_logical_map = eplb_state.policy.rebalance_experts(
@@ -83,7 +80,7 @@ def run_rebalance_experts(
     )
     assert new_physical_to_logical_map.device == torch.device("cpu")
 
-    model_state.new_physical_to_logical_map = new_physical_to_logical_map
+    return new_physical_to_logical_map
 
 
 async def transfer_run_periodically(
@@ -93,85 +90,71 @@ async def transfer_run_periodically(
     is_profile: bool = False,
 ) -> None:
     while True:
-        await asyncio.to_thread(state.rearrange_event.wait)
+        state.rearrange_event.wait(stream=cuda_stream)
         logger.info("async worker woke up for EPLB transfer")
 
         assert state.is_async
         for model_state in state.model_states.values():
+            layer_idx = 0
             # Set the async worker's CUDA stream on the communicator
             model_state.communicator.set_stream(cuda_stream)
-            rebalancing_algorithm_executed = False
-            physical_to_logical_map_cpu = None
-            current_num_layers = model_state.model.num_moe_layers
-            while (
-                model_state.rebalanced
-                and model_state.layer_to_transfer < current_num_layers
-            ):
-                if not model_state.ep_buffer_ready and model_state.rebalanced:
-                    # Polling the lock directly in the async thread avoids
-                    # the thread switch overhead of asyncio.to_thread.
-                    # This is typically faster than offloading to a worker thread.
-                    while not model_state.buffer_lock.acquire(blocking=False):
-                        await asyncio.sleep(0)
-                    try:
-                        if model_state.layer_to_transfer >= current_num_layers:
-                            break
-                        if (
-                            not rebalancing_algorithm_executed
-                            or model_state.new_physical_to_logical_map is None
-                        ):
-                            # Move the physical_to_logical_map to CPU
-                            # for rebalancing and transfer_layer.
-                            physical_to_logical_map_cpu = (
-                                model_state.physical_to_logical_map.cpu()
-                            )
-                            run_rebalance_experts(
-                                model_state, state, physical_to_logical_map_cpu
+            num_layers = model_state.model.num_moe_layers
+
+            # Snapshot the physical_to_logical_map (synchronized with
+            # rearrange_event) and copy it to CPU
+            with torch.cuda.stream(cuda_stream):
+                physical_to_logical_map_cpu = model_state.physical_to_logical_map.cpu()
+
+            new_physical_to_logical_map = run_rebalance_experts(
+                model_state, state, physical_to_logical_map_cpu, cuda_stream
             )
-                            rebalancing_algorithm_executed = True
             logger.info(
                 "Async worker computed new indices for model %s",
                 model_state.model_name,
             )
 
-                        assert model_state.new_physical_to_logical_map is not None
-                        assert physical_to_logical_map_cpu is not None
-
-                        layer_idx = model_state.layer_to_transfer
-                        old_layer_indices = physical_to_logical_map_cpu[layer_idx]
-                        new_layer_indices = model_state.new_physical_to_logical_map[
-                            layer_idx
-                        ]
-
-                        # Wait for the main thread to finish consuming the buffer
-                        # before initiating an EPLB transfer on another layer.
-                        if model_state.buffer_consumed_event is not None:
-                            cuda_stream.wait_event(model_state.buffer_consumed_event)
-                            model_state.buffer_consumed_event = None
-
+            # Execute one EPLB layer transfer per model forward pass. Each iteration
+            # of this loop will copy the new set of expert weights into
+            # model_state.expert_buffer, which will be consumed by the main thread in
+            # move_to_workspace
+            while model_state.rebalanced and layer_idx < num_layers:
                 (
-                            model_state.is_unchanged,
-                            model_state.is_received_locally,
-                            model_state.recv_metadata,
+                    is_unchanged,
+                    is_received_locally,
+                    recv_metadata,
                 ) = await transfer_layer(
-                            old_layer_indices=old_layer_indices,
-                            new_layer_indices=new_layer_indices,
+                    old_layer_indices=physical_to_logical_map_cpu[layer_idx],
+                    new_layer_indices=new_physical_to_logical_map[layer_idx],
                     expert_weights=model_state.model.expert_weights[layer_idx],
                     expert_weights_buffer=model_state.expert_buffer,
-                            ep_group=eplb_group,
                     communicator=model_state.communicator,
+                    ep_group=eplb_group,
                     is_profile=is_profile,
                     cuda_stream=cuda_stream,
                 )
-                        # block the async thread until the transfer to
-                        # the intermediate buffer is complete.
+
+                # Wait until all writes to expert_buffer have finished before making the
+                # AsyncEplbLayerResult visible to the main thread.
                 cuda_stream.synchronize()
-                        model_state.ep_buffer_ready = 1
-                    finally:
-                        model_state.buffer_lock.release()
-                else:
-                    if not model_state.rebalanced:
-                        break
-                    await asyncio.sleep(0.001)
 
-        state.rearrange_event.clear()
+                # This event guarantees that expert_buffer will not be overwritten by
+                # subsequent iterations of this loop until the main thread has consumed
+                # it. Record is called by the main thread after move_from_buffer().
+                consumed_event = CpuGpuEvent()
+
+                model_state.pending_result = AsyncEplbLayerResult(
+                    layer_idx=layer_idx,
+                    new_physical_to_logical_map=new_physical_to_logical_map[layer_idx],
+                    is_unchanged=is_unchanged,
+                    is_received_locally=is_received_locally,
+                    recv_metadata=recv_metadata,
+                    consumed_event=consumed_event,
+                )
+
+                # Block this thread until the main thread and main stream
+                # finish copying model_state.expert_buffer into
+                # model_state.model.expert_weights[layer_idx]
+                consumed_event.wait(stream=cuda_stream)
+                logger.debug("Layer %d transfer complete", layer_idx)
+                assert model_state.pending_result is None
+                layer_idx += 1

vllm/distributed/eplb/eplb_state.py

@@ -30,7 +30,6 @@ import threading
 from collections.abc import Sequence
 from dataclasses import dataclass
 
-import numpy as np
 import torch
 from torch.distributed import ProcessGroup, all_reduce
 
@@ -48,9 +47,10 @@ from vllm.model_executor.models.interfaces import MixtureOfExperts
 
 from .async_worker import start_async_worker
 from .eplb_communicator import EplbCommunicator, create_eplb_communicator
+from .eplb_utils import CpuGpuEvent
 from .policy import EPLB_POLICIES, AbstractEplbPolicy, DefaultEplbPolicy
 from .rebalance_execute import (
-    RecvMetadata,
+    AsyncEplbLayerResult,
     move_from_buffer,
     rearrange_expert_weights_inplace,
 )
@@ -174,55 +174,20 @@ class EplbModelState:
     """
     The buffer to store the expert weights during transfer.
     """
-    buffer_lock: threading.Lock
-    """
-    The lock to protect the expert buffer.
-    """
-    buffer_consumed_event: torch.cuda.Event | None
-    """
-    CUDA event recorded after the main thread finishes consuming the buffer.
-    The async worker waits on this before writing to the buffer again.
-    """
-    window_ready_event: torch.cuda.Event | None
-    """
-    CUDA event recorded after all-reduce and clone on the main thread.
-    The async worker waits on this before accessing global_expert_load_window.
-    """
-    ep_buffer_ready: int
-    """
-    The flag indicates whether the expert buffer is ready for transfer.
-    0 or 1.
-    """
-    layer_to_transfer: int
-    """
-    The layer index to transfer in async mode.
-    """
     rebalanced: bool
     """
-    The flag indicates whether the experts rebalance have been computed.
-    """
-    pending_global_ready_check: bool
-    """
-    Whether the async EPLB needs to poll peers for buffer readiness.
+    This flag is only used when running Async EPLB. It is set to True by the main thread
+    after the new expert maps have been computed. This indicates that the async worker
+    should start transferring weights. move_to_workspace sets this flag to False when
+    all weights have been transferred and the new map has been successfully committed.
+
+    rebalanced relies on the GIL to synchronize access between the main thread and
+    the async worker.
     """
     eplb_stats: EplbStats | None
     """
     EPLB stats for the model.
     """
-    is_unchanged: np.ndarray
-    """
-    intermediate variable between `move_to_buffer` and `move_to_workspace`.
-    The size is same as the num of physical experts in the current layer.
-    """
-    is_received_locally: np.ndarray
-    """
-    intermediate variable between `move_to_buffer` and `move_to_workspace`.
-    The size is same as the num of physical experts in the current layer.
-    """
-    recv_metadata: RecvMetadata
-    """
-    intermediate variable between `move_to_buffer` and `move_to_workspace`.
-    """
     cuda_device_index: int | None
     """
     CUDA device index for the async EPLB worker thread.
@@ -231,10 +196,14 @@ class EplbModelState:
     """
     The communicator for expert weight transfers.
     """
-    new_physical_to_logical_map: torch.Tensor | None = None
+    pending_result: AsyncEplbLayerResult | None = None
     """
-    intermediate variable between `move_to_buffer` and `move_to_workspace`.
-    the size is same as physical_to_logical_map
+    Set by the async worker after all writes to expert_buffer are done. Consumed
+    and reset to None by the main thread in move_to_workspace() after the contents of
+    expert_buffer have been transferred out. At most one result is pending at a time.
+
+    pending_result relies on the GIL to synchronize access between the main thread and
+    the async worker.
     """
 
 
@@ -289,7 +258,7 @@ class EplbState:
         """
         The flag indicates whether the EPLB is running in async mode.
         """
-        self.rearrange_event = threading.Event()
+        self.rearrange_event: CpuGpuEvent = CpuGpuEvent()
         """
         Event to signal when a new rearrangement is needed for the async thread.
         """
@@ -493,25 +462,10 @@ class EplbState:
             model_name=model_config.model,
             model=model,
             expert_buffer=expert_buffer,
-            buffer_lock=threading.Lock(),
-            buffer_consumed_event=None,
-            window_ready_event=None,
-            ep_buffer_ready=0,
-            layer_to_transfer=0,
             rebalanced=False,
-            pending_global_ready_check=False,
             eplb_stats=None,
-            is_unchanged=np.array([]),
-            is_received_locally=np.array([]),
-            recv_metadata=RecvMetadata(
-                recv_primary_mask=np.array([]),
-                recv_count=0,
-                recv_expert_ids=np.array([]),
-                recv_dst_rows=np.array([]),
-            ),
             cuda_device_index=self.cuda_device_index,
             communicator=communicator,
-            new_physical_to_logical_map=None,
         )
         self.model_states[model_config.compute_hash()] = model_state
         self.num_valid_physical_experts = model.num_physical_experts
@@ -622,17 +576,17 @@ class EplbState:
         self.expert_rearrangement_step += 1
 
         if self.is_async:
+            # Run _move_to_workspace if all ranks have finished transferring the
+            # new weights to the intermediate buffer
             for eplb_model_state in self.model_states.values():
-                all_ranks_buffer_ready = False
-                if eplb_model_state.pending_global_ready_check:
-                    all_ranks_buffer_ready = self._all_ranks_buffer_ready(
+                # rebalanced must remain consistent amongst all ranks otherwise the
+                # all_reduce in _all_ranks_result_ready will hang
+                if eplb_model_state.rebalanced and self._all_ranks_result_ready(
                     eplb_model_state
-                    )
-                if eplb_model_state.ep_buffer_ready and all_ranks_buffer_ready:
-                    self.move_to_workspace(
+                ):
+                    _move_to_workspace(
                         model_state=eplb_model_state,
-                        ep_group=ep_group,
-                        is_profile=is_profile,
+                        ep_rank=ep_group.rank(),
                     )
 
         if self.expert_rearrangement_step >= self.expert_rearrangement_step_interval:
@@ -846,18 +800,10 @@ class EplbState:
                     num_nodes=num_nodes,
                     num_gpus=num_gpus,
                 )
-                # Record event after clone to signal async worker
-                # that load stats data is ready
-                sync_event = torch.cuda.Event()
-                sync_event.record()
-                eplb_model_state.window_ready_event = sync_event
-
                 eplb_model_state.rebalanced = True
-                eplb_model_state.layer_to_transfer = 0
-                eplb_model_state.pending_global_ready_check = True
         # Signal async thread to start transferring layers
         if self.is_async and (not is_profile):
-            self.rearrange_event.set()
+            self.rearrange_event.record()
         return None
 
     def start_async_loop(
@@ -873,121 +819,27 @@ class EplbState:
                 is_profile=is_profile,
             )
 
-    def _all_ranks_buffer_ready(self, model_state: EplbModelState) -> bool:
+    def _all_ranks_result_ready(self, model_state: EplbModelState) -> bool:
         parallel_state = get_ep_group()
+        has_result = int(model_state.pending_result is not None)
+
         cpu_group = getattr(parallel_state, "cpu_group", None)
         if cpu_group is not None and cpu_group.size() > 1:
-            flag = torch.tensor(
-                (int(model_state.ep_buffer_ready),), dtype=torch.int32, device="cpu"
-            )
+            flag = torch.tensor((has_result,), dtype=torch.int32, device="cpu")
             all_reduce(flag, group=cpu_group)
             return int(flag.item()) == cpu_group.size()
 
         device_group = parallel_state.device_group
         if device_group.size() <= 1:
-            return bool(model_state.ep_buffer_ready)
+            return bool(has_result)
 
         device = getattr(
             parallel_state, "device", model_state.physical_to_logical_map.device
         )
-        flag = torch.tensor(
-            (int(model_state.ep_buffer_ready),), dtype=torch.int32, device=device
-        )
+        flag = torch.tensor((has_result,), dtype=torch.int32, device=device)
         all_reduce(flag, group=device_group)
         return int(flag.item()) == device_group.size()
 
-    def move_to_workspace(
-        self,
-        model_state: EplbModelState,
-        ep_group: ProcessGroup,
-        is_profile: bool = False,
-    ):
-        # We call move_to_workspace only when ep_buffer_ready is 1.
-        # It means we only need to wait for the lock for a short time.
-        max_retries = 6  # 1 minute max
-        retries = 0
-        while not model_state.buffer_lock.acquire(blocking=True, timeout=10.0):
-            retries += 1
-            if retries >= max_retries:
-                raise RuntimeError(
-                    f"Rank {ep_group.rank()}: buffer_lock timeout after "
-                    "{max_retries * 10}s"
-                )
-            logger.warning(
-                "Rank %d: EPLB buffer_lock acquire failed, retrying (%d/%d)",
-                ep_group.rank(),
-                retries,
-                max_retries,
-            )
-        try:
-            assert model_state.new_physical_to_logical_map is not None
-            expert_weights = model_state.model.expert_weights[
-                model_state.layer_to_transfer
-            ]
-            expert_weights_buffer = model_state.expert_buffer
-            new_indices = model_state.new_physical_to_logical_map[
-                model_state.layer_to_transfer
-            ].numpy()
-            move_from_buffer(
-                expert_weights=expert_weights,
-                expert_weights_buffers=expert_weights_buffer,
-                is_unchanged=model_state.is_unchanged,
-                is_received_locally=model_state.is_received_locally,
-                recv_metadata=model_state.recv_metadata,
-                new_indices=new_indices,
-                ep_rank=ep_group.rank(),
-            )
-
-            transferred_layer = model_state.layer_to_transfer
-
-            transferred_layer = model_state.layer_to_transfer
-            assert model_state.new_physical_to_logical_map is not None
-            _commit_eplb_maps_for_layer(
-                model_state,
-                new_physical_to_logical_map=model_state.new_physical_to_logical_map,
-                layer=transferred_layer,
-            )
-
-            # Record event after consuming buffer to signal async thread
-            # that it's safe to overwrite the intermediate buffer
-            consumed_event = torch.cuda.Event()
-            consumed_event.record()
-            model_state.buffer_consumed_event = consumed_event
-
-            # After the main thread consumes, advance layer_to_transfer
-            model_state.layer_to_transfer += 1
-            model_state.ep_buffer_ready = 0
-            logger.debug(
-                "model %s successfully move_to_workspace layer %d",
-                model_state.model_name,
-                transferred_layer,
-            )
-            if model_state.layer_to_transfer >= model_state.model.num_moe_layers:
-                self.post_eplb(model_state)
-                model_state.rebalanced = False
-                model_state.layer_to_transfer = 0
-                model_state.pending_global_ready_check = False
-                logger.info(
-                    "finish async transfer for model %s rank %d layer %d",
-                    model_state.model_name,
-                    ep_group.rank(),
-                    model_state.model.num_moe_layers,
-                )
-
-        finally:
-            try:
-                model_state.buffer_lock.release()
-            except Exception as e:
-                logger.error(
-                    "Rank %d: buffer_lock release failed in move_to_workspace: %s",
-                    ep_group.rank(),
-                    str(e),
-                )
-
-    def post_eplb(self, model_state: EplbModelState) -> None:
-        assert model_state.new_physical_to_logical_map is not None
-        model_state.new_physical_to_logical_map = None
-
     def _allreduce_list(self, tensor_list: list[torch.Tensor]) -> list[torch.Tensor]:
         """
         All-reduce a list of tensors.
@@ -1225,7 +1077,7 @@ def _commit_eplb_maps_for_layer(
     """
 
     # Commit physical_to_logical_map
-    src = new_physical_to_logical_map[layer]
+    src = new_physical_to_logical_map
     dst = model_state.physical_to_logical_map[layer]
     assert src.shape == dst.shape, (
         "The number of physical experts must stay the same while running Async EPLB. "
@@ -1284,3 +1136,33 @@ def _commit_eplb_maps(
     src = new_replica_count
     dst = model_state.logical_replica_count
     dst.copy_(src, non_blocking=True)
+
+
+def _move_to_workspace(
+    model_state: EplbModelState,
+    ep_rank: int,
+) -> None:
+    result = model_state.pending_result
+    assert result is not None
+    move_from_buffer(
+        expert_weights=model_state.model.expert_weights[result.layer_idx],
+        expert_weights_buffers=model_state.expert_buffer,
+        is_unchanged=result.is_unchanged,
+        is_received_locally=result.is_received_locally,
+        recv_metadata=result.recv_metadata,
+        new_indices=result.new_physical_to_logical_map.numpy(),
+        ep_rank=ep_rank,
+    )
+
+    _commit_eplb_maps_for_layer(
+        model_state,
+        new_physical_to_logical_map=result.new_physical_to_logical_map,
+        layer=result.layer_idx,
+    )
+
+    if result.layer_idx == model_state.model.num_moe_layers - 1:
+        model_state.rebalanced = False
+
+    # Reset pending_result before unblocking the async worker
+    model_state.pending_result = None
+    result.consumed_event.record()

vllm/distributed/eplb/eplb_utils.py

@@ -3,6 +3,9 @@
 """Utility functions for EPLB (Expert Parallel Load Balancing)."""
 
 import os
+import threading
+
+import torch
 
 from vllm.config import ParallelConfig
 from vllm.logger import init_logger
@@ -10,6 +13,54 @@ from vllm.logger import init_logger
 logger = init_logger(__name__)
 
 
+class CpuGpuEvent:
+    """
+    Combines a CUDA event with a CPU threading event to enforce record->wait
+    ordering across two threads.
+
+    This class is designed for exactly two threads: one producer that calls
+    record() and one consumer that calls wait(). Using it with more than two
+    threads is not supported and will produce undefined behavior.
+
+    CUDA events alone are insufficient for cross-thread synchronization because
+    waiting on an unrecorded CUDA event is a no-op. The wait will return
+    immediately instead of blocking. This class adds a threading.Event so
+    that the waiting thread blocks on the CPU side until record() is called, at
+    which point the CUDA event is guaranteed to be in-flight and event.wait() will
+    correctly synchronize the GPU stream.
+    """
+
+    def __init__(self):
+        self._event = torch.cuda.Event()
+        self._recorded = threading.Event()
+
+    def wait(self, stream: torch.cuda.Stream | None = None):
+        """
+        Blocks the calling thread until record finishes. Used to guarantee that the
+        record kernel is called before wait.
+
+        Should only be called by the Async Eplb thread.
+        """
+        self._recorded.wait()
+        self._event.wait(stream)
+        self._recorded.clear()
+
+    def record(self, stream: torch.cuda.Stream | None = None):
+        """
+        Unblocks the waiting thread after calling event.record().
+
+        Should only be called by the main thread.
+        """
+        if self._recorded.is_set():
+            raise RuntimeError(
+                "CpuGpuEvent.record() called before the previous event was "
+                "consumed by wait()"
+            )
+        self._event = torch.cuda.Event()
+        self._event.record(stream)
+        self._recorded.set()
+
+
 def override_envs_for_eplb(parallel_config: ParallelConfig) -> None:
     """
     Override environment variables for EPLB when specific conditions are met.

vllm/distributed/eplb/rebalance_execute.py

@@ -13,7 +13,11 @@ import numpy as np
 import torch
 from torch.distributed import ProcessGroup, all_gather
 
-from .eplb_communicator import EplbCommunicator
+from vllm.distributed.eplb.eplb_communicator import EplbCommunicator
+from vllm.distributed.eplb.eplb_utils import CpuGpuEvent
+from vllm.logger import init_logger
+
+logger = init_logger(__name__)
 
 
 @dataclass
@@ -34,6 +38,34 @@ class RecvMetadata:
 MoveToBufferResult = tuple[np.ndarray, np.ndarray, RecvMetadata]
 
 
+@dataclass
+class AsyncEplbLayerResult:
+    """
+    The result of one completed async EPLB layer transfer.
+    """
+
+    layer_idx: int
+    """Index of the MoE layer that was transferred."""
+    new_physical_to_logical_map: torch.Tensor
+    """
+    New physical→logical mapping for layers_idx, on CPU.
+    Shape: (num_physical_experts)
+    """
+    is_unchanged: np.ndarray
+    """Per-physical-expert flag: weight was not moved during transfer."""
+    is_received_locally: np.ndarray
+    """Per-physical-expert flag: weight was received on this rank."""
+    recv_metadata: RecvMetadata
+    """Metadata describing what was received during transfer_layer."""
+    consumed_event: CpuGpuEvent
+    """
+    Event used to synchronize access to the intermediate buffer. The async worker calls
+    wait() after it finishes transferring weights to the intermediate buffer. The main
+    thread calls record() after it finishes transferring weights out of the intermediate
+    buffer in _move_to_workspace()
+    """
+
+
 def get_ep_ranks_with_experts_batch(
     expert_ids: np.ndarray,
     num_local_experts: int,