[Misc] Make reorder batch also separate extends (#27367)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

tests/v1/attention/test_batch_reordering.py

@@ -0,0 +1,111 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from dataclasses import dataclass
+
+import numpy as np
+import pytest
+
+from vllm.v1.attention.backends.utils import reorder_batch_to_split_decodes_and_prefills
+
+
+class MockInputBatch:
+    def __init__(self, req_ids, num_computed_tokens_cpu):
+        self.req_ids = req_ids
+        self.num_computed_tokens_cpu = num_computed_tokens_cpu
+
+    def swap_states(self, i, j):
+        self.req_ids[i], self.req_ids[j] = self.req_ids[j], self.req_ids[i]
+        self.num_computed_tokens_cpu[i], self.num_computed_tokens_cpu[j] = (
+            self.num_computed_tokens_cpu[j],
+            self.num_computed_tokens_cpu[i],
+        )
+
+
+class MockSchedulerOutput:
+    def __init__(self, num_scheduled_tokens):
+        self.num_scheduled_tokens = num_scheduled_tokens
+
+
+@dataclass
+class ReorderTestCase:
+    requests: list[tuple[int, int]]  # (num_scheduled_tokens, num_computed_tokens)
+    expected_order: list[int]
+    expected_modified: bool
+    decode_threshold: int = 1
+
+
+# Test cases for batch reordering
+REORDER_TEST_CASES = {
+    "all_decodes": ReorderTestCase(
+        requests=[(1, 10), (1, 20), (1, 30)],
+        expected_order=[0, 1, 2],
+        expected_modified=False,
+    ),
+    "all_prefills": ReorderTestCase(
+        requests=[(100, 100), (200, 200), (300, 300)],
+        expected_order=[0, 1, 2],
+        expected_modified=False,
+    ),
+    "mixed_interleaved": ReorderTestCase(
+        requests=[(100, 100), (1, 10), (200, 200), (1, 20)],
+        expected_order=[3, 1, 2, 0],  # Only swap 0↔3, keep 1 and 2 in place
+        expected_modified=True,
+    ),
+    "already_ordered": ReorderTestCase(
+        requests=[(1, 10), (1, 20), (100, 100), (200, 200)],
+        expected_order=[0, 1, 2, 3],
+        expected_modified=False,
+    ),
+    "single_request": ReorderTestCase(
+        requests=[(1, 10)],
+        expected_order=[0],
+        expected_modified=False,
+    ),
+    "higher_threshold": ReorderTestCase(
+        requests=[(2, 10), (3, 20), (5, 30), (6, 40)],
+        expected_order=[0, 1, 2, 3],
+        expected_modified=False,
+        decode_threshold=4,
+    ),
+    "decodes_at_end": ReorderTestCase(
+        requests=[(100, 100), (200, 200), (1, 10), (1, 20)],
+        expected_order=[2, 3, 0, 1],
+        expected_modified=True,
+    ),
+    "decode_extend_prefill": ReorderTestCase(
+        requests=[(100, 100), (10, 50), (1, 10)],
+        expected_order=[2, 1, 0],
+        expected_modified=True,
+    ),
+    "extend_prefill_only": ReorderTestCase(
+        requests=[(100, 100), (10, 50), (200, 200), (20, 75)],
+        expected_order=[3, 1, 2, 0],  # Only swap 0↔3, keep 1 and 2 in place
+        expected_modified=True,
+    ),
+}
+
+
+@pytest.mark.parametrize(
+    "test_case", REORDER_TEST_CASES.values(), ids=REORDER_TEST_CASES.keys()
+)
+def test_reorder_batch_to_split_decodes_and_prefills(test_case: ReorderTestCase):
+    req_ids = [f"r{i}" for i in range(len(test_case.requests))]
+    num_computed_tokens = np.array([r[1] for r in test_case.requests], dtype=np.int32)
+    num_scheduled_tokens = {f"r{i}": r[0] for i, r in enumerate(test_case.requests)}
+
+    input_batch = MockInputBatch(req_ids, num_computed_tokens)
+    scheduler_output = MockSchedulerOutput(num_scheduled_tokens)
+
+    modified = reorder_batch_to_split_decodes_and_prefills(
+        input_batch, scheduler_output, decode_threshold=test_case.decode_threshold
+    )
+
+    expected_req_ids = [f"r{i}" for i in test_case.expected_order]
+
+    assert modified == test_case.expected_modified, (
+        f"Expected modified={test_case.expected_modified}, got {modified}"
+    )
+    assert input_batch.req_ids == expected_req_ids, (
+        f"Expected order {expected_req_ids}, got {input_batch.req_ids}"
+    )

vllm/v1/attention/backends/utils.py

@@ -795,51 +795,59 @@ def reorder_batch_to_split_decodes_and_prefills(
     Returns:
         True if the batch was modified, False otherwise.
     """
-    # We now want to reorder the batch so that the "decode" requests are at
-    # the front and the "prefill" requests are at the back using the least
-    # amount of swaps possible. (NOTE for now we loosely use "decode" to mean
-    # requests where attention is likely memory-bound and "prefill" to mean
-    # requests where attention is likely compute-bound, TODO(lucas): figure out
-    # a better naming here)
-    decodes = []
-    prefills = []
-    num_decode_tokens = 0
-    num_prefill_tokens = 0
+    # We now want to reorder the batch into decode → extend → prefill order
+    # where:
+    #   decode: request with num_scheduled_tokens <= decode_threshold
+    #   extend: non-decode request with existing context
+    #   prefill: non-decode request with no existing context
+    # NOTE for now we loosely use "decode" to mean requests where attention is
+    #  likely memory-bound and "prefill" to mean requests where attention is
+    #  likely compute-bound,
+    num_reqs = len(input_batch.req_ids)
+    num_scheduled_tokens = [
+        scheduler_output.num_scheduled_tokens[id] for id in input_batch.req_ids
+    ]
+    num_scheduled_tokens_np = np.array(num_scheduled_tokens)
+    num_computed_tokens_np = input_batch.num_computed_tokens_cpu[:num_reqs]
 
-    for i, req_id in enumerate(input_batch.req_ids):
-        num_tokens = scheduler_output.num_scheduled_tokens[req_id]
-        if num_tokens <= decode_threshold:
-            decodes.append(i)
-            num_decode_tokens += num_tokens
-        else:
-            prefills.append(i)
-            num_prefill_tokens += num_tokens
+    is_decode = num_scheduled_tokens_np <= decode_threshold
+    is_extend = (~is_decode) & (num_computed_tokens_np > num_scheduled_tokens_np)
+    is_prefill = (~is_decode) & (num_computed_tokens_np == num_scheduled_tokens_np)
 
-    # We hope that this is fairly minimal since decodes
-    # should be around for a number of iterations so hopefully they are
-    # relatively stationary (and new request are generally appended to the
-    # persistent batch so already should be at the back)
-    # To achieve this we loop over the decodes in descending order and
-    # the prefills in ascending order. We swap decodes from the  "back"
-    # i.e. past where the last decode should be in the reodorered with
-    # prefills from the front of the batch.
-    # `decodes` and `prefills` are already in ascending order just based on
-    # the above loop
-    num_decodes = len(decodes)
-    num_prefills = len(prefills)
-    modified_batch = False
+    # Desired order: decode → extend → prefill
+    req_regions = np.zeros(is_decode.shape, dtype=np.int32)  # 0 = decode by default
+    req_regions[is_extend] = 1
+    req_regions[is_prefill] = 2
 
-    for i in range(1, min(num_decodes, num_prefills) + 1):
-        # If the decode is at the "back" of the batch, i, we can swap it
-        # with the prefill closest to the front of the batch
-        decode_idx = decodes[num_decodes - i]
-        if decode_idx < num_decodes:
-            break
+    num_decodes = int(is_decode.sum())
+    num_extends = int(is_extend.sum())
 
-        input_batch.swap_states(prefills[i - 1], decode_idx)
-        modified_batch = True
+    target_regions = np.zeros(num_reqs, dtype=np.int32)
+    target_regions[num_decodes : num_decodes + num_extends] = 1
+    target_regions[num_decodes + num_extends :] = 2
 
-    return modified_batch
+    needs_swap = req_regions != target_regions
+
+    if not needs_swap.any():
+        return False
+
+    # Extract indices that need swapping and sort by target region
+    swap_indices = np.where(needs_swap)[0]
+    sorted_order = np.argsort(req_regions[needs_swap], kind="stable")
+    dest_indices = swap_indices[sorted_order]
+
+    src_dest_map = {int(src): int(dst) for src, dst in zip(swap_indices, dest_indices)}
+
+    for src in src_dest_map:
+        dst = src_dest_map[src]
+        while src != dst:
+            input_batch.swap_states(src, dst)
+            # Mark dst as done by updating its destination to itself
+            next_dst = src_dest_map.get(dst, dst)
+            src_dest_map[dst] = dst
+            dst = next_dst
+
+    return True
 
 
 def reshape_query_for_spec_decode(query: torch.Tensor, batch_size: int) -> torch.Tensor: