[Bugfix] get_num_blocks_to_allocate with null_block (#19031)

Signed-off-by: Chen Zhang <zhangch99@outlook.com>

tests/v1/core/test_specialized_manager.py

@@ -144,3 +144,26 @@ def test_sliding_window_remove_skipped_blocks():
     # of removed blocks should be [1003, 1002].
     manager.remove_skipped_blocks("test", 11)
     assert_block_id(block_table, [null_block_id] * 4 + original_block_ids[4:])
+
+
+def test_get_num_blocks_to_allocate():
+    block_size = 2
+    sliding_window_spec = SlidingWindowSpec(
+        block_size=block_size,
+        num_kv_heads=1,
+        head_size=1,
+        dtype=torch.float32,
+        sliding_window=4,  # Placeholder value, not related to test result
+        use_mla=False,
+    )
+
+    block_pool = BlockPool(num_gpu_blocks=100, enable_caching=True)
+    manager = get_sliding_window_manager(sliding_window_spec, block_pool)
+    cached_blocks_1 = [KVCacheBlock(i + 1) for i in range(10)]
+    cached_blocks_2 = [block_pool.null_block for _ in range(5)
+                       ] + [KVCacheBlock(i + 1) for i in range(5)]
+
+    assert manager.get_num_blocks_to_allocate("1", 20 * block_size,
+                                              cached_blocks_1) == 20
+    assert manager.get_num_blocks_to_allocate("2", 20 * block_size,
+                                              cached_blocks_2) == 15

vllm/v1/core/block_pool.py

@@ -63,6 +63,7 @@ class BlockPool:
         # The ref_cnt of null_block is not maintained, needs special care to
         # avoid freeing it.
         self.null_block = self.free_block_queue.popleft()
+        self.null_block.is_null = True
 
         self.enable_kv_cache_events = enable_kv_cache_events
         self.kv_event_queue: list[KVCacheEvent] = []
@@ -252,7 +253,7 @@ class BlockPool:
         for block in blocks:
             # ref_cnt=0 means this block is in the free list (i.e. eviction
             # candidate), so remove it.
-            if block.ref_cnt == 0 and block != self.null_block:
+            if block.ref_cnt == 0 and not block.is_null:
                 self.free_block_queue.remove(block)
             block.incr_ref()
 
@@ -267,7 +268,7 @@ class BlockPool:
         for block in ordered_blocks:
             block.decr_ref()
             # null_block should not be added to the free list.
-            if block.ref_cnt == 0 and block != self.null_block:
+            if block.ref_cnt == 0 and not block.is_null:
                 self.free_block_queue.append(block)
 
     def reset_prefix_cache(self) -> bool:

vllm/v1/core/kv_cache_utils.py

@@ -125,6 +125,9 @@ class KVCacheBlock:
     prev_free_block: Optional["KVCacheBlock"] = None
     next_free_block: Optional["KVCacheBlock"] = None
 
+    # Whether the block is a null block that should never be cached.
+    is_null: bool = False
+
     def incr_ref(self):
         self.ref_cnt += 1
 

vllm/v1/core/single_type_kv_cache_manager.py

@@ -83,7 +83,8 @@ class SingleTypeKVCacheManager(ABC):
         # free queue and ref_cnt == 0), it will be changed from a free block
         # to a computed block when the request is allocated, so we also count
         # it as needed to be allocated.
-        num_evictable_computed_blocks = sum(blk.ref_cnt == 0
+        num_evictable_computed_blocks = sum(
+            blk.ref_cnt == 0 and not blk.is_null
             for blk in new_computed_blocks)
         return ((num_new_blocks + num_evictable_computed_blocks) *
                 self.num_kv_cache_groups)