[V1] Simplify prefix caching logic by removing num_evictable_computed_blocks (vllm-project#11310)

Author: heheda12345

vllm/v1/core/kv_cache_manager.py

@@ -201,32 +201,23 @@ def allocate_slots(
                 f"num_tokens must be greater than 0, got {num_tokens}")
 
         # Touch the computed blocks to make sure they won't be evicted.
-        num_evictable_computed_blocks = 0
         if self.enable_caching:
             self._touch(computed_blocks)
-
-            # If a computed block of a request is an eviction candidate (in the
-            # free queue and ref_cnt == 0), it cannot be counted as a free block
-            # when allocating this request.
-            num_evictable_computed_blocks = len(
-                [blk for blk in computed_blocks if blk.ref_cnt == 0])
         else:
             assert not computed_blocks, (
                 "Computed blocks should be empty when "
                 "prefix caching is disabled")
 
         num_required_blocks = cdiv(num_tokens, self.block_size)
-        if (num_required_blocks > self.free_block_queue.num_free_blocks -
-                num_evictable_computed_blocks):
+        if (num_required_blocks > self.free_block_queue.num_free_blocks):
             # Cannot allocate new blocks.
             return None
 
         # Determine the number of new blocks to allocate considering
         # preallocated blocks.
         num_new_blocks = min(
             num_required_blocks + self.num_preallocate_blocks,
-            self.free_block_queue.num_free_blocks -
-            num_evictable_computed_blocks,
+            self.free_block_queue.num_free_blocks,
             # Should not exceed the maximum number of blocks per request.
             # This is especially because the block table has the shape
             # [..., max_num_blocks_per_req].