[V1] Revert `uncache_blocks` and support recaching full blocks (#12415)

Signed-off-by: Cody Yu <hao.yu.cody@gmail.com>

tests/v1/core/test_prefix_caching.py

@@ -629,33 +629,3 @@ def test_reset_prefix_cache():
     assert manager.reset_prefix_cache()
     assert not manager.cached_block_hash_to_block
     assert all([blk.block_hash is None for blk in manager.block_pool])
-
-
-def test_uncache_blocks():
-    manager = KVCacheManager(
-        block_size=16,
-        num_gpu_blocks=10,
-        max_model_len=8192,
-        sliding_window=None,
-        enable_caching=True,
-        num_preallocate_tokens=0,
-    )
-
-    req0 = make_request("0", list(range(30)))
-    blocks = manager.allocate_slots(req0, 30)
-    assert [b.block_id for b in blocks] == [0, 1]
-    assert len(manager.cached_block_hash_to_block) == 1
-
-    req0.num_computed_tokens = 30
-
-    # Simulate speculative tokens.
-    for _ in range(5):
-        req0.append_output_token_ids(8)
-    manager.allocate_slots(req0, 5)
-    assert len(manager.cached_block_hash_to_block) == 2
-
-    # After sampling, assuming only 1 token is accepted.
-    req0.num_computed_tokens = 31
-    num_uncached_blocks = manager.uncache_blocks(req0)
-    assert num_uncached_blocks == 1
-    assert len(manager.cached_block_hash_to_block) == 1

vllm/v1/core/kv_cache_manager.py

@@ -252,29 +252,6 @@ class KVCacheManager:
             if block.ref_cnt == 0:
                 self.free_block_queue.append(block)
 
-    def uncache_blocks(self, request: Request) -> int:
-        """Uncache the blocks that are no longer full based on the
-        num_computed_tokens in the given request. This happens when
-        the blocks were full and cached due to speculative tokens, but the
-        speculative tokens are not accepted.
-
-        Args:
-            request: The request.
-
-        Returns:
-            The number of uncached blocks.
-        """
-        blocks = self.req_to_blocks[request.request_id]
-        num_computed_tokens = request.num_computed_tokens
-        num_full_blocks = num_computed_tokens // self.block_size
-        num_uncached_blocks = 0
-        for block in blocks[num_full_blocks:]:
-            # If the block is not cached, the following blocks are not cached.
-            if not self._maybe_evict_cached_block(block):
-                break
-            num_uncached_blocks += 1
-        return num_uncached_blocks
-
     def reset_prefix_cache(self) -> bool:
         """Reset prefix cache. This function may be used in RLHF
         flows to invalid prefix caching after the weights are updated,
@@ -470,8 +447,22 @@ class KVCacheManager:
             assert prev_block.block_hash is not None
             prev_block_hash_value = prev_block.block_hash.hash_value
 
-        for i, blk in enumerate(full_blocks):
-            blk_idx = blk_start_idx + i
+        # Find the first uncached block. This case should only happen when
+        # speculative decoding is used.
+        offset = 0
+        for blk in full_blocks:
+            if blk.block_hash is None:
+                break
+            else:
+                prev_block_hash_value = blk.block_hash.hash_value
+                offset += 1
+        else:
+            # All blocks are cached.
+            return
+
+        for i, blk in enumerate(full_blocks[offset:]):
+            blk_idx = blk_start_idx + offset + i
+            assert blk.block_hash is None
 
             if blk_idx < num_cached_block_hashes:
                 # The block hash may already be computed in