[BUGFIX]: return fast when request requires prompt logprobs (#17251)

tests/v1/core/test_prefix_caching.py

@@ -194,7 +194,7 @@ def test_prefill_plp():
     all_token_ids = common_token_ids + unique_token_ids
     req0 = make_request("0", all_token_ids, prompt_logprobs=5)
     computed_blocks, num_computed_tokens = manager.get_computed_blocks(req0)
-    assert len(manager.req_to_block_hashes[req0.request_id]) == 3
+    assert len(manager.req_to_block_hashes[req0.request_id]) == 0
     assert not computed_blocks.blocks
     assert num_computed_tokens == 0
     blocks = manager.allocate_slots(req0, 55, computed_blocks)
@@ -256,7 +256,7 @@ def test_prefill_plp():
                         common_token_ids + unique_token_ids,
                         prompt_logprobs=5)
     computed_blocks, num_computed_tokens = manager.get_computed_blocks(req2)
-    assert len(manager.req_to_block_hashes[req2.request_id]) == 3
+    assert len(manager.req_to_block_hashes[req2.request_id]) == 0
     assert not computed_blocks.blocks
     assert num_computed_tokens == 0
     blocks = manager.allocate_slots(req2, 55, computed_blocks)

vllm/v1/core/kv_cache_manager.py

@@ -126,8 +126,11 @@ class KVCacheManager:
                 - A list of blocks that are computed for the request.
                 - The number of computed tokens.
         """
-        if not self.enable_caching:
-            # Prefix caching is disabled.
+
+        # Prefix caching is disabled or
+        # When the request requires prompt logprobs, we skip prefix caching.
+        if (not self.enable_caching
+                or request.sampling_params.prompt_logprobs is not None):
             return KVCacheBlocks.create_empty(), 0
 
         # The block hashes for the request may already be computed
@@ -141,9 +144,6 @@ class KVCacheManager:
         if self.log_stats:
             assert self.prefix_cache_stats is not None
             self.prefix_cache_stats.requests += 1
-        # When the request requires prompt logprobs, we skip prefix caching.
-        if request.sampling_params.prompt_logprobs is not None:
-            return KVCacheBlocks.create_empty(), 0
 
         if len(block_hashes) * self.block_size == request.num_tokens:
             # When prompt length is divisible by the block size and all