fix(v1/kv_cache): resolve async KV transfer bug in cascade attention (#23485)

Signed-off-by: Ayush Satyam <ayushsatyam146@gmail.com>

Author: ayushsatyam146

vllm/v1/core/kv_cache_coordinator.py

@@ -148,27 +148,22 @@ def free(self, request_id: str) -> None:
         for manager in self.single_type_managers:
             manager.free(request_id)
 
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> list[int]:
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> list[int]:
         """
-        Get the number of common prefix blocks for all requests in the RUNNING
-        state for each kv cache group.
+        Get the number of common prefix blocks for all requests with allocated
+        KV cache for each kv cache group.
 
         Args:
-            request_id: The request ID.
-            num_running_requests: The total number of requests in the RUNNING
-                state.
+            running_request_id: The request ID of any running request, used to
+                identify the common prefix blocks.
 
         Returns:
-            list[int]: The number of common prefix blocks for all requests in
-                the RUNNING state for each kv cache group.
+            list[int]: The number of common prefix blocks for each kv cache group.
         """
-        num_blocks_per_group = [
-            manager.get_num_common_prefix_blocks(request_id, num_running_requests)
+        return [
+            manager.get_num_common_prefix_blocks(running_request_id)
             for manager in self.single_type_managers
         ]
-        return num_blocks_per_group
 
     def remove_skipped_blocks(self, request_id: str, num_computed_tokens: int) -> None:
         """
@@ -226,9 +221,7 @@ def __init__(
         )
         self.num_single_type_manager = len(self.single_type_managers)
 
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> list[int]:
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> list[int]:
         return [0] * self.num_single_type_manager
 
     def find_longest_cache_hit(

vllm/v1/core/kv_cache_manager.py

@@ -10,7 +10,7 @@
 from vllm.v1.core.kv_cache_utils import KVCacheBlock
 from vllm.v1.kv_cache_interface import KVCacheConfig
 from vllm.v1.metrics.stats import PrefixCacheStats
-from vllm.v1.request import Request, RequestStatus
+from vllm.v1.request import Request
 
 logger = init_logger(__name__)
 
@@ -344,49 +344,39 @@ def reset_prefix_cache(self) -> bool:
             self.prefix_cache_stats.reset = True
         return True
 
-    def get_num_common_prefix_blocks(
-        self,
-        request: Request,
-        num_running_requests: int,
-    ) -> list[int]:
-        """Calculate the number of common prefix blocks shared by all requests
-        in the RUNNING state for each kv cache group.
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> list[int]:
+        """Calculate the number of common prefix blocks for each kv cache group.
 
-        The function determines this by selecting any request and iterating
-        through its blocks.  A block is considered a common prefix block if its
-        `ref_cnt` equals the total number of requests in the RUNNING state.
+        The function selects a running request and iterates through its blocks.
+        A block is considered a common prefix block if ALL requests with
+        allocated KV cache share it (i.e., ref_cnt equals the number of entries
+        in req_to_blocks).
 
-        NOTE(woosuk): The number of requests in the RUNNING state is **greater
+        NOTE(woosuk): The number of requests with allocated KV cache is **greater
         than or equal to** the number of requests scheduled in the current step.
-        This is because the RUNNING state only indicates that:
+        This is because having allocated KV cache only indicates that:
         1. The request has not yet finished, and
         2. The request holds its blocks unfreed.
 
-        While all scheduled requests must be in the RUNNING state, the inverse
-        is not necessarily true. There may be RUNNING requests that are not
-        scheduled in the current step.
+        While all scheduled requests must have allocated KV cache, the inverse
+        is not necessarily true. There may be requests with allocated KV cache
+        that are not scheduled in the current step.
 
         This can result in an edge case where the number of common prefix blocks
         is 0, even though all scheduled requests share a common prefix. This
-        occurs because there may be unscheduled RUNNING requests that do not
-        share the common prefix. Currently, this case cannot be easily detected,
-        so the function returns 0 in such cases.
+        occurs because there may be unscheduled requests that do not share the
+        common prefix. Currently, this case cannot be easily detected, so the
+        function returns 0 in such cases.
 
         Args:
-            request: Any request in the RUNNING state, used to identify the
-                common prefix blocks.
-            num_running_requests: The total number of requests in the RUNNING
-                state. This can be different from the number of scheduled
-                requests in the current step.
+            running_request_id: The request ID of any running request, used to
+                identify the common prefix blocks.
 
         Returns:
             list[int]: The number of common prefix blocks for each kv cache
             group.
         """
-        assert request.status == RequestStatus.RUNNING
-        return self.coordinator.get_num_common_prefix_blocks(
-            request.request_id, num_running_requests
-        )
+        return self.coordinator.get_num_common_prefix_blocks(running_request_id)
 
     def take_events(self) -> list[KVCacheEvent]:
         """Take the KV cache events from the block pool.

vllm/v1/core/sched/scheduler.py

@@ -597,7 +597,7 @@ def schedule(self) -> SchedulerOutput:
             any_request = self.running[0]
             num_common_prefix_blocks = (
                 self.kv_cache_manager.get_num_common_prefix_blocks(
-                    any_request, len(self.running)
+                    any_request.request_id
                 )
             )
 

vllm/v1/core/single_type_kv_cache_manager.py

@@ -182,21 +182,17 @@ def free(self, request_id: str) -> None:
         self.num_cached_block.pop(request_id, None)
 
     @abstractmethod
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> int:
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> int:
         """
-        Get the number of common prefix blocks for all requests in the RUNNING
-        state.
+        Get the number of common prefix blocks for all requests with allocated
+        KV cache.
 
         Args:
-            request_id: The request ID.
-            num_running_requests: The total number of requests in the RUNNING
-                state.
+            running_request_id: The request ID.
 
         Returns:
-            The number of common prefix blocks for all requests in the RUNNING
-                state.
+            The number of common prefix blocks for all requests with allocated
+            KV cache.
         """
 
         raise NotImplementedError
@@ -302,13 +298,11 @@ def remove_skipped_blocks(self, request_id: str, num_computed_tokens: int) -> No
         # No need to remove blocks for full attention.
         pass
 
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> int:
-        blocks = self.req_to_blocks[request_id]
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> int:
+        blocks = self.req_to_blocks[running_request_id]
         num_common_blocks = 0
         for block in blocks:
-            if block.ref_cnt == num_running_requests:
+            if block.ref_cnt == len(self.req_to_blocks):
                 num_common_blocks += 1
             else:
                 break
@@ -408,9 +402,7 @@ def remove_skipped_blocks(self, request_id: str, num_computed_tokens: int) -> No
             blocks[i] = self._null_block
         self.block_pool.free_blocks(removed_blocks)
 
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> int:
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> int:
         """
         NOTE(Chen): The prefix blocks are null blocks for sliding window layers.
         So it's not correct to count ref_cnt like FullAttentionManager. Return
@@ -544,9 +536,7 @@ def remove_skipped_blocks(self, request_id: str, num_computed_tokens: int) -> No
             blocks[i] = self._null_block
         self.block_pool.free_blocks(removed_blocks)
 
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> int:
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> int:
         """
         cascade attention is not supported by chunked local attention.
         """
@@ -596,9 +586,7 @@ def remove_skipped_blocks(self, request_id: str, num_computed_tokens: int) -> No
         #  (for which find_longest_cache_hit returns block_pool.null_block)
         pass
 
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> int:
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> int:
         """
         cascade attention is not supported by mamba
         """
@@ -648,9 +636,7 @@ def cache_blocks(self, request: Request, num_tokens: int) -> None:
         # requests, so this method is not relevant.
         raise ValueError("Should not be called as prefix caching is disabled.")
 
-    def get_num_common_prefix_blocks(
-        self, request_id: str, num_running_requests: int
-    ) -> int:
+    def get_num_common_prefix_blocks(self, running_request_id: str) -> int:
         # Cross-attention blocks contain request-specific encoder states
         # and are not shared between different requests
         return 0