[Hybrid] Pass kernel block size to builders (#27753)

Signed-off-by: Thomas Parnell <tpa@zurich.ibm.com>

vllm/v1/attention/backends/flash_attn.py

@@ -62,7 +62,11 @@ class FlashAttentionBackend(AttentionBackend):
 
     @staticmethod
     def get_supported_kernel_block_size() -> list[int | MultipleOf]:
-        return [MultipleOf(16)]
+        # NOTE(tdoublep): while in principle, FA supports
+        # MultipleOf(16), these are the block sizes that do not
+        # suffer from the NaN propagation problem described here:
+        # https://github.com/Dao-AILab/flash-attention/issues/1974
+        return [16, 32, 64]
 
     @classmethod
     def validate_head_size(cls, head_size: int) -> None:

vllm/v1/kv_cache_interface.py

@@ -2,7 +2,7 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
 import copy
-from dataclasses import dataclass, fields
+from dataclasses import dataclass, fields, replace
 from math import prod
 
 import torch
@@ -44,6 +44,12 @@ class KVCacheSpec:
         """
         raise NotImplementedError
 
+    def copy_with_new_block_size(self, block_size: int) -> Self:
+        """
+        Create a new KVCacheSpec from self but replacing the block size.
+        """
+        return replace(self, block_size=block_size)
+
     @classmethod
     def merge(cls, specs: list[Self]) -> Self:
         """

vllm/v1/worker/gpu_model_runner.py

@@ -4039,16 +4039,11 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         ) -> list[AttentionGroup]:
             attn_groups: list[AttentionGroup] = []
             for (attn_backend, kv_cache_spec), layer_names in attn_backends_map.items():
-                attn_group = AttentionGroup.create_with_metadata_builders(
+                attn_group = AttentionGroup(
                     attn_backend,
                     layer_names,
                     kv_cache_spec,
-                    self.vllm_config,
-                    self.device,
                     kv_cache_group_id,
-                    num_metadata_builders=1
-                    if not self.parallel_config.enable_dbo
-                    else 2,
                 )
 
                 attn_groups.append(attn_group)
@@ -4067,7 +4062,27 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         for i, attn_backend_map in enumerate(attention_backend_maps):
             self.attn_groups.append(create_attn_groups(attn_backend_map, i))
 
+    def initialize_metadata_builders(
+        self, kv_cache_config: KVCacheConfig, kernel_block_sizes: list[int]
+    ) -> None:
+        """
+        Create the metadata builders for all KV cache groups and attn groups.
+        """
+        for kv_cache_group_id in range(len(kv_cache_config.kv_cache_groups)):
+            for attn_group in self.attn_groups[kv_cache_group_id]:
+                attn_group.create_metadata_builders(
+                    self.vllm_config,
+                    self.device,
+                    kernel_block_sizes[kv_cache_group_id]
+                    if kv_cache_group_id < len(kernel_block_sizes)
+                    else None,
+                    num_metadata_builders=1
+                    if not self.parallel_config.enable_dbo
+                    else 2,
+                )
         # Calculate reorder batch threshold (if needed)
+        # Note (tdoublep): do this *after* constructing builders,
+        # because some of them change the threshold at init time.
         self.calculate_reorder_batch_threshold()
 
     def _check_and_update_cudagraph_mode(
@@ -4633,6 +4648,10 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         # kernel_block_size 64 and split the 256-token-block to 4 blocks with 64
         # tokens each.
         kernel_block_sizes = self._prepare_kernel_block_sizes(kv_cache_config)
+
+        # create metadata builders
+        self.initialize_metadata_builders(kv_cache_config, kernel_block_sizes)
+
         # Reinitialize need to after initialize_attn_backend
         self.may_reinitialize_input_batch(kv_cache_config, kernel_block_sizes)
         kv_caches = self.initialize_kv_cache_tensors(

vllm/v1/worker/utils.py

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 from collections import defaultdict
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import TYPE_CHECKING
 
 import torch
@@ -134,31 +134,37 @@ class MultiModalBudget:
 @dataclass
 class AttentionGroup:
     backend: type[AttentionBackend]
-    # When ubatching is enabled we will have a metadata builder for each ubatch
-    # so that if they use internal persistant buffers for cudagraphs, and they
-    # won't have to worry about conflicting with the other ubatches.
-    metadata_builders: list[AttentionMetadataBuilder]
     layer_names: list[str]
     kv_cache_spec: KVCacheSpec
     kv_cache_group_id: int
+    # When ubatching is enabled we will have a metadata builder for each ubatch
+    # so that if they use internal persistant buffers for cudagraphs, and they
+    # won't have to worry about conflicting with the other ubatches.
+    metadata_builders: list[AttentionMetadataBuilder] = field(
+        default_factory=lambda: []
+    )
 
-    @staticmethod
-    def create_with_metadata_builders(
-        backend: type[AttentionBackend],
-        layer_names: list[str],
-        kv_cache_spec: KVCacheSpec,
-        vllm_config: VllmConfig,
-        device: torch.device,
-        kv_cache_group_id: int,
+    def create_metadata_builders(
+        self,
+        vllm_config,
+        device,
+        kernel_block_size: int | None,
         num_metadata_builders: int = 1,
-    ) -> "AttentionGroup":
-        metadata_builders = [
-            backend.get_builder_cls()(kv_cache_spec, layer_names, vllm_config, device)
+    ):
+        kv_cache_spec_builder = (
+            self.kv_cache_spec.copy_with_new_block_size(kernel_block_size)
+            if kernel_block_size is not None
+            else self.kv_cache_spec
+        )
+        self.metadata_builders = [
+            self.backend.get_builder_cls()(
+                kv_cache_spec_builder,
+                self.layer_names,
+                vllm_config,
+                device,
+            )
             for _ in range(num_metadata_builders)
         ]
-        return AttentionGroup(
-            backend, metadata_builders, layer_names, kv_cache_spec, kv_cache_group_id
-        )
 
     def get_metadata_builder(self, ubatch_id: int = 0) -> AttentionMetadataBuilder:
         assert len(self.metadata_builders) > ubatch_id