Updates to Flex + VLLm integration (#21416)

Signed-off-by: drisspg <drisspguessous@gmail.com>

tests/kernels/test_flex_attention.py

@@ -9,12 +9,17 @@ import pytest
 import torch
 from packaging import version
 
-from vllm import SamplingParams
+from tests.v1.attention.utils import (BatchSpec, create_common_attn_metadata,
+                                      create_standard_kv_cache_spec,
+                                      create_vllm_config)
+from vllm.v1.attention.backends.flex_attention import (
+    FlexAttentionMetadataBuilder)
 
-from ..models.utils import check_embeddings_close
+from ..models.utils import check_embeddings_close, check_logprobs_close
 
 TORCH_VERSION = version.parse(torch.__version__)
 MINIMUM_TORCH_VERSION = version.parse("2.7.0")
+DIRECT_BUILD_VERSION = version.parse("2.9.dev0")
 
 
 def set_seed(seed):
@@ -34,22 +39,18 @@ def test_flex_attention_vs_default_backend(vllm_runner, monkeypatch):
     """Test that FlexAttention produces the same outputs as the default backend.
 
     This test compares the outputs from the FlexAttention backend with
-    the default backend, ensuring they are identical when using the same seed.
+    the default backend, ensuring they are similar when using the same seed.
     """
     model_name = "Qwen/Qwen2.5-1.5B-Instruct"
     seed = 42
     max_tokens = 24
+    num_logprobs = 5
     prompts = [
         "Hello, my name is",
         "The president of the United States is",
         "The capital of France is",
     ]
 
-    sampling_params = SamplingParams(temperature=0.0,
-                                     top_p=1.0,
-                                     seed=seed,
-                                     max_tokens=max_tokens)
-
     # Run with flex attention
     with monkeypatch.context() as m:
         m.setenv("VLLM_USE_V1", "1")
@@ -61,7 +62,8 @@ def test_flex_attention_vs_default_backend(vllm_runner, monkeypatch):
                          tensor_parallel_size=1,
                          num_gpu_blocks_override=128,
                          enforce_eager=True) as llm_flex:
-            output_flex = llm_flex.generate(prompts, sampling_params)
+            output_flex = llm_flex.generate_greedy_logprobs(
+                prompts, max_tokens, num_logprobs)
 
     # Run with default backend
     with monkeypatch.context() as m:
@@ -71,20 +73,17 @@ def test_flex_attention_vs_default_backend(vllm_runner, monkeypatch):
                          runner="generate",
                          tensor_parallel_size=1,
                          num_gpu_blocks_override=128,
-                         enforce_eager=True) as llm_default:
-            output_default = llm_default.generate(prompts, sampling_params)
-
-    # Compare outputs from both backends
-    for i, (flex_result,
-            default_result) in enumerate(zip(output_flex, output_default)):
-        prompt = prompts[i]
-        flex_text = flex_result[1][0]
-        default_text = default_result[1][0]
-
-        assert flex_text == default_text, (
-            f"FlexAttention output doesn't match default for: {prompt!r}\n"
-            f"FlexAttention: {flex_text!r}\n"
-            f"Default: {default_text!r}")
+                         enforce_eager=True,
+                         gpu_memory_utilization=0.85) as llm_default:
+            output_default = llm_default.generate_greedy_logprobs(
+                prompts, max_tokens, num_logprobs)
+
+    check_logprobs_close(
+        outputs_0_lst=output_flex,
+        outputs_1_lst=output_default,
+        name_0="flex",
+        name_1="default",
+    )
 
 
 @pytest.mark.skipif(
@@ -136,5 +135,70 @@ def test_encoder_flex_attention_vs_default_backend(vllm_runner, monkeypatch):
     )
 
 
+@pytest.mark.skipif(
+    not torch.cuda.is_available() or TORCH_VERSION < DIRECT_BUILD_VERSION,
+    reason="CUDA not available or PyTorch version < 2.7",
+)
+def test_block_mask_direct_vs_slow_path():
+    """Test that direct path block mask is a superset of slow path.
+
+    The direct path may include extra blocks for performance (over-estimation),
+    but must include all blocks that the slow path determines are necessary.
+    """
+    device = torch.device("cuda")
+
+    vllm_config = create_vllm_config(model_name="meta-llama/Meta-Llama-3-8B",
+                                     block_size=16,
+                                     max_model_len=1024)
+    kv_cache_spec = create_standard_kv_cache_spec(vllm_config)
+
+    # Use a mixed batch that will create groups spanning multiple sequences
+    batch_spec = BatchSpec(seq_lens=[35, 64, 128, 256],
+                           query_lens=[33, 5, 32, 64],
+                           name="test_mixed_batch")
+
+    common_attn_metadata = create_common_attn_metadata(
+        batch_spec, vllm_config.cache_config.block_size, device)
+
+    builder = FlexAttentionMetadataBuilder(kv_cache_spec, [], vllm_config,
+                                           device)
+
+    metadata_direct = builder.build(common_prefix_len=0,
+                                    common_attn_metadata=common_attn_metadata)
+    builder.direct_build = False
+    metadata_slow = builder.build(common_prefix_len=0,
+                                  common_attn_metadata=common_attn_metadata)
+
+    assert metadata_direct.block_mask is not None
+    assert metadata_slow.block_mask is not None
+
+    # Extract block indices for comparison, B, H are the same
+    direct_indices = metadata_direct.block_mask.kv_indices[0, 0]
+    slow_indices = metadata_slow.block_mask.kv_indices[0, 0]
+    direct_num = metadata_direct.block_mask.kv_num_blocks[0, 0]
+    slow_num = metadata_slow.block_mask.kv_num_blocks[0, 0]
+
+    # main test: every block needed by slow path must be in direct path
+    num_groups = direct_num.shape[0]
+    all_contained = True
+    missing_details = []
+
+    for group_idx in range(num_groups):
+        direct_blocks = set(
+            direct_indices[group_idx, :direct_num[group_idx]].tolist())
+        slow_blocks = set(
+            slow_indices[group_idx, :slow_num[group_idx]].tolist())
+
+        missing_blocks = slow_blocks - direct_blocks
+        if missing_blocks:
+            all_contained = False
+            missing_details.append(
+                f"Group {group_idx}: missing {sorted(missing_blocks)}")
+
+    assert all_contained, (
+        "Direct path is missing blocks required by slow path:\n" +
+        "\n".join(missing_details))
+
+
 if __name__ == "__main__":
     pytest.main([__file__])

tests/v1/attention/test_attention_backends.py

@@ -10,14 +10,15 @@ from tests.v1.attention.utils import (BatchSpec, _Backend,
                                       create_standard_kv_cache_spec,
                                       create_vllm_config,
                                       get_attention_backend)
-from vllm.utils import STR_DTYPE_TO_TORCH_DTYPE, cdiv
+from vllm.utils import STR_DTYPE_TO_TORCH_DTYPE, cdiv, is_torch_equal_or_newer
 from vllm.v1.attention.backends.utils import (CommonAttentionMetadata,
                                               set_kv_cache_layout)
 from vllm.v1.kv_cache_interface import FullAttentionSpec
 
 BACKENDS_TO_TEST = [
     _Backend.FLASH_ATTN_VLLM_V1, _Backend.FLASHINFER_VLLM_V1,
-    _Backend.FLEX_ATTENTION, _Backend.TRITON_ATTN_VLLM_V1, _Backend.TREE_ATTN
+    _Backend.FLEX_ATTENTION, _Backend.TRITON_ATTN_VLLM_V1, _Backend.TREE_ATTN,
+    "FLEX_ATTENTION_SLOW"
 ]
 
 # Remove flashinfer from the list if it's not available
@@ -206,10 +207,18 @@ def run_attention_backend(backend: _Backend, kv_cache_spec: FullAttentionSpec,
                           kv_cache: torch.Tensor) -> torch.Tensor:
     """Run attention computation using the specified backend's AttentionImpl."""
 
-    builder_cls, impl_cls = get_attention_backend(backend)
+    # Handle special case for FLEX_ATTENTION_SLOW
+    actual_backend = backend
+
+    use_direct_block_mask = is_torch_equal_or_newer("2.9.0.dev0")
+    if backend == "FLEX_ATTENTION_SLOW":
+        actual_backend = _Backend.FLEX_ATTENTION
+        use_direct_block_mask = False
+
+    builder_cls, impl_cls = get_attention_backend(actual_backend)
 
     # Mock flashinfer's get_per_layer_parameters if needed
-    if backend == _Backend.FLASHINFER_VLLM_V1:
+    if actual_backend == _Backend.FLASHINFER_VLLM_V1:
         import unittest.mock
 
         from vllm.v1.attention.backends.utils import PerLayerParameters
@@ -239,6 +248,8 @@ def run_attention_backend(backend: _Backend, kv_cache_spec: FullAttentionSpec,
     else:
         # Build metadata
         builder = builder_cls(kv_cache_spec, layer_names, vllm_config, device)
+        if actual_backend == _Backend.FLEX_ATTENTION:
+            builder.direct_build = use_direct_block_mask
         attn_metadata = builder.build(
             common_prefix_len=0,
             common_attn_metadata=common_attn_metadata,
@@ -453,11 +464,6 @@ def test_backend_correctness(batch_spec_name: str, model: str):
         rtol = 1e-2
         atol = 5e-3
 
-        if backend_name == _Backend.FLEX_ATTENTION:
-            atol = 5e-1  # TODO: figure out why flex_attention has such large
-            # numerical differences for medium_decode, medium_prefill,
-            # mixed_medium
-
         max_diff = torch.max(torch.abs(backend_output - sdpa_output)).item()
         max_rel_diff = torch.max(
             torch.abs(backend_output - sdpa_output) /