[Misc] Take user preference in attention selector (#4960)

tests/kernels/test_attention_selector.py

@@ -0,0 +1,84 @@
+import os
+from unittest.mock import patch
+
+import pytest
+import torch
+
+from vllm.attention.selector import which_attn_to_use
+
+
+@pytest.mark.parametrize(
+    "name", ["TORCH_SDPA", "ROCM_FLASH", "XFORMERS", "FLASHINFER"])
+@pytest.mark.parametrize("device", ["cpu", "hip", "cuda"])
+def test_env(name: str, device: str):
+    """Test that the attention selector can be set via environment variable.
+    Note that we do not test FlashAttn because it is the default backend.
+    """
+    name_backup = os.environ.get("VLLM_ATTENTION_BACKEND", None)
+    os.environ["VLLM_ATTENTION_BACKEND"] = name
+
+    if device == "cpu":
+        with patch("vllm.attention.selector.is_cpu", return_value=True):
+            backend = which_attn_to_use(8, 16, 8, None, torch.float16,
+                                        torch.float16, 16)
+        assert backend.name == "TORCH_SDPA"
+    elif device == "hip":
+        with patch("vllm.attention.selector.is_hip", return_value=True):
+            backend = which_attn_to_use(8, 16, 8, None, torch.float16,
+                                        torch.float16, 16)
+        assert backend.name == "ROCM_FLASH"
+    else:
+        backend = which_attn_to_use(8, 16, 8, None, torch.float16,
+                                    torch.float16, 16)
+        assert backend.name == name
+
+    if name_backup is not None:
+        os.environ["VLLM_ATTENTION_BACKEND"] = name_backup
+
+
+def test_flash_attn():
+    """Test FlashAttn validation."""
+    name_backup = os.environ.get("VLLM_ATTENTION_BACKEND", None)
+    os.environ["VLLM_ATTENTION_BACKEND"] = "FLASH_ATTN"
+
+    # Unsupported CUDA arch
+    with patch("torch.cuda.get_device_capability", return_value=[7, 5]):
+        backend = which_attn_to_use(8, 16, 8, None, torch.float16, None, 16)
+        assert backend.name != "FLASH_ATTN"
+
+    # Unsupported data type
+    backend = which_attn_to_use(8, 16, 8, None, torch.float8_e4m3fn, None, 16)
+    assert backend.name != "FLASH_ATTN"
+
+    # Unsupported kv cache data type
+    backend = which_attn_to_use(8, 16, 8, None, torch.float16, "fp8", 16)
+    assert backend.name != "FLASH_ATTN"
+
+    # Unsupported block size
+    backend = which_attn_to_use(8, 16, 8, None, torch.float16, None, 8)
+    assert backend.name != "FLASH_ATTN"
+
+    # Unsupported sliding window
+    backend = which_attn_to_use(8, 16, 8, 1, torch.float16, None, 16)
+    assert backend.name != "FLASH_ATTN"
+
+    # flash-attn is not installed
+    with patch.dict('sys.modules', {'vllm_flash_attn': None}):
+        backend = which_attn_to_use(8, 16, 8, None, torch.float16, None, 16)
+        assert backend.name != "FLASH_ATTN"
+
+    # Unsupported head size
+    backend = which_attn_to_use(8, 17, 8, None, torch.float16, None, 16)
+    assert backend.name != "FLASH_ATTN"
+
+    if name_backup is not None:
+        os.environ["VLLM_ATTENTION_BACKEND"] = name_backup
+
+
+def test_invalid_env():
+    """Throw an exception if the backend name is invalid."""
+    name_backup = os.environ.get("VLLM_ATTENTION_BACKEND", None)
+    os.environ["VLLM_ATTENTION_BACKEND"] = "INVALID"
+    with pytest.raises(ValueError):
+        which_attn_to_use(8, 16, 8, None, torch.float16, None, 16)
+    os.environ["VLLM_ATTENTION_BACKEND"] = name_backup

vllm/attention/backends/flashinfer.py

@@ -218,6 +218,7 @@ class FlashInferImpl(AttentionImpl):
             )
 
         if prefill_meta := attn_metadata.prefill_metadata:
+            # Prompt run.
             assert prefill_meta.block_tables is not None
             if kv_cache is None or prefill_meta.block_tables.numel() == 0:
                 output = flash_attn_varlen_func(

vllm/attention/selector.py

@@ -30,24 +30,16 @@ def get_attn_backend(
     kv_cache_dtype: Optional[str],
     block_size: int,
 ) -> Type[AttentionBackend]:
-    backend = _which_attn_to_use(num_heads, head_size, num_kv_heads,
+    """Determine which attention backend to use and only import
+    the selected backend module.
+    """
+    backend = which_attn_to_use(num_heads, head_size, num_kv_heads,
                                 sliding_window, dtype, kv_cache_dtype,
                                 block_size)
     if backend == _Backend.FLASH_ATTN:
         from vllm.attention.backends.flash_attn import (  # noqa: F401
             FlashAttentionBackend)
-
-        # We check it here not in _which_attn_to_use because we cannot know
-        # the head size until we import FlashAttentionBackend.
-        supported_head_sizes = FlashAttentionBackend.get_supported_head_sizes()
-        if head_size in supported_head_sizes:
-            logger.info("Using FlashAttention-2 backend.")
         return FlashAttentionBackend
-        logger.info(
-            "Cannot use FlashAttention-2 backend for head size %d. "
-            "Using XFormers backend instead.", head_size)
-        backend = _Backend.XFORMERS
-
     if backend == _Backend.XFORMERS:
         logger.info("Using XFormers backend.")
         from vllm.attention.backends.xformers import (  # noqa: F401
@@ -64,14 +56,15 @@ def get_attn_backend(
         return TorchSDPABackend
     elif backend == _Backend.FLASHINFER:
         logger.info("Using Flashinfer backend.")
-        logger.warning("Eager mode is enforced for the Flashinfer backend.")
+        logger.warning("Eager mode is required for the Flashinfer backend. "
+                       "Please make sure --enforce-eager is set.")
         from vllm.attention.backends.flashinfer import FlashInferBackend
         return FlashInferBackend
     else:
         raise ValueError("Invalid attention backend.")
 
 
-def _which_attn_to_use(
+def which_attn_to_use(
     num_heads: int,
     head_size: int,
     num_kv_heads: int,
@@ -81,54 +74,84 @@ def _which_attn_to_use(
     block_size: int,
 ) -> _Backend:
     """Returns which flash attention backend to use."""
+
+    # Default case.
+    selected_backend = _Backend.FLASH_ATTN
+
+    # Check the environment variable and override if specified
+    backend_by_env_var: Optional[str] = envs.VLLM_ATTENTION_BACKEND
+    if backend_by_env_var is not None:
+        backend_members = _Backend.__members__
+        if backend_by_env_var not in backend_members:
+            raise ValueError(
+                f"Invalid attention backend '{backend_by_env_var}'. "
+                f"Available backends: {', '.join(backend_members)} "
+                "(case-sensitive).")
+        selected_backend = _Backend[backend_by_env_var]
+
     if is_cpu():
+        if selected_backend != _Backend.TORCH_SDPA:
+            logger.info("Cannot use %s backend on CPU.", selected_backend)
         return _Backend.TORCH_SDPA
 
     if is_hip():
         # AMD GPUs.
+        selected_backend = (_Backend.ROCM_FLASH if selected_backend
+                            == _Backend.FLASH_ATTN else selected_backend)
+        if selected_backend == _Backend.ROCM_FLASH:
             if torch.cuda.get_device_capability()[0] != 9:
                 # not Instinct series GPUs.
-            logger.info("flash_atten is not supported on NAVI GPUs.")
+                logger.info("flash_attn is not supported on NAVI GPUs.")
+        else:
+            logger.info("%s is not supported in AMD GPUs.", selected_backend)
         return _Backend.ROCM_FLASH
 
-    # NVIDIA GPUs.
+    # FlashAttn in NVIDIA GPUs.
+    if selected_backend == _Backend.FLASH_ATTN:
         if torch.cuda.get_device_capability()[0] < 8:
             # Volta and Turing NVIDIA GPUs.
-        logger.info("Cannot use FlashAttention-2 backend for Volta and Turing "
+            logger.info(
+                "Cannot use FlashAttention-2 backend for Volta and Turing "
                 "GPUs.")
-        return _Backend.XFORMERS
+            selected_backend = _Backend.XFORMERS
-
+        elif dtype not in (torch.float16, torch.bfloat16):
-    if dtype not in (torch.float16, torch.bfloat16):
+            logger.info(
-        logger.info("Cannot use FlashAttention-2 backend for dtype other than "
+                "Cannot use FlashAttention-2 backend for dtype other than "
                 "torch.float16 or torch.bfloat16.")
-        return _Backend.XFORMERS
+            selected_backend = _Backend.XFORMERS
-
+        elif kv_cache_dtype is not None and kv_cache_dtype.startswith("fp8"):
-    if kv_cache_dtype is not None and kv_cache_dtype.startswith("fp8"):
+            logger.info(
-        logger.info("Cannot use FlashAttention-2 backend for FP8 KV cache.")
+                "Cannot use FlashAttention-2 backend for FP8 KV cache.")
-        return _Backend.XFORMERS
+            selected_backend = _Backend.XFORMERS
-
+        elif block_size % 16 != 0:
-    if block_size % 16 != 0:
+            logger.info(
-        logger.info("Cannot use FlashAttention-2 backend for block size not "
+                "Cannot use FlashAttention-2 backend for block size not "
                 "divisible by 16.")
-        return _Backend.XFORMERS
+            selected_backend = _Backend.XFORMERS
-
+        elif sliding_window is not None:
-    if sliding_window is not None:
             logger.info(
                 "Cannot use FlashAttention-2 backend due to sliding window.")
-        return _Backend.XFORMERS
+            selected_backend = _Backend.XFORMERS
 
+    # FlashAttn is valid for the model, checking if the package is installed.
+    if selected_backend == _Backend.FLASH_ATTN:
         try:
             import vllm_flash_attn  # noqa: F401
+
+            from vllm.attention.backends.flash_attn import (  # noqa: F401
+                FlashAttentionBackend)
+
+            supported_sizes = FlashAttentionBackend.get_supported_head_sizes()
+            if head_size not in supported_sizes:
+                logger.info(
+                    "Cannot use FlashAttention-2 backend for head size %d.",
+                    head_size)
+                selected_backend = _Backend.XFORMERS
         except ImportError:
             logger.info(
-            "Cannot use FlashAttention-2 backend because the vllm_flash_attn "
+                "Cannot use FlashAttention-2 backend because the "
-            "package is not found. `pip install vllm-flash-attn` for better "
+                "vllm_flash_attn package is not found. "
-            "performance.")
+                "`pip install vllm-flash-attn` for better performance.")
-        return _Backend.XFORMERS
+            selected_backend = _Backend.XFORMERS
 
-    backend_by_env_var = envs.VLLM_ATTENTION_BACKEND
+    return selected_backend
-    if backend_by_env_var is not None:
-        return _Backend[backend_by_env_var]
-
-    # Default case.
-    return _Backend.FLASH_ATTN