[Bugfix] [ROCm] [UX] Reorganize ROCm Backend Selection Logic (#26980)

Signed-off-by: vllmellm <vllm.ellm@embeddedllm.com>
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@ -0,0 +1,337 @@
# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
"""Tests for attention backend selectors."""
from unittest.mock import MagicMock, patch
import pytest
import torch
from vllm.attention.backends.registry import AttentionBackendEnum
from vllm.platforms import current_platform
# ROCm-specific attention backend selection tests
pytestmark = pytest.mark.skipif(
not current_platform.is_rocm(), reason="ROCm-specific tests"
)
@pytest.fixture
def mock_vllm_config():
"""Create a mock VllmConfig for testing."""
config = MagicMock()
config.model_config.dtype = torch.float16
config.model_config.hf_config.architectures = ["LlamaForCausalLM"]
config.cache_config.block_size = 16
return config
@pytest.fixture
def mock_on_gfx9():
"""Mock the on_gfx9 function to return True."""
with patch("vllm.platforms.rocm.on_gfx9", return_value=True):
yield
@pytest.mark.parametrize(
"env_vars, selected_backend, expected_backend_path",
[
# Test Case 1: Default (no env vars, no explicit backend)
(
{},
None,
AttentionBackendEnum.TRITON_ATTN.get_path(),
),
# Test Case 2: Explicit TRITON_ATTN backend
(
{},
"TRITON_ATTN",
AttentionBackendEnum.TRITON_ATTN.get_path(),
),
# Test Case 3: Explicit ROCM_ATTN backend
(
{},
"ROCM_ATTN",
AttentionBackendEnum.ROCM_ATTN.get_path(),
),
# Test Case 4: Explicit ROCM_AITER_FA backend
(
{},
"ROCM_AITER_FA",
AttentionBackendEnum.ROCM_AITER_FA.get_path(),
),
# Test Case 5: Explicit ROCM_AITER_UNIFIED_ATTN backend
(
{},
"ROCM_AITER_UNIFIED_ATTN",
AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN.get_path(),
),
# Test Case 6: VLLM_ROCM_USE_AITER=1
# (defaults to AITER FA when MHA not explicitly disabled)
(
{"VLLM_ROCM_USE_AITER": "1"},
None,
AttentionBackendEnum.ROCM_AITER_FA.get_path(),
),
# Test Case 7: VLLM_ROCM_USE_AITER=1 + VLLM_ROCM_USE_AITER_MHA=1
(
{"VLLM_ROCM_USE_AITER": "1", "VLLM_ROCM_USE_AITER_MHA": "1"},
None,
AttentionBackendEnum.ROCM_AITER_FA.get_path(),
),
# Test Case 8: VLLM_ROCM_USE_AITER=1 + VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION=1
(
{
"VLLM_ROCM_USE_AITER": "1",
"VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION": "1",
},
None,
AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN.get_path(),
),
# Test Case 9: VLLM_V1_USE_PREFILL_DECODE_ATTENTION=1
(
{"VLLM_V1_USE_PREFILL_DECODE_ATTENTION": "1"},
None,
AttentionBackendEnum.ROCM_ATTN.get_path(),
),
# Test Case 10: VLLM_ROCM_USE_AITER=1 + explicit TRITON_ATTN
(
{"VLLM_ROCM_USE_AITER": "1"},
"TRITON_ATTN",
AttentionBackendEnum.TRITON_ATTN.get_path(),
),
# Test Case 11: VLLM_ROCM_USE_AITER=1 + VLLM_ROCM_USE_AITER_MHA=0
# (explicitly disabled)
(
{"VLLM_ROCM_USE_AITER": "1", "VLLM_ROCM_USE_AITER_MHA": "0"},
None,
AttentionBackendEnum.TRITON_ATTN.get_path(),
),
# Test Case 12: VLLM_ROCM_USE_AITER=1 + explicit ROCM_ATTN
(
{"VLLM_ROCM_USE_AITER": "1"},
"ROCM_ATTN",
AttentionBackendEnum.ROCM_ATTN.get_path(),
),
],
)
def test_standard_attention_backend_selection(
env_vars,
selected_backend,
expected_backend_path,
mock_vllm_config,
mock_on_gfx9,
monkeypatch,
):
"""Test standard attention backend selection with various configurations."""
# Set environment variables
for key, value in env_vars.items():
monkeypatch.setenv(key, value)
# Import after setting env vars to ensure they're picked up
# Reload envs to pick up new environment variables
import importlib
import vllm.envs as envs
from vllm.attention.backends.registry import _Backend
importlib.reload(envs)
# Convert string backend to enum if provided
backend_enum = None
if selected_backend:
backend_enum = getattr(_Backend, selected_backend)
# Get the backend class path
from vllm.platforms.rocm import RocmPlatform
backend_path = RocmPlatform.get_attn_backend_cls(
selected_backend=backend_enum,
head_size=128,
dtype=torch.float16,
kv_cache_dtype="auto",
block_size=16,
use_mla=False,
has_sink=False,
use_sparse=False,
)
assert backend_path == expected_backend_path
@pytest.mark.parametrize(
"env_vars, selected_backend, block_size, expected_backend_path, should_raise",
[
# Test Case 1: TRITON_MLA with block_size != 1
(
{},
"TRITON_MLA",
16,
AttentionBackendEnum.TRITON_MLA.get_path(),
False,
),
# Test Case 2: TRITON_MLA with block_size == 1 (should raise)
(
{},
"TRITON_MLA",
1,
None,
True,
),
# Test Case 3: ROCM_AITER_MLA with block_size == 1
(
{},
"ROCM_AITER_MLA",
1,
AttentionBackendEnum.ROCM_AITER_MLA.get_path(),
False,
),
# Test Case 4: ROCM_AITER_MLA with block_size != 1 (should raise)
(
{},
"ROCM_AITER_MLA",
16,
AttentionBackendEnum.ROCM_AITER_MLA.get_path(),
False,
),
# Test Case 5: VLLM_ROCM_USE_AITER=1 with block_size == 1
(
{"VLLM_ROCM_USE_AITER": "1"},
None,
1,
AttentionBackendEnum.ROCM_AITER_MLA.get_path(),
False,
),
# Test Case 6: VLLM_ROCM_USE_AITER=1 with block_size == 16
# (should use ROCM_AITER_MLA now, as it supports block_size 16)
(
{"VLLM_ROCM_USE_AITER": "1"},
None,
16,
AttentionBackendEnum.ROCM_AITER_MLA.get_path(),
False,
),
# Test Case 7: VLLM_ROCM_USE_AITER=1 + explicit TRITON_MLA
(
{"VLLM_ROCM_USE_AITER": "1"},
"TRITON_MLA",
16,
AttentionBackendEnum.TRITON_MLA.get_path(),
False,
),
# Test Case 8: Explicit ROCM_AITER_TRITON_MLA
(
{},
"ROCM_AITER_TRITON_MLA",
16,
AttentionBackendEnum.ROCM_AITER_TRITON_MLA.get_path(),
False,
),
],
)
def test_mla_backend_selection(
env_vars,
selected_backend,
block_size,
expected_backend_path,
should_raise,
mock_vllm_config,
monkeypatch,
):
"""Test MLA backend selection with various configurations."""
# Set environment variables
for key, value in env_vars.items():
monkeypatch.setenv(key, value)
# Import after setting env vars
# Reload envs
import importlib
import vllm.envs as envs
from vllm.attention.backends.registry import _Backend
importlib.reload(envs)
# Mock is_aiter_mla_enabled based on env vars and block_size
aiter_enabled = env_vars.get("VLLM_ROCM_USE_AITER") == "1"
mock_rocm_ops = MagicMock()
mock_rocm_ops.is_mla_enabled.return_value = aiter_enabled
mock_aiter_module = MagicMock()
mock_aiter_module.rocm_aiter_ops = mock_rocm_ops
with patch.dict("sys.modules", {"vllm._aiter_ops": mock_aiter_module}):
# Convert string backend to enum if provided
backend_enum = None
if selected_backend:
backend_enum = getattr(_Backend, selected_backend)
from vllm.platforms.rocm import RocmPlatform
if should_raise:
with pytest.raises(ValueError):
RocmPlatform.get_attn_backend_cls(
selected_backend=backend_enum,
head_size=128,
dtype=torch.float16,
kv_cache_dtype="auto",
block_size=block_size,
use_mla=True,
has_sink=False,
use_sparse=False,
)
else:
backend_path = RocmPlatform.get_attn_backend_cls(
selected_backend=backend_enum,
head_size=128,
dtype=torch.float16,
kv_cache_dtype="auto",
block_size=block_size,
use_mla=True,
has_sink=False,
use_sparse=False,
)
assert backend_path == expected_backend_path
def test_aiter_fa_requires_gfx9(mock_vllm_config):
"""Test that ROCM_AITER_FA requires gfx9 architecture."""
from vllm.attention.backends.registry import _Backend
from vllm.platforms.rocm import RocmPlatform
# Mock on_gfx9 to return False
with (
patch("vllm.platforms.rocm.on_gfx9", return_value=False),
pytest.raises(
ValueError,
match="only supported on gfx9",
),
):
RocmPlatform.get_attn_backend_cls(
selected_backend=_Backend.ROCM_AITER_FA,
head_size=128,
dtype=torch.float16,
kv_cache_dtype="auto",
block_size=16,
use_mla=False,
has_sink=False,
use_sparse=False,
)
def test_sparse_not_supported(mock_vllm_config):
"""Test that sparse attention is not supported on ROCm."""
from vllm.platforms.rocm import RocmPlatform
with pytest.raises(
AssertionError, match="Sparse MLA backend on ROCm only supports block size 1"
):
RocmPlatform.get_attn_backend_cls(
selected_backend=None,
head_size=128,
dtype=torch.float16,
kv_cache_dtype="auto",
block_size=16,
use_mla=False,
has_sink=False,
use_sparse=True,
)

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@ -262,30 +262,64 @@ class RocmPlatform(Platform):
f"is not MLA type while requested for MLA backend."
)
if selected_backend == AttentionBackendEnum.FLEX_ATTENTION:
logger.info("Using FlexAttention backend.")
return "vllm.v1.attention.backends.flex_attention.FlexAttentionBackend"
if (
rocm_aiter_ops.is_mha_enabled()
) or selected_backend == AttentionBackendEnum.ROCM_AITER_FA:
logger.info("Using Aiter Flash Attention backend.")
return AttentionBackendEnum.ROCM_AITER_FA.get_path()
if (
rocm_aiter_ops.is_triton_unified_attn_enabled()
) or selected_backend == AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN:
logger.info("Using Aiter Unified Attention backend.")
return AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN.get_path()
if (
envs.VLLM_V1_USE_PREFILL_DECODE_ATTENTION
or selected_backend == AttentionBackendEnum.ROCM_ATTN
):
# rocm specific backend, with aiter and/or
# triton prefix-prefill
logger.info("Using Rocm Attention backend.")
if selected_backend == AttentionBackendEnum.TRITON_ATTN:
logger.info("Using Triton Attention backend on V1 engine.")
return AttentionBackendEnum.TRITON_ATTN.get_path()
if selected_backend == AttentionBackendEnum.ROCM_ATTN:
logger.info("Using Rocm Attention backend on V1 engine.")
return AttentionBackendEnum.ROCM_ATTN.get_path()
# default case, using triton unified attention
logger.info("Using Triton Attention backend.")
return AttentionBackendEnum.TRITON_ATTN.get_path()
if selected_backend == AttentionBackendEnum.ROCM_AITER_FA:
if on_gfx9():
logger.info("Using Aiter Flash Attention backend on V1 engine.")
return AttentionBackendEnum.ROCM_AITER_FA.get_path()
else:
raise ValueError(
f"The selected backend, {selected_backend.name}, "
"is only supported on gfx9 architectures."
)
if selected_backend == AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN:
logger.info("Using Aiter Unified Attention backend on V1 engine.")
return AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN.get_path()
# Handle automatic backend selection based on environment variables
if selected_backend is None:
# Priority 1: Check for AITER Unified Attention (must check before MHA)
if envs.VLLM_ROCM_USE_AITER and envs.VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION:
logger.info("Using Aiter Unified Attention backend on V1 engine.")
return AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN.get_path()
# Priority 2: Check for AITER MHA (Flash Attention)
# Only use if explicitly enabled (not just VLLM_ROCM_USE_AITER=1)
if envs.VLLM_ROCM_USE_AITER and envs.VLLM_ROCM_USE_AITER_MHA and on_gfx9():
logger.info("Using Aiter Flash Attention backend on V1 engine.")
return AttentionBackendEnum.ROCM_AITER_FA.get_path()
# Priority 3: Check for ROCM_ATTN (prefill-decode split)
if envs.VLLM_V1_USE_PREFILL_DECODE_ATTENTION:
logger.info("Using Rocm Attention backend on V1 engine.")
return AttentionBackendEnum.ROCM_ATTN.get_path()
# Priority 4: Check for AITER enabled without specific flags
# This defaults to AITER FA only if MHA is not explicitly disabled
if (
envs.VLLM_ROCM_USE_AITER
and on_gfx9()
and envs.VLLM_ROCM_USE_AITER_MHA is not False
):
logger.info("Using Aiter Flash Attention backend on V1 engine.")
return AttentionBackendEnum.ROCM_AITER_FA.get_path()
# Default: Triton Unified Attention
logger.info("Using Triton Attention backend on V1 engine.")
return AttentionBackendEnum.TRITON_ATTN.get_path()
raise RuntimeError(
"V0 attention backends have been removed. Set VLLM_USE_V1=1 "
"to select a supported backend."
)
@classmethod
def set_device(cls, device: torch.device) -> None: