[Bugfix] Make Attention Backend Auto-Selection Batch-Invariance-Aware (#40193)

Signed-off-by: Srreyansh Sethi <srreyansh.sethi@gmail.com>
Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>
Co-authored-by: Matthew Bonanni <mbonanni@redhat.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

examples/rl/rlhf_async_new_apis.py

@@ -131,16 +131,9 @@ class TrainModel:
         from vllm.model_executor.layers.batch_invariant import (
             init_batch_invariance,
         )
-        from vllm.platforms import current_platform
-        from vllm.v1.attention.backends.registry import AttentionBackendEnum
 
         # need to init all env vars for batch invariance which affect nccl ops
-        attn_backend = (
-            AttentionBackendEnum.TRITON_ATTN
-            if current_platform.is_rocm()
-            else AttentionBackendEnum.FLASH_ATTN
-        )
-        init_batch_invariance(attn_backend)
+        init_batch_invariance()
 
         self.model = AutoModelForCausalLM.from_pretrained(
             model_name, dtype=torch.bfloat16

vllm/model_executor/layers/batch_invariant.py

@@ -14,7 +14,6 @@ from vllm.triton_utils import tl, triton
 from vllm.utils.mem_utils import get_max_shared_memory_bytes
 from vllm.utils.platform_utils import num_compute_units
 from vllm.utils.torch_utils import is_torch_equal_or_newer
-from vllm.v1.attention.backends.registry import AttentionBackendEnum
 
 logger = init_logger(__name__)
 
@@ -991,40 +990,7 @@ def enable_batch_invariant_mode():
     torch.backends.cuda.preferred_blas_library(backend="cublaslt")
 
 
-def override_envs_for_invariance(
-    attention_backend: AttentionBackendEnum | None,
-):
-    decode_invariant_backends = [
-        AttentionBackendEnum.FLASH_ATTN,  # best supported backend
-        AttentionBackendEnum.TRITON_ATTN,
-    ]
-    supported_backends = decode_invariant_backends + [
-        # FlashInfer temporarily disabled due to invariant CTA sizes.
-        # See FlashInfer issue #2424
-        # AttentionBackendEnum.FLASHINFER,
-        AttentionBackendEnum.FLASH_ATTN_MLA,
-        AttentionBackendEnum.TRITON_MLA,
-        # Not yet supported MLA backends
-        # AttentionBackendEnum.FLASHMLA,
-        # AttentionBackendEnum.FLEX_ATTENTION,  # IMA issue
-        # AttentionBackendEnum.FLASHINFER_MLA,  # PR #28967
-    ]
-    if attention_backend not in supported_backends:
-        supported_names = [b.name for b in supported_backends]
-        backend_name = attention_backend.name if attention_backend else None
-        error = (
-            "VLLM batch_invariant mode requires an attention backend in "
-            f"{supported_names}, but got '{backend_name}'. "
-            "Please use --attention-backend or attention_config to set "
-            "one of the supported backends before enabling batch_invariant."
-        )
-        raise RuntimeError(error)
-    if attention_backend not in decode_invariant_backends:
-        warning = (
-            "You are using a non-decode-invariant form of batch invariance. "
-            "This will not be invariant between prefill and decode."
-        )
-        logger.warning_once(warning)
+def override_envs_for_invariance():
     os.environ["VLLM_ALLREDUCE_USE_SYMM_MEM"] = "0"
 
     os.environ["CUBLAS_WORKSPACE_CONFIG"] = ":4096:8"
@@ -1045,12 +1011,10 @@ def override_envs_for_invariance(
     os.environ["VLLM_USE_AOT_COMPILE"] = "0"
 
 
-def init_batch_invariance(
-    attention_backend: AttentionBackendEnum | None,
-):
+def init_batch_invariance():
     # this will hit all the csrc overrides as well
     if envs.VLLM_BATCH_INVARIANT:
-        override_envs_for_invariance(attention_backend)
+        override_envs_for_invariance()
         enable_batch_invariant_mode()
 
         # Disable TF32 for batch invariance - it causes non-deterministic rounding

vllm/v1/attention/backend.py

@@ -236,6 +236,10 @@ class AttentionBackend(ABC):
         """
         return False
 
+    @classmethod
+    def supports_batch_invariance(cls) -> bool:
+        return False
+
     @classmethod
     def supports_attn_type(cls, attn_type: str) -> bool:
         """Check if backend supports a given attention type.
@@ -278,6 +282,7 @@ class AttentionBackend(ABC):
         device_capability: "DeviceCapability",
         attn_type: str,
         use_non_causal: bool = False,
+        use_batch_invariant: bool = False,
     ) -> list[str]:
         invalid_reasons = []
         if not cls.supports_head_size(head_size):
@@ -312,6 +317,8 @@ class AttentionBackend(ABC):
             invalid_reasons.append(f"attention type {attn_type} not supported")
         if use_non_causal and not cls.supports_non_causal():
             invalid_reasons.append("non-causal attention not supported")
+        if use_batch_invariant and not cls.supports_batch_invariance():
+            invalid_reasons.append("batch invariance not supported")
         combination_reason = cls.supports_combination(
             head_size,
             dtype,

vllm/v1/attention/backends/flash_attn.py

@@ -103,6 +103,10 @@ class FlashAttentionBackend(AttentionBackend):
     def get_name() -> str:
         return "FLASH_ATTN"
 
+    @classmethod
+    def supports_batch_invariance(cls) -> bool:
+        return True
+
     @classmethod
     def supports_non_causal(cls) -> bool:
         return True

vllm/v1/attention/backends/mla/flashattn_mla.py

@@ -56,6 +56,10 @@ class FlashAttnMLABackend(MLACommonBackend):
     def get_name() -> str:
         return "FLASH_ATTN_MLA"
 
+    @classmethod
+    def supports_batch_invariance(cls) -> bool:
+        return True
+
     @staticmethod
     def get_builder_cls() -> type["FlashAttnMLAMetadataBuilder"]:
         return FlashAttnMLAMetadataBuilder

vllm/v1/attention/backends/mla/triton_mla.py

@@ -55,6 +55,10 @@ class TritonMLABackend(MLACommonBackend):
     def get_name() -> str:
         return "TRITON_MLA"
 
+    @classmethod
+    def supports_batch_invariance(cls) -> bool:
+        return True
+
     @staticmethod
     def get_impl_cls() -> type["TritonMLAImpl"]:
         return TritonMLAImpl

vllm/v1/attention/backends/triton_attn.py

@@ -296,6 +296,10 @@ class TritonAttentionBackend(AttentionBackend):
     def get_name() -> str:
         return "TRITON_ATTN"
 
+    @classmethod
+    def supports_batch_invariance(cls) -> bool:
+        return True
+
     @staticmethod
     def get_impl_cls() -> type["TritonAttentionImpl"]:
         return TritonAttentionImpl

vllm/v1/attention/selector.py

@@ -6,6 +6,7 @@ from typing import NamedTuple, cast, get_args
 
 import torch
 
+import vllm.envs as envs
 from vllm.config.cache import CacheDType
 from vllm.logger import init_logger
 from vllm.utils.import_utils import resolve_obj_by_qualname
@@ -30,6 +31,7 @@ class AttentionSelectorConfig(NamedTuple):
     use_per_head_quant_scales: bool = False
     attn_type: str = AttentionType.DECODER
     use_non_causal: bool = False
+    use_batch_invariant: bool = False
 
     def __repr__(self):
         return (
@@ -43,7 +45,8 @@ class AttentionSelectorConfig(NamedTuple):
             f"use_mm_prefix={self.use_mm_prefix}, "
             f"use_per_head_quant_scales={self.use_per_head_quant_scales}, "
             f"attn_type={self.attn_type}, "
-            f"use_non_causal={self.use_non_causal})"
+            f"use_non_causal={self.use_non_causal}, "
+            f"use_batch_invariant={self.use_batch_invariant})"
         )
 
 
@@ -95,6 +98,7 @@ def get_attn_backend(
         use_per_head_quant_scales=use_per_head_quant_scales,
         attn_type=attn_type or AttentionType.DECODER,
         use_non_causal=use_non_causal,
+        use_batch_invariant=envs.VLLM_BATCH_INVARIANT,
     )
 
     return _cached_get_attn_backend(
@@ -162,4 +166,9 @@ def _cached_get_mamba_attn_backend(
         ) from e
 
     mamba_attn_backend = selected_backend.get_class()
+    if envs.VLLM_BATCH_INVARIANT and not mamba_attn_backend.supports_batch_invariance():
+        raise RuntimeError(
+            "VLLM batch_invariant mode is not supported for "
+            f"{mamba_attn_backend.get_name()}."
+        )
     return mamba_attn_backend

vllm/v1/worker/gpu_worker.py

@@ -1027,11 +1027,10 @@ def init_worker_distributed_environment(
     backend: str = "nccl",
 ) -> None:
     """Initialize the distributed environment."""
-    attention_config = vllm_config.attention_config
     parallel_config = vllm_config.parallel_config
     from vllm.model_executor.layers.batch_invariant import init_batch_invariance
 
-    init_batch_invariance(attention_config.backend)
+    init_batch_invariance()
     override_envs_for_eplb(parallel_config)
     set_custom_all_reduce(not parallel_config.disable_custom_all_reduce)