[Bugfix][ROCm][MoE] Fall back instead of crashing when AITER MoE is requested for a non-gated (is_act_and_mul=False) model (#56590)

Signed-off-by: Shantipriya Parida <shantipriya.parida@amd.com>

tests/kernels/moe/test_unquantized_backend_selection.py

@@ -6,6 +6,7 @@ import pytest
 import torch
 
 from tests.kernels.moe.utils import make_dummy_moe_config
+from vllm.model_executor.layers.fused_moe.activation import MoEActivation
 from vllm.model_executor.layers.fused_moe.config import RoutingMethodType
 from vllm.model_executor.layers.fused_moe.oracle.unquantized import (
     UnquantizedMoeBackend,
@@ -228,6 +229,65 @@ def test_select_rocm_aiter_backend(mock_aiter_enabled, mock_has_flashinfer):
         assert expert_cls is not None
 
 
+@patch(
+    "vllm.utils.flashinfer.has_flashinfer",
+    return_value=False,
+)
+@patch(
+    "vllm.model_executor.layers.fused_moe.oracle.unquantized.rocm_aiter_ops."
+    "is_fused_moe_enabled",
+    return_value=True,
+)
+@patch(
+    "vllm.model_executor.layers.fused_moe.oracle.unquantized.rocm_aiter_ops."
+    "is_rdna_aiter_enabled",
+    return_value=False,
+)
+@pytest.mark.skipif(
+    not current_platform.is_rocm(), reason="ROCm-specific backend selection test"
+)
+def test_select_rocm_aiter_backend_non_gated_activation_falls_back(
+    mock_rdna_disabled, mock_aiter_enabled, mock_has_flashinfer, monkeypatch
+):
+    """Test ROCm backend selection falls back (not raises) for non-gated MoE."""
+    monkeypatch.setenv("VLLM_ROCM_USE_AITER", "1")
+    monkeypatch.setenv("VLLM_ROCM_USE_AITER_MOE", "1")
+
+    with patch(
+        "vllm.model_executor.layers.fused_moe.oracle.unquantized.current_platform"
+    ) as mock_platform:
+        mock_platform.is_cuda.return_value = False
+        mock_platform.is_rocm.return_value = True
+        mock_platform.is_cpu.return_value = False
+        mock_platform.is_xpu.return_value = False
+        mock_platform.is_tpu.return_value = False
+        mock_platform.is_out_of_tree.return_value = False
+
+        moe_config = make_dummy_moe_config(activation=MoEActivation.SILU_NO_MUL)
+        assert moe_config.is_act_and_mul is False
+
+        selected_backend, expert_cls = select_unquantized_moe_backend(
+            moe_config=moe_config,
+        )
+
+        assert selected_backend != UnquantizedMoeBackend.AITER
+        assert expert_cls is not None
+
+
+@pytest.mark.skipif(
+    not current_platform.is_rocm(), reason="ROCm-specific backend selection test"
+)
+def test_explicit_aiter_backend_non_gated_activation_still_raises():
+    """Explicit `--moe-backend aiter` still raises for non-gated MoE;
+    only the env-var opt-in path falls back."""
+    moe_config = make_dummy_moe_config(activation=MoEActivation.SILU_NO_MUL)
+    moe_config.moe_backend = "aiter"  # explicit pin, not "auto"
+    assert moe_config.is_act_and_mul is False
+
+    with pytest.raises(ValueError):
+        select_unquantized_moe_backend(moe_config=moe_config)
+
+
 @patch(
     "vllm.model_executor.layers.fused_moe.experts.trtllm_bf16_moe.TrtLlmBf16ExpertsMonolithic.is_supported_config",
     return_value=(True, None),

vllm/model_executor/layers/fused_moe/oracle/unquantized.py

@@ -305,7 +305,33 @@ def select_unquantized_moe_backend(
                 AVAILABLE_BACKENDS.remove(UnquantizedMoeBackend.AITER)
         else:
             backend = UnquantizedMoeBackend.AITER
-            return _return_or_raise(backend, moe_config, activation_format)
+            reason = None
+            for k_cls in backend_to_kernel_cls(backend):
+                supported, reason = k_cls.is_supported_config(
+                    k_cls, moe_config, None, None, activation_format
+                )
+                if supported:
+                    logger.info_once(_make_log_backend(backend))
+                    return backend, k_cls
+            # AITER was explicitly requested but does not support this
+            # deployment configuration (e.g. a non-gated MoE activation,
+            # which AiterExperts._supports_no_act_and_mul() rejects
+            # unconditionally). Rather than hard-crashing at engine init,
+            # warn — explicitly stating that this is falling back, and
+            # which backends will be tried next — and fall back to the
+            # remaining backends in priority order below.
+            logger.warning_once(
+                "VLLM_ROCM_USE_AITER_MOE=1 was requested, but %s "
+                "Falling back to try the remaining available MoE backends: %s.",
+                _make_log_unsupported(backend, reason),
+                ", ".join(
+                    b.value
+                    for b in AVAILABLE_BACKENDS
+                    if b != UnquantizedMoeBackend.AITER
+                ),
+            )
+            if UnquantizedMoeBackend.AITER in AVAILABLE_BACKENDS:
+                AVAILABLE_BACKENDS.remove(UnquantizedMoeBackend.AITER)
 
     for backend in AVAILABLE_BACKENDS:
         for k_cls in backend_to_kernel_cls(backend):