[DSV4/SM120] Allow fused MHC opt-in with standalone TileLang pre disabled (#30954)

Signed-off-by: David Orman <ormandj@corenode.com>
Co-authored-by: Mohammad Miadh Angkad <176301910+mmangkad@users.noreply.github.com>

python/sglang/srt/models/deepseek_v4.py

@@ -153,6 +153,7 @@ from sglang.srt.utils import (
     log_info_on_rank0,
     make_layers,
 )
+from sglang.srt.utils.common import is_sm120_supported
 from sglang.srt.utils.custom_op import register_custom_op
 from sglang.srt.utils.hf_transformers_utils import get_rope_config
 
@@ -204,12 +205,13 @@ DEEPSEEK_V4_STACKED_PARAMS_MAPPING: List[Tuple[str, str, int]] = [
 
 
 def _is_fused_mhc_post_pre_enabled() -> bool:
-    # The fused path directly reuses TileLang mhc_post/mhc_pre kernels and their
-    # tensor layout assumptions, so keep it disabled when either dependency is off.
+    # SM120 disables the standalone TileLang pre path. mhc_fused_post_pre does
+    # not read that flag and dispatches independently for both small and large
+    # token batches, so the standalone pre flag must not veto the fused opt-in.
     return (
         envs.SGLANG_OPT_FUSE_MHC_POST_PRE.get()
-        and envs.SGLANG_OPT_USE_TILELANG_MHC_PRE.get()
         and envs.SGLANG_OPT_USE_TILELANG_MHC_POST.get()
+        and (envs.SGLANG_OPT_USE_TILELANG_MHC_PRE.get() or is_sm120_supported())
     )
 
 

test/registered/unit/models/test_deepseek_v4_fused_mhc_policy.py

@@ -0,0 +1,79 @@
+"""Unit tests for the DeepSeek-V4 fused-MHC enable policy."""
+
+import unittest
+from unittest.mock import patch
+
+import sglang.srt.models.deepseek_v4 as deepseek_v4
+from sglang.srt.environ import envs
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=2, suite="base-a-test-cpu")
+
+
+class TestDeepseekV4FusedMHCPolicy(CustomTestCase):
+    def _is_enabled(
+        self,
+        *,
+        fuse: bool,
+        tilelang_pre: bool,
+        tilelang_post: bool,
+        sm120: bool,
+    ) -> bool:
+        with (
+            envs.SGLANG_OPT_FUSE_MHC_POST_PRE.override(fuse),
+            envs.SGLANG_OPT_USE_TILELANG_MHC_PRE.override(tilelang_pre),
+            envs.SGLANG_OPT_USE_TILELANG_MHC_POST.override(tilelang_post),
+            patch.object(deepseek_v4, "is_sm120_supported", return_value=sm120),
+        ):
+            return deepseek_v4._is_fused_mhc_post_pre_enabled()
+
+    def test_sm120_allows_fused_opt_in_with_standalone_pre_disabled(self):
+        self.assertTrue(
+            self._is_enabled(
+                fuse=True,
+                tilelang_pre=False,
+                tilelang_post=True,
+                sm120=True,
+            )
+        )
+
+    def test_other_platform_still_requires_tilelang_pre(self):
+        self.assertFalse(
+            self._is_enabled(
+                fuse=True,
+                tilelang_pre=False,
+                tilelang_post=True,
+                sm120=False,
+            )
+        )
+        self.assertTrue(
+            self._is_enabled(
+                fuse=True,
+                tilelang_pre=True,
+                tilelang_post=True,
+                sm120=False,
+            )
+        )
+
+    def test_fusion_opt_in_and_tilelang_post_remain_required(self):
+        self.assertFalse(
+            self._is_enabled(
+                fuse=False,
+                tilelang_pre=False,
+                tilelang_post=True,
+                sm120=True,
+            )
+        )
+        self.assertFalse(
+            self._is_enabled(
+                fuse=True,
+                tilelang_pre=False,
+                tilelang_post=False,
+                sm120=True,
+            )
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()