Fix MiniMax MSA fallback when fmha plan is unavailable (#29250)

python/sglang/srt/layers/attention/minimax_sparse_ops/minimax_sparse.py

@@ -1,5 +1,6 @@
 # Copyright 2025 XunhaoLai. All rights reserved.
 
+import logging
 from typing import Callable, List, Optional, Tuple
 
 import torch
@@ -11,6 +12,20 @@ from .decode.topk_sparse import flash_decode_with_gqa_share_sparse
 from .prefill.flash_with_topk_idx import flash_prefill_with_topk_index
 from .prefill.topk_sparse import flash_prefill_with_gqa_share_sparse
 
+logger = logging.getLogger(__name__)
+_msa_fallback_warned = False
+
+
+def _warn_msa_fallback(err: Exception) -> None:
+    global _msa_fallback_warned
+    if _msa_fallback_warned:
+        return
+    logger.warning(
+        "MiniMax MSA backend is unavailable (%s); falling back to Triton sparse attention.",
+        err,
+    )
+    _msa_fallback_warned = True
+
 
 def minimax_sparse_prefill(
     q: torch.Tensor,  # [total_extend_tokens, num_q_heads, qk_head_dim]
@@ -96,8 +111,9 @@ def minimax_sparse_prefill(
     # replaces this step; the indexer above is unchanged. MSA has no attn-sink
     # input, so keep the Triton path when sink is present.
     if use_msa and sink is None:
-        from .msa import msa_sparse_prefill_main
+        from .msa import MSAUnavailableError, msa_sparse_prefill_main
 
+        try:
             o = msa_sparse_prefill_main(
                 q=q,
                 k_cache=k_cache,
@@ -111,6 +127,26 @@ def minimax_sparse_prefill(
                 block_size_k=block_size_k,
                 sm_scale=sm_scale,
             )
+        except MSAUnavailableError as err:
+            _warn_msa_fallback(err)
+            o = flash_prefill_with_gqa_share_sparse(
+                q=q,
+                k_cache=k_cache,
+                v_cache=v_cache,
+                sink=sink,
+                req_to_token=req_to_token,
+                slot_ids=slot_ids,
+                topk_idx=topk_idx,
+                block_size_q=block_size_q,
+                block_size_k=block_size_k,
+                cu_seqlens=cu_seqlens,
+                seq_lens=seq_lens,
+                prefix_lens=prefix_lens,
+                max_seqlen_q=max_seqlen_q,
+                sm_scale=sm_scale,
+                cu_seqblocks_q=cu_seqblocks_q,
+                max_seqblock_q=max_seqblock_q,
+            )
     else:
         o = flash_prefill_with_gqa_share_sparse(
             q=q,
@@ -203,8 +239,9 @@ def minimax_sparse_decode(
         # Step 3: Sparse attention using topk index (main head). The MSA path
         # only replaces this step; keep the Triton path when sink is present.
         if use_msa and sink is None:
-            from .msa import msa_sparse_decode_main
+            from .msa import MSAUnavailableError, msa_sparse_decode_main
 
+            try:
                 o = msa_sparse_decode_main(
                     q=q,
                     k_cache=k_cache,
@@ -218,6 +255,20 @@ def minimax_sparse_decode(
                     kv_indices=msa_kv_indices,
                     plan=msa_plan,
                 )
+            except MSAUnavailableError as err:
+                _warn_msa_fallback(err)
+                o = flash_decode_with_gqa_share_sparse(
+                    q=q,
+                    sink=sink,
+                    k_cache=k_cache,
+                    v_cache=v_cache,
+                    req_to_token=req_to_token,
+                    seq_lens=seq_lens,
+                    slot_ids=slot_ids,
+                    block_size=block_size_k,
+                    topk_idx=topk_idx,
+                    sm_scale=sm_scale,
+                )
         else:
             o = flash_decode_with_gqa_share_sparse(
                 q=q,

python/sglang/srt/layers/attention/minimax_sparse_ops/msa.py

@@ -12,9 +12,34 @@ from typing import Optional
 import torch
 
 
+class MSAUnavailableError(RuntimeError):
+    """Raised when fmha_sm100 cannot serve the MiniMax MSA path."""
+
+
+@functools.lru_cache(maxsize=1)
+def _load_fmha_sm100():
+    try:
+        from fmha_sm100 import fmha_sm100, fmha_sm100_plan
+    except Exception as err:
+        raise MSAUnavailableError(
+            "fmha_sm100 or fmha_sm100_plan is not importable"
+        ) from err
+    if not callable(fmha_sm100) or not callable(fmha_sm100_plan):
+        raise MSAUnavailableError("fmha_sm100 exports must be callable")
+    return fmha_sm100, fmha_sm100_plan
+
+
+def _run_fmha_sm100_plan(*args, **kwargs):
+    _, fmha_sm100_plan = _load_fmha_sm100()
+    try:
+        return fmha_sm100_plan(*args, **kwargs)
+    except (AttributeError, RuntimeError, TypeError) as err:
+        raise MSAUnavailableError("fmha_sm100_plan failed") from err
+
+
 @functools.lru_cache(maxsize=1)
 def msa_available() -> bool:
-    """True iff the fmha_sm100 sparse kernels are importable on this device."""
+    """True iff the fmha_sm100 sparse kernels and plan API are usable here."""
     try:
         cap = torch.cuda.get_device_capability()
     except Exception:
@@ -24,10 +49,9 @@ def msa_available() -> bool:
     if cap[0] != 10 or cap[1] not in (0, 3):
         return False
     try:
-        import fmha_sm100  # noqa: F401
+        _load_fmha_sm100()
-
         return True
-    except Exception:
+    except MSAUnavailableError:
         return False
 
 
@@ -82,7 +106,7 @@ def msa_sparse_prefill_main(
 
     Returns o [total_q, num_q_heads, head_dim].
     """
-    from fmha_sm100 import fmha_sm100, fmha_sm100_plan
+    fmha_sm100, _ = _load_fmha_sm100()
 
     max_slots, num_kv_heads, head_dim = k_cache.shape
     num_q_heads = q.shape[1]
@@ -105,7 +129,7 @@ def msa_sparse_prefill_main(
     # topk_idx [Hkv, total_q, topk] -> kv_block_indexes [total_q, Hkv, topk].
     kv_block_indexes = topk_idx.permute(1, 0, 2).contiguous().to(torch.int32)
 
-    plan = fmha_sm100_plan(
+    plan = _run_fmha_sm100_plan(
         qo_segment_lens,
         seq_lens.to(torch.int32),
         num_q_heads,
@@ -147,8 +171,6 @@ def build_msa_decode_meta(
     Used only by the standalone parity harnesses; the serving backend builds eager-decode
     metadata via ``build_msa_decode_cg_plan`` + ``update_msa_decode_cg_meta`` instead.
     """
-    from fmha_sm100 import fmha_sm100_plan
-
     max_slots, num_kv_heads, _ = k_cache.shape
     P = block_size_k
     if max_slots % P != 0:
@@ -156,7 +178,7 @@ def build_msa_decode_meta(
     B = slot_ids.shape[0]
     kv_indices = _build_page_table(req_to_token, slot_ids, seq_lens, P)
     seq_lens_i32 = seq_lens.to(torch.int32)
-    plan = fmha_sm100_plan(
+    plan = _run_fmha_sm100_plan(
         torch.ones(B, dtype=torch.int32),
         seq_lens_i32,
         num_q_heads,
@@ -199,14 +221,14 @@ def _check_cg_plan_layout(plan) -> None:
     assumes (dict at tuple index 3 holding the four length tensors) — these are
     undocumented fmha_sm100 internals."""
     if not (isinstance(plan, tuple) and len(plan) > 3 and isinstance(plan[3], dict)):
-        raise RuntimeError(
+        raise MSAUnavailableError(
             "fmha_sm100_plan no longer returns a tuple with a metadata dict at index 3; "
             "the MSA CUDA-graph decode path must be revalidated against this fmha_sm100 "
             "version. Set SGLANG_DISABLE_MSA=1 to use the Triton path meanwhile."
         )
     missing = [k for k in _MSA_CG_LEN_KEYS if not torch.is_tensor(plan[3].get(k))]
     if missing:
-        raise RuntimeError(
+        raise MSAUnavailableError(
             f"fmha_sm100 plan is missing length tensors {missing}; the MSA CUDA-graph "
             "decode path must be revalidated against this fmha_sm100 version. "
             "Set SGLANG_DISABLE_MSA=1 to use the Triton path meanwhile."
@@ -228,13 +250,11 @@ def build_msa_decode_cg_plan(
     every topk block valid; the length-dependent tensors are overwritten each step by
     ``update_msa_decode_cg_meta``. Returns the plan tuple to pass to ``fmha_sm100``.
     """
-    from fmha_sm100 import fmha_sm100_plan
-
     P = block_size_k
     ref_len = topk * P  # length at which all topk blocks exist -> full worklist
     qo = torch.ones(batch_size, dtype=torch.int32)
     kv = torch.full((batch_size,), ref_len, dtype=torch.int32)
-    plan = fmha_sm100_plan(
+    plan = _run_fmha_sm100_plan(
         qo,
         kv,
         num_q_heads,
@@ -266,8 +286,6 @@ def update_msa_decode_cg_meta(
     capture — i.e. only from ``init_forward_metadata_out_graph``. The captured graph then
     reads the same plan-tensor and ``kv_indices_buf`` addresses on replay.
     """
-    from fmha_sm100 import fmha_sm100_plan
-
     P = block_size_k
     B = seq_lens.shape[0]
     seq_lens_i32 = seq_lens.to(torch.int32)
@@ -276,7 +294,7 @@ def update_msa_decode_cg_meta(
     # worklist is identical to the persistent one (topk*P based) and is discarded.
     # qo_offset is clamped: graph replay pads the batch with seq_len==0 slots
     # (masked via kv_segment_lens==0, but seq_len-1 would be -1).
-    fresh = fmha_sm100_plan(
+    fresh = _run_fmha_sm100_plan(
         torch.ones(B, dtype=torch.int32),
         seq_lens_i32,
         num_q_heads,
@@ -320,7 +338,7 @@ def msa_sparse_decode_main(
     (eager decode only) and passes them in. When omitted (only the standalone parity
     harnesses) they are built here via ``build_msa_decode_meta``.
     """
-    from fmha_sm100 import fmha_sm100
+    fmha_sm100, _ = _load_fmha_sm100()
 
     max_slots, num_kv_heads, head_dim = k_cache.shape
     H = q.shape[1]