[Nemotron] Hoist mamba track-mask host syncs out of the per-layer prefill path (#32589)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

python/sglang/srt/layers/attention/hybrid_linear_attn_backend.py

@@ -99,6 +99,11 @@ class MambaAttnBackendBase(AttentionBackend):
             forward_batch.mamba_track_indices = self._translate_mamba_indices(
                 forward_batch.mamba_track_indices
             )
+        # Resolve the tracked-row selection once per forward
+        has_mamba_track_mask = bool(
+            forward_batch.mamba_track_mask is not None
+            and forward_batch.mamba_track_mask.any()
+        )
         _real_bs = forward_batch._original_batch_size
         if _real_bs is not None and _real_bs < mamba_cache_indices.shape[0]:
             mamba_cache_indices = mamba_cache_indices.clone()
@@ -198,10 +203,7 @@ class MambaAttnBackendBase(AttentionBackend):
                     forward_batch.extend_start_loc[-1]
                     + forward_batch.extend_seq_lens[-1]
                 )
-                if (
-                    forward_batch.mamba_track_mask is not None
-                    and forward_batch.mamba_track_mask.any()
-                ):
+                if has_mamba_track_mask:
                     track_conv_indices = self._init_track_conv_indices(
                         query_start_loc, forward_batch
                     )
@@ -215,11 +217,6 @@ class MambaAttnBackendBase(AttentionBackend):
         else:
             raise ValueError(f"Invalid forward mode: {forward_batch.forward_mode=}")
 
-        has_mamba_track_mask = bool(
-            forward_batch.mamba_track_mask is not None
-            and forward_batch.mamba_track_mask.any()
-        )
-
         return ForwardMetadata(
             query_start_loc=query_start_loc,
             mamba_cache_indices=mamba_cache_indices,
@@ -800,6 +797,10 @@ class Mamba2AttnBackend(MambaAttnBackendBase):
             is_target_verify=forward_batch.forward_mode.is_target_verify(),
             draft_token_num=draft_token_num,
         )
+        # `forward` slices the track destinations from ([-num_decodes:])
+        assert (
+            self.forward_metadata.num_decodes == forward_batch.batch_size
+        ), f"{self.forward_metadata.num_decodes=} != {forward_batch.batch_size=}"
 
     def init_forward_metadata(self, forward_batch: ForwardBatch):
         metadata = self._forward_metadata(forward_batch)
@@ -831,17 +832,12 @@ class Mamba2AttnBackend(MambaAttnBackendBase):
             output=output,
             layer_cache=layer_cache,
             metadata=self.forward_metadata,
-            forward_batch=forward_batch,
             mup_vector=mup_vector,
             use_triton_causal_conv=use_triton_causal_conv,
         )
 
         if forward_batch.mamba_track_mask is not None:
-            if (
-                intermediate_states is not None
-                and forward_batch.mamba_track_mask is not None
-                and forward_batch.mamba_track_mask.any()
-            ):
+            if intermediate_states is not None:
                 self._track_mamba_state_extend(
                     forward_batch,
                     intermediate_states,

python/sglang/srt/layers/attention/mamba/mamba.py

@@ -27,7 +27,6 @@ from sglang.srt.layers.linear import (
 )
 from sglang.srt.layers.quantization.base_config import QuantizationConfig
 from sglang.srt.mem_cache.memory_pool import MambaPool
-from sglang.srt.model_executor.forward_batch_info import ForwardBatch
 from sglang.srt.model_loader.weight_utils import (
     composed_weight_loader,
     sharded_weight_loader,
@@ -445,7 +444,6 @@ class MambaMixer2(torch.nn.Module):
         output: Optional[torch.Tensor] = None,
         layer_cache: MambaPool.State,
         metadata: Mamba2Metadata,
-        forward_batch: ForwardBatch,
         mup_vector: Optional[torch.Tensor] = None,
         use_triton_causal_conv: bool = False,
     ):
@@ -564,14 +562,13 @@ class MambaMixer2(torch.nn.Module):
             x = hidden_states_B_C_p.transpose(
                 0, 1
             )  # this is the form that causal-conv see
+            # Runs once per mamba layer
             if (
-                forward_batch.mamba_track_mask is not None
-                and forward_batch.mamba_track_mask.any()
+                metadata.has_mamba_track_mask
                 and metadata.track_conv_indices is not None
             ):
                 x_to_track = x[:, metadata.track_conv_indices].transpose(0, 1)
-                mask_indices = forward_batch.mamba_track_mask.nonzero(as_tuple=True)[0]
-                conv_state[forward_batch.mamba_track_indices[mask_indices]] = x_to_track
+                conv_state[metadata.conv_states_mask_indices] = x_to_track
             ccfn = (
                 causal_conv1d_fn
                 if not use_triton_causal_conv

python/sglang/srt/layers/attention/mamba/mamba2_metadata.py

@@ -275,6 +275,16 @@ class Mamba2Metadata(ForwardMetadata):
             if forward_batch.spec_info is not None
             else 1
         )
+        # Resolve the tracked-row selection once per forward
+        mamba_track_mask_indices = None
+        conv_states_mask_indices = None
+        if forward_metadata.has_mamba_track_mask:
+            mamba_track_mask_indices = forward_batch.mamba_track_mask.nonzero(
+                as_tuple=True
+            )[0]
+            conv_states_mask_indices = forward_batch.mamba_track_indices[
+                mamba_track_mask_indices
+            ]
         return Mamba2Metadata(
             query_start_loc=query_start_loc,
             mamba_cache_indices=forward_metadata.mamba_cache_indices,
@@ -288,6 +298,8 @@ class Mamba2Metadata(ForwardMetadata):
             track_ssm_final_src=forward_metadata.track_ssm_final_src,
             track_ssm_final_dst=forward_metadata.track_ssm_final_dst,
             has_mamba_track_mask=forward_metadata.has_mamba_track_mask,
+            mamba_track_mask_indices=mamba_track_mask_indices,
+            conv_states_mask_indices=conv_states_mask_indices,
             num_prefills=num_prefills,
             num_prefill_tokens=num_prefill_tokens,
             num_decodes=num_decodes,

python/sglang/srt/model_executor/runner/decode_cuda_graph_runner.py

@@ -176,8 +176,12 @@ def build_replay_fb_view(
         # The mamba-track registry slot (VIRTUAL ids) is the v2p translate SOURCE
         # for the backend, which copies the result into its own static buffer and
         # reads THAT in the decode track-save — this slot is never mutated. None
-        # when mamba-track is disabled.
-        mamba_track_indices=getattr(buffers, "mamba_track_indices", None),
+        # when mamba-track is disabled, slice to [:bs] like every other buffer
+        mamba_track_indices=(
+            None
+            if buffers.mamba_track_indices is None
+            else buffers.mamba_track_indices[:bs]
+        ),
         spec_info=forward_batch.spec_info,
     )