Remove q concat in FA3 backend for DeepSeek decode (#5638)

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

@@ -62,6 +62,7 @@ class AttentionBackend(ABC):
         layer: RadixAttention,
         forward_batch: ForwardBatch,
         save_kv_cache: bool = True,
+        **kwargs,
     ):
         """Run forward on an attention layer."""
         if forward_batch.forward_mode.is_decode():
@@ -72,6 +73,7 @@ class AttentionBackend(ABC):
                 layer,
                 forward_batch,
                 save_kv_cache=save_kv_cache,
+                **kwargs,
             )
         else:
             return self.forward_extend(
@@ -81,6 +83,7 @@ class AttentionBackend(ABC):
                 layer,
                 forward_batch,
                 save_kv_cache=save_kv_cache,
+                **kwargs,
             )
 
     def forward_decode(

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

@@ -623,6 +623,8 @@ class FlashAttentionBackend(AttentionBackend):
         layer: RadixAttention,
         forward_batch: ForwardBatch,
         save_kv_cache=True,
+        # For multi-head latent attention
+        q_rope: Optional[torch.Tensor] = None,
     ):
         if k is not None:
             assert v is not None
@@ -815,9 +817,15 @@ class FlashAttentionBackend(AttentionBackend):
                 c_kv_cache = c_kv.view(
                     -1, self.page_size, layer.tp_v_head_num, layer.v_head_dim
                 )
-                q_all = q.contiguous().view(-1, layer.tp_q_head_num, layer.head_dim)
-                q_nope = q_all[:, :, : layer.v_head_dim]
-                q_rope = q_all[:, :, layer.v_head_dim :]
+                if q_rope is not None:
+                    q_nope = q.view(-1, layer.tp_q_head_num, layer.v_head_dim)
+                    q_rope = q_rope.view(
+                        -1, layer.tp_q_head_num, layer.head_dim - layer.v_head_dim
+                    )
+                else:
+                    q_all = q.contiguous().view(-1, layer.tp_q_head_num, layer.head_dim)
+                    q_nope = q_all[:, :, : layer.v_head_dim]
+                    q_rope = q_all[:, :, layer.v_head_dim :]
 
                 result = flash_attn_with_kvcache(
                     q=q_rope,
@@ -877,6 +885,8 @@ class FlashAttentionBackend(AttentionBackend):
         layer: RadixAttention,
         forward_batch: ForwardBatch,
         save_kv_cache=True,
+        # For multi-head latent attention
+        q_rope: Optional[torch.Tensor] = None,
     ) -> torch.Tensor:
         if k is not None:
             assert v is not None
@@ -1047,9 +1057,15 @@ class FlashAttentionBackend(AttentionBackend):
                 -1, self.page_size, layer.tp_v_head_num, layer.v_head_dim
             )
 
-            q_all = q.contiguous().view(-1, layer.tp_q_head_num, layer.head_dim)
-            q_nope = q_all[:, :, : layer.v_head_dim]
-            q_rope = q_all[:, :, layer.v_head_dim :]
+            if q_rope is not None:
+                q_nope = q.view(-1, layer.tp_q_head_num, layer.v_head_dim)
+                q_rope = q_rope.view(
+                    -1, layer.tp_q_head_num, layer.head_dim - layer.v_head_dim
+                )
+            else:
+                q_all = q.contiguous().view(-1, layer.tp_q_head_num, layer.head_dim)
+                q_nope = q_all[:, :, : layer.v_head_dim]
+                q_rope = q_all[:, :, layer.v_head_dim :]
             max_seqlen_q = metadata.max_seq_len_q
 
             result = flash_attn_with_kvcache(

python/sglang/srt/layers/radix_attention.py

@@ -87,6 +87,7 @@ class RadixAttention(nn.Module):
         v,
         forward_batch: ForwardBatch,
         save_kv_cache: bool = True,
+        **kwargs,
     ):
         if k is not None:
             # For cross-layer sharing, kv can be None
@@ -95,5 +96,11 @@ class RadixAttention(nn.Module):
             v = v.view(-1, self.tp_v_head_num, self.v_head_dim)
 
         return forward_batch.attn_backend.forward(
-            q, k, v, self, forward_batch, save_kv_cache
+            q,
+            k,
+            v,
+            self,
+            forward_batch,
+            save_kv_cache,
+            **kwargs,
         )

python/sglang/srt/models/deepseek_v2.py

@@ -751,10 +751,15 @@ class DeepseekV2AttentionMLA(nn.Module):
 
         q_pe, k_pe = self.rotary_emb(positions, q_pe, k_pe)
 
-        q = torch.cat([q_nope_out, q_pe], dim=-1)
         k = torch.cat([k_nope, k_pe], dim=-1)
 
-        attn_output = self.attn_mqa(q, k, k_nope, forward_batch)
+        if self.attention_backend == "fa3":
+            attn_output = self.attn_mqa(
+                q_nope_out, k, k_nope, forward_batch, q_rope=q_pe
+            )
+        else:
+            q = torch.cat([q_nope_out, q_pe], dim=-1)
+            attn_output = self.attn_mqa(q, k, k_nope, forward_batch)
         attn_output = attn_output.view(-1, self.num_local_heads, self.kv_lora_rank)
 
         if self.use_deep_gemm_bmm: