Re-introduce `get_cuda_graph_seq_len_fill_value` (#1783)

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

@@ -41,6 +41,10 @@ class AttentionBackend(ABC):
         """Init the metadata for a forward pass for replying a cuda graph."""
         raise NotImplementedError()
 
+    def get_cuda_graph_seq_len_fill_value(self):
+        """Get the fill value for padded seq lens. Typically, it is 0 or 1."""
+        raise NotImplementedError()
+
     def forward(
         self,
         q: torch.Tensor,

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

@@ -161,6 +161,9 @@ class DoubleSparseAttnBackend(AttentionBackend):
         self.cuda_graph_start_loc.zero_()
         self.cuda_graph_start_loc[1:bs] = torch.cumsum(seq_lens[: bs - 1], dim=0)
 
+    def get_cuda_graph_seq_len_fill_value(self):
+        return 1
+
     def forward_extend(
         self, q, k, v, layer: RadixAttention, forward_batch: ForwardBatch
     ):

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

@@ -210,6 +210,9 @@ class FlashInferAttnBackend(AttentionBackend):
             encoder_lens=encoder_lens[:bs] if encoder_lens is not None else None,
         )
 
+    def get_cuda_graph_seq_len_fill_value(self):
+        return 0
+
     def forward_extend(
         self, q, k, v, layer: RadixAttention, forward_batch: ForwardBatch
     ):

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

@@ -108,6 +108,9 @@ class TritonAttnBackend(AttentionBackend):
         self.cuda_graph_start_loc.zero_()
         self.cuda_graph_start_loc[1:bs] = torch.cumsum(seq_lens[: bs - 1], dim=0)
 
+    def get_cuda_graph_seq_len_fill_value(self):
+        return 1
+
     def forward_extend(
         self, q, k, v, layer: RadixAttention, forward_batch: ForwardBatch
     ):

python/sglang/srt/model_executor/cuda_graph_runner.py

@@ -134,7 +134,11 @@ class CudaGraphRunner:
         self.max_bs = max(self.capture_bs)
         self.model_runner.attn_backend.init_cuda_graph_state(self.max_bs)
 
-        self.seq_len_fill_value = 1
+        self.seq_len_fill_value = (
+            self.model_runner.attn_backend.get_cuda_graph_seq_len_fill_value()
+        )
+
+        # FIXME(lsyin): leave it here for now, I don't know whether it is necessary
         self.encoder_len_fill_value = 0
 
         if self.use_torch_compile:
@@ -287,7 +291,7 @@ class CudaGraphRunner:
         index = bisect.bisect_left(self.capture_bs, raw_bs)
         bs = self.capture_bs[index]
         if bs != raw_bs:
-            self.seq_lens.fill_(self.seq_len_fill_value)
+            self.seq_lens.fill_(1)
             self.out_cache_loc.zero_()
 
         # Common inputs