Optimize Triton Draft Backend (#11556)

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

 from __future__ import annotations
 
 from dataclasses import dataclass
-from typing import TYPE_CHECKING, Optional, Union
+from typing import TYPE_CHECKING, List, Optional
 
 import torch
 import triton
@@ -12,6 +12,7 @@ from sglang.srt.layers.attention.utils import create_flashinfer_kv_indices_trito
 from sglang.srt.layers.dp_attention import get_attention_tp_size
 from sglang.srt.layers.radix_attention import AttentionType
 from sglang.srt.model_executor.forward_batch_info import ForwardBatch, ForwardMode
+from sglang.srt.speculative.spec_utils import generate_draft_decode_kv_indices
 from sglang.srt.utils import (
     get_bool_env_var,
     get_device_core_count,
@@ -423,6 +424,7 @@ class TritonAttnBackend(AttentionBackend):
         max_bs: int,
         max_num_tokens: int,
         kv_indices_buf: Optional[torch.Tensor] = None,
+        cuda_graph_num_kv_splits_buf: Optional[torch.Tensor] = None,
     ):
         self.cuda_graph_attn_logits = torch.zeros(
             (max_num_tokens, self.num_head, self.max_kv_splits, self.v_head_dim),
@@ -434,9 +436,17 @@ class TritonAttnBackend(AttentionBackend):
             dtype=torch.float32,
             device=self.device,
         )
-        self.cuda_graph_num_kv_splits = torch.full(
-            (max_num_tokens,), self.max_kv_splits, dtype=torch.int32, device=self.device
-        )
+
+        if cuda_graph_num_kv_splits_buf is None:
+            self.cuda_graph_num_kv_splits = torch.full(
+                (max_num_tokens,),
+                self.max_kv_splits,
+                dtype=torch.int32,
+                device=self.device,
+            )
+        else:
+            self.cuda_graph_num_kv_splits = cuda_graph_num_kv_splits_buf
+
         if kv_indices_buf is None:
             self.cuda_graph_kv_indices = torch.zeros(
                 (max_num_tokens * self.max_context_len),
@@ -683,9 +693,7 @@ class TritonAttnBackend(AttentionBackend):
                     )
 
             else:
-                kv_indptr[: spec_info.kv_indptr.shape[0]] = spec_info.kv_indptr
-                kv_indices[: spec_info.kv_indices.shape[0]] = spec_info.kv_indices
-                num_token = spec_info.kv_indptr.shape[0] - 1
+                assert False, "Multi-step cuda graph init is not done here."
             self.get_num_kv_splits(num_kv_splits[:num_token], seq_lens[:bs])
 
         elif forward_mode.is_target_verify():
@@ -898,11 +906,8 @@ class TritonMultiStepDraftBackend:
         topk: int,
         speculative_num_steps: int,
     ):
-        from sglang.srt.speculative.spec_utils import generate_draft_decode_kv_indices
-
         self.topk = topk
         self.speculative_num_steps = speculative_num_steps
-        self.generate_draft_decode_kv_indices = generate_draft_decode_kv_indices
         max_bs = model_runner.req_to_token_pool.size * self.topk
         self.kv_indptr = torch.zeros(
             (
@@ -912,7 +917,7 @@ class TritonMultiStepDraftBackend:
             dtype=torch.int32,
             device=model_runner.device,
         )
-        self.attn_backends = []
+        self.attn_backends: List[TritonAttnBackend] = []
         for i in range(self.speculative_num_steps):
             self.attn_backends.append(
                 TritonAttnBackend(
@@ -931,13 +936,19 @@ class TritonMultiStepDraftBackend:
         self.page_size = model_runner.server_args.page_size
 
     def common_template(
-        self, forward_batch: ForwardBatch, kv_indices_buffer: torch.Tensor, call_fn: int
+        self,
+        forward_batch: ForwardBatch,
+        kv_indices_buffer: Optional[torch.Tensor],
+        call_fn: int,
     ):
+        if kv_indices_buffer is None:
+            kv_indices_buffer = self.cuda_graph_kv_indices
+
         num_seqs = forward_batch.batch_size
         bs = self.topk * num_seqs
         seq_lens_sum = forward_batch.seq_lens_sum
 
-        self.generate_draft_decode_kv_indices[
+        generate_draft_decode_kv_indices[
             (self.speculative_num_steps, num_seqs, self.topk)
         ](
             forward_batch.req_pool_indices,
@@ -955,6 +966,9 @@ class TritonMultiStepDraftBackend:
             self.page_size,
         )
 
+        if call_fn is None:
+            return
+
         for i in range(self.speculative_num_steps):
             forward_batch.spec_info.kv_indptr = self.kv_indptr[i, : bs + 1]
             forward_batch.spec_info.kv_indices = kv_indices_buffer[i][
@@ -989,9 +1003,18 @@ class TritonMultiStepDraftBackend:
             dtype=torch.int64,
             device=self.device,
         )
+        self.cuda_graph_num_kv_splits = torch.full(
+            (max_num_tokens,),
+            self.attn_backends[0].max_kv_splits,
+            dtype=torch.int32,
+            device=self.device,
+        )
         for i in range(self.speculative_num_steps):
             self.attn_backends[i].init_cuda_graph_state(
-                max_bs, max_num_tokens, kv_indices_buf=self.cuda_graph_kv_indices[i]
+                max_bs,
+                max_num_tokens,
+                kv_indices_buf=self.cuda_graph_kv_indices[i],
+                cuda_graph_num_kv_splits_buf=self.cuda_graph_num_kv_splits,
             )
 
     def init_forward_metadata_capture_cuda_graph(self, forward_batch: ForwardBatch):
@@ -1006,24 +1029,24 @@ class TritonMultiStepDraftBackend:
                 spec_info=forward_batch.spec_info,
             )
 
-        self.common_template(forward_batch, self.cuda_graph_kv_indices, call_fn)
+        self.common_template(forward_batch, None, call_fn)
 
     def init_forward_metadata_replay_cuda_graph(
         self, forward_batch: ForwardBatch, bs: int
     ):
-        def call_fn(i, forward_batch):
-            self.attn_backends[i].init_forward_metadata_replay_cuda_graph(
-                bs,
-                forward_batch.req_pool_indices,
-                forward_batch.seq_lens,
-                seq_lens_sum=-1,
-                encoder_lens=None,
-                forward_mode=ForwardMode.DECODE,
-                spec_info=forward_batch.spec_info,
-                seq_lens_cpu=None,
-            )
-
-        self.common_template(forward_batch, self.cuda_graph_kv_indices, call_fn)
+        self.common_template(forward_batch, None, None)
+
+        # NOTE: Multi-step's attention backends use the slice of
+        # - kv_indptr buffer (cuda graph and non-cuda graph)
+        # - kv_indices buffer (cuda graph only)
+        # So we don't need to assign the KV indices inside the attention backend.
+
+        # Compute num_kv_splits only once
+        num_token = forward_batch.batch_size * self.topk
+        self.attn_backends[-1].get_num_kv_splits(
+            self.attn_backends[-1].cuda_graph_num_kv_splits[:num_token],
+            forward_batch.seq_lens[:bs],
+        )
 
 
 @triton.jit