Refector prepare_inputs in model_runner_v1.py (vllm-project#2750)

### What this PR does / why we need it?
Refector prepare_inputs in model_runner_v1.py for more easy read.

### Does this PR introduce _any_ user-facing change?
NO
### How was this patch tested?
PASS CI

- vLLM version: v0.10.1.1
- vLLM main:
vllm-project/vllm@e599e2c

---------

Signed-off-by: ChenTaoyu-SJTU <ctynb@qq.com>

Author: ChenTaoyu-SJTU

vllm_ascend/worker/model_runner_v1.py

@@ -880,6 +880,26 @@ def _gather_mm_embeddings(
                 mm_embeds.append(mm_embeds_item)
         return mm_embeds
 
+    def _get_cumsum_and_arange(
+        self,
+        num_tokens: np.ndarray,
+        cumsum_dtype: Optional[np.dtype] = None,
+    ) -> tuple[np.ndarray, np.ndarray]:
+        """Get the cumulative sum and batched arange of the given array.
+        # E.g., [2, 5, 3] -> ([2, 7, 10], [0, 1, 0, 1, 2, 3, 4, 0, 1, 2])
+        # Equivalent to but faster than:
+        # np.concatenate([np.arange(n) for n in num_tokens])
+        """
+        # Step 1. [2, 5, 3] -> [2, 7, 10]
+        cu_num_tokens = np.cumsum(num_tokens, dtype=cumsum_dtype)
+        total_num_tokens = cu_num_tokens[-1]
+        # Step 2. [2, 7, 10] -> [0, 0, 2, 2, 2, 2, 2, 7, 7, 7]
+        cumsums_offsets = np.repeat(cu_num_tokens - num_tokens, num_tokens)
+        # Step 3. [0, 1, 0, 1, 2, 3, 4, 0, 1, 2]
+        arange = self.arange_np[:total_num_tokens] - cumsums_offsets
+
+        return cu_num_tokens, arange
+
     def _prepare_inputs(
         self,
         scheduler_output: "SchedulerOutput",
@@ -901,17 +921,16 @@ def _prepare_inputs(
         self.input_batch.block_table.commit_block_table(num_reqs)
 
         # Get the number of scheduled tokens for each request.
-        # TODO: The Python loop can be slow. Optimize.
-        num_scheduled_tokens = np.empty(num_reqs, dtype=np.int32)
-        num_valid_tokens = np.empty(num_reqs, dtype=np.int32)
-        max_num_scheduled_tokens = 0
-        for i, req_id in enumerate(self.input_batch.req_ids):
-            num_tokens = scheduler_output.num_scheduled_tokens[req_id]
-            num_scheduled_tokens[i] = num_tokens
-            num_valid_tokens[i] = num_tokens - \
-                len(scheduler_output.scheduled_spec_decode_tokens.get(req_id, []))
-            max_num_scheduled_tokens = max(max_num_scheduled_tokens,
-                                           num_tokens)
+        req_ids = self.input_batch.req_ids
+        tokens = [scheduler_output.num_scheduled_tokens[i] for i in req_ids]
+        num_scheduled_tokens = np.array(tokens, dtype=np.int32)
+        max_num_scheduled_tokens = max(tokens)
+        num_valid_tokens = np.array([
+            num_tokens -
+            len(scheduler_output.scheduled_spec_decode_tokens.get(i, []))
+            for num_tokens, i in zip(tokens, req_ids)
+        ],
+                                    dtype=np.int32)
 
         if (self.use_aclgraph and total_num_scheduled_tokens
                 <= self.aclgraph_batch_sizes[-1]):
@@ -952,13 +971,15 @@ def _prepare_inputs(
         if self.lora_config:
             self.set_active_loras(self.input_batch, num_scheduled_tokens)
 
-        # Prepare positions
+        # Get request indices.
+        # E.g., [2, 5, 3] -> [0, 0, 1, 1, 1, 1, 1, 2, 2, 2]
         req_indices = np.repeat(self.arange_np[:num_reqs],
                                 num_scheduled_tokens)
-        cu_num_tokens = np.cumsum(num_scheduled_tokens)
-        cumsums_offsets = np.repeat(cu_num_tokens - num_scheduled_tokens,
-                                    num_scheduled_tokens)
-        arange = self.arange_np[:total_num_scheduled_tokens] - cumsums_offsets
+
+        # cu_num_tokens: [2, 5, 3] -> [2, 7, 10]
+        # arange: [0, 1, 0, 1, 2, 3, 4, 0, 1, 2]
+        cu_num_tokens, arange = self._get_cumsum_and_arange(
+            num_scheduled_tokens)
 
         positions_np = self.positions_np[:total_num_scheduled_tokens]
         np.add(self.input_batch.num_computed_tokens_cpu[req_indices],
@@ -975,50 +996,73 @@ def _prepare_inputs(
                 self.mrope_positions_cpu[:, :total_num_scheduled_tokens],
                 non_blocking=True)
 
-        self.positions_cpu[total_num_scheduled_tokens:num_input_tokens].zero_()
-        self.positions[:num_input_tokens].copy_(
-            self.positions_cpu[:num_input_tokens], non_blocking=True)
-        positions_cpu = self.positions_cpu[:num_input_tokens]
-        positions = self.positions[:num_input_tokens]
-        self.query_lens = torch.from_numpy(num_scheduled_tokens)
+        # Get token indices.
+        # E.g., [0, 1, 0, 1, 2, 3, 4, 0, 1, 2]
+        # -> [0, 1, M, M + 1, M + 2, M + 3, M + 4, 2 * M, 2 * M + 1, 2 * M + 2]
+        # where M is the max_model_len.
+        token_indices = (positions_np +
+                         req_indices * self.input_batch.token_ids_cpu.shape[1])
+
+        # Prepare input_ids.
+        # NOTE(woosuk): We use torch.index_select instead of np.take here
+        # because torch.index_select is much faster than np.take for large
+        # tensors.
+        torch.index_select(self.input_batch.token_ids_cpu_tensor.flatten(),
+                           0,
+                           torch.from_numpy(token_indices),
+                           out=self.input_ids_cpu[:total_num_scheduled_tokens])
+
+        # Prepare some information for building Attention-Metadata
+        # Compute and commit slot mapping
+        self.input_batch.block_table.compute_slot_mapping(
+            req_indices, positions_np)
+        self.input_batch.block_table.commit_slot_mapping(
+            total_num_scheduled_tokens)
+        self.slot_mapping_cpu[:total_num_scheduled_tokens].copy_(
+            self.input_batch.block_table[0].
+            slot_mapping_cpu[:total_num_scheduled_tokens])
+
+        self.query_start_loc_np[0] = 0
+        self.query_start_loc_np[1:num_reqs + 1] = cu_num_tokens
+        self.query_start_loc[:num_reqs + 1].copy_(
+            self.query_start_loc_cpu[:num_reqs + 1], non_blocking=True)
 
         self.seq_lens_np[:num_reqs] = (
             self.input_batch.num_computed_tokens_cpu[:num_reqs] +
             num_scheduled_tokens)
-        seq_lens_cpu = self.seq_lens_cpu[:num_reqs]
+        self.seq_lens[:num_reqs].copy_(self.seq_lens_cpu[:num_reqs],
+                                       non_blocking=True)
 
-        block_table_indices = (req_indices * self.max_num_blocks_per_req +
-                               positions_np // self.block_size)
+        # Fill unused with -1. Needed for reshape_and_cache
+        self.query_start_loc[num_reqs + 1:].fill_(-1)
+        self.seq_lens[num_reqs:].fill_(0)
 
-        block_table_cpu = self.input_batch.block_table[0].get_cpu_tensor()
-        block_numbers = block_table_cpu.flatten()[block_table_indices].numpy()
-        block_offsets = positions_np % self.block_size
-        np.add(block_numbers * self.block_size,
-               block_offsets,
-               out=self.slot_mapping_np[:total_num_scheduled_tokens])
+        self.query_lens = torch.from_numpy(num_scheduled_tokens)
 
+        # Copy the tensors to the NPU.
+        self.input_ids[:total_num_scheduled_tokens].copy_(
+            self.input_ids_cpu[:total_num_scheduled_tokens], non_blocking=True)
+
+        self.positions_cpu[total_num_scheduled_tokens:num_input_tokens].zero_()
+        self.positions[:num_input_tokens].copy_(
+            self.positions_cpu[:num_input_tokens], non_blocking=True)
+
+        # Make Attention metadata
+        positions_cpu = self.positions_cpu[:num_input_tokens]
+        positions = self.positions[:num_input_tokens]
+        seq_lens_cpu = self.seq_lens_cpu[:num_reqs]
         attn_state = self._build_attn_state(num_reqs, num_scheduled_tokens,
                                             num_valid_tokens)
-
         self.attn_mask = self._make_attention_mask(seq_lens=seq_lens_cpu,
                                                    position=positions_cpu,
                                                    attn_state=attn_state)
         self.attn_state = attn_state  # type: ignore
 
-        self.query_start_loc_np[0] = 0
-        self.query_start_loc_np[1:num_reqs + 1] = cu_num_tokens
-        self.query_start_loc[:num_reqs + 1].copy_(
-            self.query_start_loc_cpu[:num_reqs + 1], non_blocking=True)
-        self.seq_lens[:num_reqs].copy_(self.seq_lens_cpu[:num_reqs],
-                                       non_blocking=True)
-
-        # Fill unused with -1. Needed for reshape_and_cache
-        self.seq_lens[num_reqs:].fill_(0)
-        self.query_start_loc[num_reqs + 1:].fill_(-1)
-
         self.with_prefill = with_prefill
         self.num_tokens_across_dp = num_tokens_across_dp
         self._update_graph_pad_size(with_prefill, maybe_padded_num_tokens)
+
+        # Make AscendCommonAttentionMetadata
         common_attn_metadata = AscendCommonAttentionMetadata(
             query_start_loc=self.query_start_loc[:num_reqs + 1],
             query_start_loc_cpu=self.query_start_loc_cpu[:num_reqs + 1],
@@ -1044,19 +1088,8 @@ def _prepare_inputs(
         if self.vllm_config.model_config.use_mla:
             attn_metadata.num_input_tokens = num_input_tokens
 
-        # Prepare input_ids
-        token_indices = (positions_np +
-                         req_indices * self.input_batch.token_ids_cpu.shape[1])
-        torch.index_select(self.input_batch.token_ids_cpu_tensor.flatten(),
-                           0,
-                           torch.from_numpy(token_indices),
-                           out=self.input_ids_cpu[:total_num_scheduled_tokens])
-        # Copy the tensors to the NPU.
-        self.input_ids[:total_num_scheduled_tokens].copy_(
-            self.input_ids_cpu[:total_num_scheduled_tokens], non_blocking=True)
-
-        # _prepare_inputs may reorder the batch, so we must gather multi
-        # modal outputs after that to ensure the correct order
+        # _prepare_inputs may reorder the batch, so we must gather
+        # multi-modal outputs after that to ensure the correct order
         if self.is_multimodal_model:
             # Run the multimodal encoder if any.
             self._execute_mm_encoder(scheduler_output)