[Model Runner V2] Skip kernel launch for penalties & logit_bias (#32634)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

vllm/v1/worker/gpu/sample/logit_bias.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import numpy as np
 import torch
 
 from vllm.sampling_params import SamplingParams
@@ -49,12 +50,18 @@ class LogitBiasState:
             device=device,
         )
 
+        # Using any of the above.
+        self.use_logit_bias = np.zeros(max_num_reqs, dtype=bool)
+
     def add_request(
         self,
         req_idx: int,
         prompt_len: int,
         sampling_params: SamplingParams,
     ) -> None:
+        # Using any logit bias.
+        use_logit_bias = False
+
         # Allowed token IDs.
         allowed_token_ids = sampling_params.allowed_token_ids
         if allowed_token_ids:
@@ -66,6 +73,7 @@ class LogitBiasState:
                 )
             self.num_allowed_token_ids.np[req_idx] = num_allowed_token_ids
             self.allowed_token_ids.stage_write(req_idx, 0, allowed_token_ids)
+            use_logit_bias = True
         else:
             self.num_allowed_token_ids.np[req_idx] = 0
 
@@ -81,6 +89,7 @@ class LogitBiasState:
             self.num_logit_bias.np[req_idx] = num_logit_bias
             self.logit_bias_token_ids.stage_write(req_idx, 0, logit_bias.keys())
             self.logit_bias.stage_write(req_idx, 0, logit_bias.values())
+            use_logit_bias = True
         else:
             self.num_logit_bias.np[req_idx] = 0
 
@@ -89,7 +98,7 @@ class LogitBiasState:
         min_len = prompt_len + min_tokens
         self.min_lens.np[req_idx] = min_len
         stop_token_ids = sampling_params.all_stop_token_ids
-        if stop_token_ids:
+        if min_tokens > 0 and stop_token_ids:
             num_stop_token_ids = len(stop_token_ids)
             if num_stop_token_ids > MAX_NUM_STOP_TOKEN_IDS:
                 raise ValueError(
@@ -98,9 +107,12 @@ class LogitBiasState:
                 )
             self.num_stop_token_ids.np[req_idx] = num_stop_token_ids
             self.stop_token_ids.stage_write(req_idx, 0, stop_token_ids)
+            use_logit_bias = True
         else:
             self.num_stop_token_ids.np[req_idx] = 0
 
+        self.use_logit_bias[req_idx] = use_logit_bias
+
     def apply_staged_writes(self) -> None:
         self.num_allowed_token_ids.copy_to_uva()
         self.allowed_token_ids.apply_write()
@@ -117,8 +129,13 @@ class LogitBiasState:
         self,
         logits: torch.Tensor,
         idx_mapping: torch.Tensor,
+        idx_mapping_np: np.ndarray,
         pos: torch.Tensor,
     ) -> None:
+        if not np.any(self.use_logit_bias[idx_mapping_np]):
+            # No request uses logit bias. Skip the kernel launch.
+            return
+
         apply_logit_bias(
             logits,
             idx_mapping,

vllm/v1/worker/gpu/sample/penalties.py

@@ -18,6 +18,7 @@ class PenaltiesState:
         self.repetition_penalty = UvaBackedTensor(max_num_reqs, dtype=torch.float32)
         self.frequency_penalty = UvaBackedTensor(max_num_reqs, dtype=torch.float32)
         self.presence_penalty = UvaBackedTensor(max_num_reqs, dtype=torch.float32)
+        self.use_penalty = np.zeros(max_num_reqs, dtype=bool)
 
         # Initialize repetition penalty manually because 0 is an invalid value for it.
         self.repetition_penalty.np.fill(1.0)
@@ -42,7 +43,10 @@ class PenaltiesState:
         self.repetition_penalty.np[req_idx] = sampling_params.repetition_penalty
         self.frequency_penalty.np[req_idx] = sampling_params.frequency_penalty
         self.presence_penalty.np[req_idx] = sampling_params.presence_penalty
-        if use_penalty(sampling_params):
+
+        do_penalty = use_penalty(sampling_params)
+        self.use_penalty[req_idx] = do_penalty
+        if do_penalty:
             self._penalties_reqs.append(req_idx)
 
     def apply_staged_writes(
@@ -66,7 +70,16 @@ class PenaltiesState:
         self.frequency_penalty.copy_to_uva()
         self.presence_penalty.copy_to_uva()
 
-    def apply_penalties(self, logits: torch.Tensor, idx_mapping: torch.Tensor) -> None:
+    def apply_penalties(
+        self,
+        logits: torch.Tensor,
+        idx_mapping: torch.Tensor,
+        idx_mapping_np: np.ndarray,
+    ) -> None:
+        if not np.any(self.use_penalty[idx_mapping_np]):
+            # No request uses penalties. Skip the kernel launch.
+            return
+
         apply_penalties(
             logits,
             idx_mapping,

vllm/v1/worker/gpu/sample/sampler.py

@@ -104,10 +104,10 @@ class Sampler:
         logits = torch.empty_like(logits, dtype=torch.float32).copy_(logits)
 
         # Apply logit bias (e.g., allowed_token_ids, min_tokens) in place.
-        self.logit_bias_state.apply_logit_bias(logits, idx_mapping, pos)
+        self.logit_bias_state.apply_logit_bias(logits, idx_mapping, idx_mapping_np, pos)
 
         # Apply penalties in place.
-        self.penalties_state.apply_penalties(logits, idx_mapping)
+        self.penalties_state.apply_penalties(logits, idx_mapping, idx_mapping_np)
 
         # Apply temperature in place.
         apply_temperature(logits, idx_mapping, self.sampling_states.temperature.gpu)