[Bugfix] Fix Random Dataset Prefix Length Inaccuracy (#33907)

Signed-off-by: frankwang28 <frank.wbb@hotmail.com>
Co-authored-by: Roger Wang <hey@rogerw.io>

vllm/benchmarks/datasets.py

@@ -380,7 +380,7 @@ def gen_prompt_decode_to_target_len(
     max_retry: int = 10,
     add_special_tokens: bool = False,
     rng: np.random.Generator | None = None,
-) -> tuple[str, list[int]]:
+) -> tuple[str, list[int], int]:
     """
     Ensure decoded-then-encoded prompt length matches the target token length.
 
@@ -392,7 +392,9 @@ def gen_prompt_decode_to_target_len(
     [6880, 6881] -> ['Ġcalls', 'here'] ->
     [1650, 939, 486] -> ['Ġcall', 'sh', 'ere']
 
-    Returns a tuple of the final prompt string and the adjusted token sequence.
+    Returns a tuple of the final prompt string, the adjusted token sequence,
+    and the token mismatch (final_len - target_token_len) if the retry budget
+    is exhausted.
     """
     remain_num_try = max_retry
     token_mismatch = 0
@@ -499,7 +501,7 @@ class RandomDataset(BenchmarkDataset):
         allowed_tokens = np.array(list(set(all_tokens) - set(prohibited_tokens)))
 
         # Generate prefix once
-        prefix_token_ids = self.get_prefix(allowed_tokens, prefix_len)
+        prefix_token_ids = self.get_prefix(tokenizer, allowed_tokens, prefix_len)
 
         requests = []
         token_mismatch_total = 0
@@ -554,19 +556,36 @@ class RandomDataset(BenchmarkDataset):
 
     def get_prefix(
         self,
+        tokenizer: TokenizerLike,
         allowed_tokens: np.ndarray,
         prefix_len: int,
     ) -> list[int]:
         """
         Get the prefix for the dataset.
         """
-        return (
-            allowed_tokens[
-                self._rng.integers(0, len(allowed_tokens), size=prefix_len)
-            ].tolist()
-            if prefix_len > 0
-            else []
+        if prefix_len <= 0:
+            return []
+
+        prefix_tokens = allowed_tokens[
+            self._rng.integers(0, len(allowed_tokens), size=prefix_len)
+        ].tolist()
+        _, adjusted_tokens, token_mismatch = gen_prompt_decode_to_target_len(
+            tokenizer=tokenizer,
+            token_sequence=prefix_tokens,
+            target_token_len=prefix_len,
+            add_special_tokens=False,
+            rng=self._rng,
         )
+        if token_mismatch != 0:
+            sign = "more" if token_mismatch > 0 else "fewer"
+            logger.warning(
+                "Prefix tokenization produced %d %s tokens than expected "
+                "after decoding and re-encoding. This is expected due to "
+                "the imperfect nature of the sampling procedure",
+                abs(token_mismatch),
+                sign,
+            )
+        return adjusted_tokens
 
     def get_sampling_params(
         self,
@@ -1128,7 +1147,7 @@ class RandomMultiModalDataset(RandomDataset):
             "Sampling from %d out of %d (vocab size)", len(allowed_tokens), vocab_size
         )
         # Generate prefix once
-        prefix_token_ids = self.get_prefix(allowed_tokens, prefix_len)
+        prefix_token_ids = self.get_prefix(tokenizer, allowed_tokens, prefix_len)
         # Add synthetic multimodal items to each request
         mm_requests = []
         token_mismatch_total = 0