Fix prefill latency performance drop of bench serving (#14592)

python/sglang/bench_serving.py

@@ -1534,6 +1534,12 @@ def sample_random_requests(
         num=num_prompts,
     )
 
+    if return_text:
+        # Need to truncate input_len as server encode will add special token.
+        num_special_tokens = int(tokenizer.num_special_tokens_to_add())
+        for i in range(num_prompts):
+            input_lens[i] = max(0, input_lens[i] - num_special_tokens)
+
     if random_sample:
         # Sample token ids from ShareGPT and repeat/truncate them to satisfy the input_lens