[Bugfix] Re-enable prefill of max model length (#24446)

Signed-off-by: Yannick Schnider <yannick.schnider1@ibm.com>

tests/v1/e2e/test_context_length.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+end-to-end tests for context length corner cases of vLLM v1 model runner
+versus HuggingFace's transformers.
+
+This test verifies the following behavior: allow a prefill that fills the
+model's maximum context length and then request a single new token.
+
+Test strategy
+- Build a textual prompt that tokenizes to exactly ``max_model_len`` tokens.
+- Run vLLM generation requesting a single new token (max_tokens=1).
+- Run HF generation on the same prompt requesting a single token too.
+- Assert both return the same number of generated tokens and the same ids.
+
+"""
+
+import pytest
+import torch
+from transformers import AutoModelForCausalLM
+
+from tests.models.utils import check_outputs_equal
+from tests.utils import create_new_process_for_each_test
+from vllm import LLM, SamplingParams
+from vllm.inputs import TokensPrompt
+
+
+@create_new_process_for_each_test()
+@pytest.mark.parametrize("model", ["JackFram/llama-160m"])
+@pytest.mark.parametrize("max_model_len", [2048])
+@pytest.mark.parametrize("max_tokens", [1])
+def test_prefill_max_context_length(
+    model: str,
+    max_model_len: int,
+    max_tokens: int,
+) -> None:
+    """Compare vLLM and HuggingFace when the prompt already fills the
+    model's maximum context length and we request a single new token.
+
+    The test ensures vLLM does not raise the "Sampled token IDs exceed the
+    max model length" assertion and that both vLLM and HF produce the same
+    single token when given the same inputs.
+    """
+
+    # Construct a prompt of size max_model_len
+    prompt_ids = [[43] * max_model_len]
+
+    # Generate max_tokens new tokens deterministically.
+    sampling_params = [
+        SamplingParams(max_tokens=max_tokens, temperature=0.0, ignore_eos=True)
+    ]
+
+    # --- vLLM generation ---
+    llm = LLM(
+        model=model,
+        tokenizer=model,
+        max_num_seqs=1,
+        tensor_parallel_size=1,
+    )
+
+    vllm_token_prompts = [TokensPrompt(prompt_token_ids=prompt_ids[0])]
+    vllm_results = llm.generate(vllm_token_prompts, sampling_params)
+
+    vllm_output_ids = vllm_results[0].outputs[0].token_ids
+
+    # --- HuggingFace generation ---
+    with torch.no_grad():
+        hf_model = AutoModelForCausalLM.from_pretrained(model)
+
+        # HF expects a tensor of input ids shaped (batch, seq_len).
+        hf_input_tokens = torch.tensor(prompt_ids[0]).unsqueeze(0)
+
+        # Generate max_tokens new tokens deterministically.
+        hf_generated = hf_model.generate(
+            hf_input_tokens,
+            do_sample=False,
+            min_new_tokens=max_tokens,
+            max_new_tokens=max_tokens,
+        )
+
+        # HF returns the prompt + generated tokens. Slice off the prompt.
+        hf_output_ids = hf_generated.cpu().tolist()[0][len(prompt_ids[0]):]
+
+    # check that vLLM outputs (token ids) match HF outputs
+    # Note: for simplicity don't pass detokenized string
+    check_outputs_equal(
+        outputs_0_lst=[(hf_output_ids, "")],
+        outputs_1_lst=[(vllm_output_ids, "")],
+        name_0="hf",
+        name_1="vllm",
+    )

vllm/v1/worker/gpu_model_runner.py

@@ -2247,14 +2247,28 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
 
             start_idx = self.input_batch.num_tokens_no_spec[req_idx]
             end_idx = start_idx + len(sampled_ids)
-            assert end_idx <= self.max_model_len, (
-                "Sampled token IDs exceed the max model length. "
-                f"Total number of tokens: {end_idx} > max_model_len: "
-                f"{self.max_model_len}")
-
-            self.input_batch.token_ids_cpu[req_idx,
-                                           start_idx:end_idx] = sampled_ids
-            self.input_batch.is_token_ids[req_idx, start_idx:end_idx] = True
+            assert end_idx <= self.max_model_len + 1, (
+                "Sampled token IDs exceed the max model length + 1. "
+                f"Total number of tokens: {end_idx} > max_model_len + 1: "
+                f"{self.max_model_len + 1}")
+
+            n_tokens_cache = len(sampled_ids)
+
+            # Sampled token IDs exceed the max model length by 1. This is
+            # legitimate as we can still sample 1 last token when the context
+            # length equals the max model length. Note that we do not need to
+            # cache this token ID as the sequence finishes after this step.
+            # Additionally, the buffers token_ids_cpu and is_token_ids are of
+            # size max model length only.
+            if end_idx == self.max_model_len + 1:
+                n_tokens_cache -= 1
+
+            self.input_batch.token_ids_cpu[req_idx, start_idx:(
+                start_idx + n_tokens_cache)] = sampled_ids[:n_tokens_cache]
+            self.input_batch.is_token_ids[req_idx,
+                                          start_idx:(start_idx +
+                                                     n_tokens_cache)] = True
+
             self.input_batch.num_tokens_no_spec[req_idx] = end_idx
             self.input_batch.num_tokens[req_idx] = end_idx