[BugFix] Avoid prefix cache hit in the same schedule step for mamba layers (#29387)

Signed-off-by: Chen Zhang <zhangch99@outlook.com>

tests/models/language/generation/test_hybrid.py

@@ -7,6 +7,7 @@ import pytest
 
 from tests.models.registry import HF_EXAMPLE_MODELS
 from tests.utils import multi_gpu_test
+from vllm import LLM
 from vllm.engine.arg_utils import EngineArgs
 from vllm.platforms import current_platform
 from vllm.sampling_params import SamplingParams
@@ -769,3 +770,30 @@ def test_apc_multiple_prompts_partial_cached_outputs(
             name_0="vllm_no_cache",
             name_1=f"vllm_cache_it_{r_idx + 1}",
         )
+
+
+# we have to use a real large model to get reasonable results
+# the model can't be a hybrid model as we need block_size 16
+@pytest.mark.parametrize("model", ["tiiuae/falcon-mamba-7b"])
+def test_apc_common_prefix_same_batch(
+    model: str,
+    monkeypatch,
+) -> None:
+    # Required to put the two requests in the same batch
+    monkeypatch.setenv("VLLM_ENABLE_V1_MULTIPROCESSING", "0")
+    llm = LLM(
+        model=model,
+        enforce_eager=True,
+        block_size=16,
+        mamba_block_size=16,
+        enable_prefix_caching=True,
+        seed=42,
+    )
+    prompts = [
+        "hello what is one plus one what is one plus one what is one plus one the answer is",  # noqa: E501
+        "hello what is one plus one what is one plus one what is one plus one the answer is",  # noqa: E501
+    ]
+    sampling_params = SamplingParams(temperature=0.8, top_p=0.95, max_tokens=20)
+    outputs = llm.generate(prompts, sampling_params)
+    for output in outputs:
+        assert "two" in output.outputs[0].text