[aot_compile]change VLLM backend to read fake args from example_value (#29104)

Signed-off-by: Laith Sakka <lsakka@meta.com>

tests/compile/test_aot_compile.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import functools
+import multiprocessing
 import tempfile
 from contextlib import contextmanager
 
@@ -137,3 +139,67 @@ def test_shape_env(monkeypatch: pytest.MonkeyPatch):
                 artifacts = compiled_mod.aot_compiled_fn._artifacts
                 guards_string = artifacts.compiled_fn.shape_env.format_guards()
                 assert guards_string == " - s77 <= 42\n - Eq(Mod(s77, 2), 0)"
+
+
+@pytest.mark.skipif(
+    not is_torch_equal_or_newer("2.10.0.dev"), reason="requires torch 2.10"
+)
+@use_vllm_config(make_vllm_config())
+def test_gpt2_cache_hit(monkeypatch: pytest.MonkeyPatch):
+    """
+    Test that compiling gpt2 twice results in a cache hit and
+    capture torch dynamic symbol creations to ensure make_symbol
+    not called on cache hit.
+    """
+
+    import torch.fx.experimental.symbolic_shapes as symbolic_shapes_module
+    from torch.utils._sympy.symbol import make_symbol
+
+    from vllm import LLM
+
+    create_symbol_counter = multiprocessing.Value("i", 0)
+    original_make_symbol = make_symbol
+
+    @functools.wraps(original_make_symbol)
+    def counting_make_symbol(prefix, idx, **kwargs):
+        with create_symbol_counter.get_lock():
+            create_symbol_counter.value += 1
+        return original_make_symbol(prefix, idx, **kwargs)
+
+    symbolic_shapes_module.make_symbol = counting_make_symbol
+    try:
+        with monkeypatch.context() as m, tempfile.TemporaryDirectory() as tmpdirname:
+            m.setenv("VLLM_CACHE_ROOT", tmpdirname)
+            m.setenv("VLLM_USE_AOT_COMPILE", "1")
+            # First compilation - initialize model and generate
+            llm_model = LLM(
+                model="gpt2",
+                compilation_config=CompilationConfig(
+                    mode=CompilationMode.VLLM_COMPILE,
+                ),
+                max_model_len=256,
+            )
+
+            llm_model.generate("Hello, my name is")
+            assert create_symbol_counter.value == 2
+            create_symbol_counter.value = 0
+
+            # Clean up first model
+            del llm_model
+
+            # Second compilation - should hit cache
+            m.setenv("VLLM_FORCE_AOT_LOAD", "1")
+            llm_model = LLM(
+                model="gpt2",
+                compilation_config=CompilationConfig(
+                    mode=CompilationMode.VLLM_COMPILE,
+                ),
+                max_model_len=256,
+            )
+            llm_model.generate("Hello, my name is")
+
+            assert create_symbol_counter.value == 0
+
+    finally:
+        # Restore original method
+        symbolic_shapes_module.make_symbol = original_make_symbol

vllm/compilation/backends.py

@@ -402,6 +402,7 @@ class PiecewiseCompileInterpreter(torch.fx.Interpreter):
         self.extra_traceback = False
 
     def run(self, *args):
+        # maybe instead just assert inputs are fake?
         fake_args = [
             self.fake_mode.from_tensor(t) if isinstance(t, torch.Tensor) else t
             for t in args
@@ -416,11 +417,13 @@ class PiecewiseCompileInterpreter(torch.fx.Interpreter):
         kwargs: dict[str, Any],
     ) -> Any:
         assert isinstance(target, str)
+
         output = super().call_module(target, args, kwargs)
 
         if target in self.compile_submod_names:
             index = self.compile_submod_names.index(target)
             submod = self.fetch_attr(target)
+
             sym_shape_indices = [
                 i for i, x in enumerate(args) if isinstance(x, torch.SymInt)
             ]
@@ -746,11 +749,21 @@ class VllmBackend:
             if not item.is_splitting_graph
         ]
 
+        # Extract fake values from the graph to use them when needed.
+        all_fake_values = []
+        for i in graph.graph.find_nodes(op="placeholder"):
+            all_fake_values.append(i.meta["example_value"])
+
+        fake_args = [
+            all_fake_values[i] if isinstance(t, torch.Tensor) else t
+            for i, t in enumerate(example_inputs)
+        ]
+
         # propagate the split graph to the piecewise backend,
         # compile submodules with symbolic shapes
         PiecewiseCompileInterpreter(
             self.split_gm, submod_names_to_compile, self.vllm_config, self
-        ).run(*example_inputs)
+        ).run(*fake_args)
 
         graph_path = os.path.join(local_cache_dir, "computation_graph.py")
         if not os.path.exists(graph_path):
@@ -780,14 +793,7 @@ class VllmBackend:
             )
 
         # if we need to copy input buffers for cudagraph
-        from torch._guards import detect_fake_mode
-
-        fake_mode = detect_fake_mode()
-        fake_args = [
-            fake_mode.from_tensor(t) if isinstance(t, torch.Tensor) else t
-            for t in example_inputs
-        ]
-
+        #
         # index of tensors that have symbolic shapes (batch size)
         # for weights and static buffers, they will have concrete shapes.
         # symbolic shape only happens for input tensors.

vllm/compilation/decorators.py

@@ -433,7 +433,6 @@ def _support_torch_compile(
             return TorchCompileWithNoGuardsWrapper.__call__(self, *args, **kwargs)
 
         # This is the path for the first compilation.
-
         # the first compilation needs to have dynamic shapes marked
         _mark_dynamic_inputs(
             self,