[Bugfix] Fix FA3 full cuda graph correctness (#19106)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

.buildkite/test-pipeline.yaml

@@ -320,6 +320,7 @@ steps:
   # these tests need to be separated, cannot combine
   - pytest -v -s compile/piecewise/test_simple.py
   - pytest -v -s compile/piecewise/test_toy_llama.py
+  - pytest -v -s compile/piecewise/test_full_cudagraph.py
 
 - label: PyTorch Fullgraph Test # 18min
   mirror_hardwares: [amdexperimental, amdproduction]

tests/compile/piecewise/test_full_cudagraph.py

@@ -7,6 +7,7 @@ import pytest
 
 from vllm import LLM, SamplingParams
 from vllm.config import CompilationConfig
+from vllm.platforms import current_platform
 
 MODEL = "Qwen/Qwen2-1.5B-Instruct"
 
@@ -37,7 +38,7 @@ def full_cudagraph_llm():
             "VLLM_FLASH_ATTN_VERSION": "3"
     }):
         return LLM(model=MODEL,
-                   gpu_memory_utilization=0.2,
+                   gpu_memory_utilization=0.3,
                    compilation_config=CompilationConfig(full_cuda_graph=True))
 
 
@@ -48,7 +49,7 @@ def piecewise_llm():
             "VLLM_FLASH_ATTN_VERSION": "3"
     }):
         return LLM(model=MODEL,
-                   gpu_memory_utilization=0.5,
+                   gpu_memory_utilization=0.6,
                    compilation_config=CompilationConfig())
 
 
@@ -61,6 +62,8 @@ def generate_text(llm: LLM, batch_size: int, max_tokens: int):
     return llm.generate(prompts, sampling_params)
 
 
+@pytest.mark.skipif(current_platform.get_device_capability() != (9, 0),
+                    reason="Only Hopper GPUs support FlashAttention 3")
 @pytest.mark.parametrize(("batch_size", "max_tokens"), [(1, 10), (7, 10),
                                                         (16, 10), (25, 10),
                                                         (32, 10), (45, 10),

vllm/v1/attention/backends/flash_attn.py

@@ -307,13 +307,14 @@ class FlashAttentionMetadataBuilder:
         self.kv_cache_spec = kv_cache_spec
         self.block_table = block_table
 
-        if get_flash_attn_version() == 3:
-            self.aot_schedule = not compilation_config.full_cuda_graph
-            if not self.aot_schedule:
-                logger.warning(
-                    "AOT Schedule is disabled when using full_cuda_graph")
-        else:
-            self.aot_schedule = False
+        self.aot_schedule = (get_flash_attn_version() == 3)
+        self.use_full_cuda_graph = compilation_config.full_cuda_graph
+        if self.use_full_cuda_graph and not self.aot_schedule:
+            raise ValueError("Full CUDA graph mode requires AOT scheduling, "
+                             "which requires FlashAttention 3.")
+        self.scheduler_metadata = torch.zeros(self.runner.max_num_reqs + 1,
+                                              dtype=torch.int32,
+                                              device=self.runner.device)
 
         # Sliding window size to be used with the AOT scheduler will be
         # populated on first build() call.
@@ -326,7 +327,7 @@ class FlashAttentionMetadataBuilder:
     def build(self, num_reqs: int, num_actual_tokens: int, max_query_len: int,
               common_prefix_len: int,
               common_attn_metadata: CommonAttentionMetadata):
-        max_seq_len = self.runner.seq_lens_np[:num_reqs].max()
+        max_seq_len = int(self.runner.seq_lens_np[:num_reqs].max())
         query_start_loc = common_attn_metadata.query_start_loc
         seq_lens = common_attn_metadata.seq_lens
         block_table = self.block_table
@@ -448,6 +449,18 @@ class FlashAttentionMetadataBuilder:
                                           max_seq_len=max_seq_len,
                                           causal=True)
 
+        if self.use_full_cuda_graph:
+            assert scheduler_metadata is not None
+            n = scheduler_metadata.shape[0]
+            self.scheduler_metadata[:n].copy_(scheduler_metadata,
+                                              non_blocking=True)
+            # NOTE(woosuk): We should zero out the rest of the scheduler
+            # metadata to guarantee the correctness. Otherwise, some thread
+            # blocks may use the invalid scheduler metadata and overwrite the
+            # output buffer.
+            self.scheduler_metadata[n:] = 0
+            scheduler_metadata = self.scheduler_metadata[:n]
+
         attn_metadata = FlashAttentionMetadata(
             num_actual_tokens=num_actual_tokens,
             max_query_len=max_query_len,

vllm/v1/worker/gpu_model_runner.py

@@ -1750,6 +1750,11 @@ class GPUModelRunner(LoRAModelRunnerMixin):
             attn_metadata: Optional[dict[str, Any]] = None
         else:
             query_start_loc = self.query_start_loc[:num_reqs + 1]
+            # Make sure max_model_len is used at the graph capture time.
+            self.seq_lens_np[:num_reqs] = self.max_model_len
+            self.seq_lens_np[num_reqs:] = 0
+            self.seq_lens[:num_reqs].copy_(self.seq_lens_cpu[:num_reqs],
+                                           non_blocking=True)
             seq_lens = self.seq_lens[:num_reqs]
 
             common_attn_metadata = CommonAttentionMetadata(