[https://nvbugs/5517404][fix] Use the correct cuda graph for dynamic spec dec

tensorrt_llm/_torch/pyexecutor/cuda_graph_runner.py

@@ -40,9 +40,6 @@ def __init__(self, engine: "PyTorchModelEngine"):
         self.max_beam_width = engine.max_beam_width
         self.spec_config = engine.spec_config
 
-        self.max_possible_draft_len = (self.spec_config.max_draft_len
-                                       if self.enable_spec_decode else 0)
-
         self.graphs: Dict[Tuple[int, int], torch.cuda.CUDAGraph] = {}
         self.graph_outputs: Dict[Tuple[int, int],
                                  Callable[[], Optional[torch.Tensor]]] = {}
@@ -58,7 +55,7 @@ def _create_shared_static_tensors(self):
         """Allocates static tensors sized for the largest possible batch."""
         engine = self._get_engine()
 
-        token_per_request = self.max_possible_draft_len + 1
+        token_per_request = self.draft_len + 1
         max_total_tokens = (self.max_supported_batch_size *
                             self.max_beam_width * token_per_request)
         max_total_tokens = min(max_total_tokens, engine.max_num_tokens)
@@ -78,7 +75,7 @@ def _create_shared_static_tensors(self):
 
     @property
     def enable_spec_decode(self):
-        return self._get_engine().is_spec_decode
+        return self._get_engine().enable_spec_decode
 
     @property
     def draft_len(self):
@@ -174,7 +171,7 @@ def capture(self,
         # [CUDA graph spec decode padding]
         # We pad input IDs/position IDs to the maximum draft length (token per request).
         # We're forced to do this because we cannot reallocate inputs over many graph runs.
-        token_per_request = self.max_possible_draft_len + 1
+        token_per_request = self.draft_len + 1
         num_tokens_for_capture = (batch_size * self.max_beam_width *
                                   token_per_request)
 

tensorrt_llm/_torch/pyexecutor/py_executor.py

@@ -1198,13 +1198,19 @@ def _executor_loop_overlap(self):
                     previous_tensors = self.previous_batch and self.previous_batch.sample_state
                     target_inputs = None
                     draft_outputs = None
-                    if self.drafter is not None and self.use_spec_decode:
+                    # If there are previous draft tokens, we need to update the target requests to accept some draft tokens.
+                    # When there's any accepted tokens, we can't directly use the previous batch's outputs in this iteration for the target model,
+                    # so we'll set the target model's input to None and skip updating the target requests after target model forward.
+                    update_target_requests_before_forward = self.has_previous_draft_tokens
+                    if self.drafter is not None and (
+                            self.use_spec_decode
+                            or update_target_requests_before_forward):
                         target_inputs, draft_outputs, draft_batch = self._handle_speculative_decoding(
                             scheduled_batch, previous_tensors)
 
                     # Use the draft_model's outputs if we've launched the draft model.
                     # Otherwise, use the previous batch's outputs.
-                    if target_inputs is not None:
+                    if target_inputs is not None or update_target_requests_before_forward:
                         previous_tensors_device = target_inputs
                     else:
                         previous_tensors_device = self.previous_batch and self.previous_batch.sample_state and self.previous_batch.sample_state.device
@@ -1215,7 +1221,7 @@ def _executor_loop_overlap(self):
                     if target_inputs is not None:
                         self._process_draft_results(scheduled_batch,
                                                     draft_outputs, draft_batch)
-                    elif self.previous_batch is not None:
+                    elif self.previous_batch is not None and not update_target_requests_before_forward:
                         self._update_requests(self.previous_batch.sample_state)
 
                     if self.guided_decoder is not None:
@@ -1973,14 +1979,15 @@ def _handle_speculative_decoding(self, scheduled_batch, previous_tensors):
             # If needed, the overlap should happen between the target requests and the draft requests.
             # Otherwise, we can still do overlap between the previous target requests and the current target requests.
             has_draft_batch = (
-                self.previous_batch is not None
+                self.previous_batch is not None and self.use_spec_decode
                 and self.drafter.should_forward_draft_model(scheduled_batch))
 
-            if has_draft_batch:
+            if has_draft_batch or self.has_previous_draft_tokens:
                 self._update_requests(self.previous_batch.sample_state)
                 if self.has_previous_draft_tokens:
                     self._prepare_draft_requests()
 
+            if has_draft_batch:
                 target_inputs, draft_outputs, draft_batch = self.drafter.generate_draft_tokens_with_overlap(
                     scheduled_batch, self.resource_manager,
                     previous_tensors.device if previous_tensors else None)

tensorrt_llm/_torch/pyexecutor/py_executor_creator.py

@@ -41,11 +41,9 @@
 from .py_executor import PyExecutor
 
 
-# Development flag to control chain drafter feature
+# Development function to control chain drafter feature
 def _get_allow_chain_drafter() -> bool:
-    """Get the chain drafter flag from environment variable."""
-    # Use environment variable for cross-process compatibility
-    return os.getenv("TRTLLM_ALLOW_CHAIN_DRAFTER", "0") == "1"
+    return True
 
 
 class _ExecutorCreationStage(enum.Enum):

tensorrt_llm/_torch/pyexecutor/sampler.py

@@ -563,6 +563,9 @@ def update_requests(self, state: SampleState) -> None:
             if get_draft_token_length(req) > 0:
                 req.py_num_accepted_draft_tokens = num_accepted
                 req.py_rewind_len = req.py_draft_pages_allocated - num_accepted
+            else:
+                req.py_num_accepted_draft_tokens = 0
+                req.py_rewind_len = 0
             processed += num_accepted
             self.handle_logprobs(req, state, beam=self.BEAM, count=processed)
             req.py_decoding_iter += 1

tensorrt_llm/_torch/speculative/model_drafter.py

@@ -396,6 +396,7 @@ def _convert_draft_tensors(
         new_tokens_lens = None
         next_draft_tokens = None
         has_draft_tokens = False
+        batch_size = new_tokens.shape[1]
         # Iterate through generation requests and copy tokens based on accepted draft tokens
         for request in scheduled_batch.all_requests():
             idx = request.py_seq_slot
@@ -411,9 +412,8 @@ def _convert_draft_tensors(
 
         if has_draft_tokens:
             # We already updated the target state, so the new_tokens_lens should be all ones.
-            new_tokens_lens = torch.ones(scheduled_batch.batch_size,
-                                         device=device)
-            next_draft_tokens = torch.zeros(scheduled_batch.batch_size,
+            new_tokens_lens = torch.ones(batch_size, device=device)
+            next_draft_tokens = torch.zeros(batch_size,
                                             self.max_draft_tokens,
                                             device=device)
 
@@ -438,13 +438,13 @@ def _update_target_inputs_with_draft_tokens(
         Update target inputs with new draft tokens from sample state.
         """
         if draft_tensors is not None:
-            for request in draft_batch.all_requests():
+            for req_idx, request in enumerate(draft_batch.all_requests()):
                 # Skip prefill requests
                 if target_inputs.next_draft_tokens is None:
                     continue
 
                 # Get the index of the draft/target tokens in the device tensor
-                draft_idx = request.py_seq_slot
+                draft_idx = req_idx if self.use_static_draft_loop else request.py_seq_slot
                 target_idx = req_id_to_old_request[
                     request.py_request_id].py_seq_slot
                 target_inputs.new_tokens[draft_position + 1:draft_position +

tests/unittest/_torch/helpers.py

@@ -188,6 +188,7 @@ def create_mock_engine(batch_size: int):
         max_beam_width=1,
         max_num_tokens=8192,
         is_spec_decode=False,
+        enable_spec_decode=False,
         spec_config=None,
         _cuda_graph_mem_pool=None,
         use_mrope=False,

tests/unittest/_torch/speculative/test_dynamic_spec_decode.py

@@ -1,22 +1,32 @@
 import os
 import sys
 import unittest
-from unittest.mock import patch
+from unittest.mock import Mock, patch
 
 import pytest
 import torch
 from utils.llm_data import llm_models_root
 
 from tensorrt_llm import LLM, SamplingParams
+from tensorrt_llm._torch.pyexecutor.llm_request import LlmRequestState
 from tensorrt_llm.llmapi import (CudaGraphConfig, EagleDecodingConfig,
                                  KvCacheConfig)
 
 sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
 
 
+@pytest.fixture(scope="function")
+def enforce_single_worker(monkeypatch):
+    monkeypatch.setenv("TLLM_WORKER_USE_SINGLE_PROCESS", "1")
+    yield
+
+
 @pytest.mark.parametrize("disable_overlap_scheduler", [True, False])
 @pytest.mark.high_cuda_memory
-def test_dynamic_spec_decode(disable_overlap_scheduler: bool):
+def test_dynamic_spec_decode(enforce_single_worker,
+                             disable_overlap_scheduler: bool):
+    # mock_should_use_spec_decode doesn't work with multiple processes,
+    # so we use the enforce_single_worker fixture to set the environment variable.
     total_mem_gb = torch.cuda.get_device_properties(0).total_memory / 1e9
     if total_mem_gb < 35:
         pytest.skip("Not enough memory to load target + draft model")
@@ -51,32 +61,42 @@ def test_dynamic_spec_decode(disable_overlap_scheduler: bool):
         eagle3_one_model=False,
     )
 
-    # Mock should_use_spec_decode to return True for first two calls, then False
+    llm_spec = LLM(**llm_common_config, speculative_config=spec_config)
+    sampling_params = SamplingParams(max_tokens=128, temperature=0)
+
+    # Output tests
+    prompts = [
+        "The president of the United States is",
+    ]
+    sampling_params = SamplingParams(max_tokens=20, temperature=0)
+
+    # Mock should_use_spec_decode to turn on/off spec decode dynamically.
     def mock_should_use_spec_decode(requests, max_batch_size, max_num_tokens,
                                     max_draft_len):
-        if not hasattr(mock_should_use_spec_decode, 'call_count'):
-            mock_should_use_spec_decode.call_count = 0
-        mock_should_use_spec_decode.call_count += 1
-        return mock_should_use_spec_decode.call_count <= 2
+        for req in requests:
+            if req.state != LlmRequestState.GENERATION_IN_PROGRESS:
+                continue
+
+            mock_should_use_spec_decode.call_count += 1
+            # Turn off spec decode when we've called it 5 times.
+            # In the current case, at the 5th call, there are 2 accepted draft tokens,
+            # so we can have better coverage for the switching between spec decode on and off.
+            if mock_should_use_spec_decode.call_count > 5:
+                return False
+        return True
+
+    # Create a Mock object with the mock function as side_effect
+    mock_should_use_spec_decode = Mock(side_effect=mock_should_use_spec_decode)
+    # Reset mock state before using it
+    mock_should_use_spec_decode.reset_mock()
+    mock_should_use_spec_decode.call_count = 0
 
     with patch(
             'tensorrt_llm._torch.speculative.model_drafter.ModelDrafter.should_use_spec_decode',
-            side_effect=mock_should_use_spec_decode):
-        llm_spec = LLM(**llm_common_config, speculative_config=spec_config)
-        sampling_params = SamplingParams(max_tokens=128, temperature=0)
-
-        # Output tests
-        prompts = [
-            "The capital of France is",
-            "The president of the United States is",
-        ]
-        sampling_params = SamplingParams(max_tokens=10, temperature=0)
-
+            mock_should_use_spec_decode):
         results_spec = llm_spec.generate(prompts, sampling_params)
-        generated_text_spec = [
-            result.outputs[0].text for result in results_spec
-        ]
-        llm_spec.shutdown()
+    generated_text_spec = [result.outputs[0].text for result in results_spec]
+    llm_spec.shutdown()
 
     llm_ref = LLM(**llm_common_config)
     results_ref = llm_ref.generate(prompts, sampling_params)