[BugFix] Fix returned logprobs with spec decode + prefill chunking (#29216)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/v1/sample/test_logprobs.py

@@ -521,8 +521,8 @@ def test_logprobs_mode(logprobs_mode: LogprobsMode):
         pytest.param(
             (
                 "eagle",
-                "meta-llama/Llama-3.1-8B-Instruct",
-                "yuhuili/EAGLE-LLaMA3.1-Instruct-8B",
+                "meta-llama/Llama-3.2-1B-Instruct",
+                "nm-testing/Llama3_2_1B_speculator.eagle3",
             ),
             marks=large_gpu_mark(min_gb=32),
         ),
@@ -541,7 +541,7 @@ def test_spec_decode_logprobs(
     """
     from vllm import LLM
 
-    prompt = "Hello world"
+    prompt = "Hello world " * 50
     sampling_params = SamplingParams(
         temperature=0, logprobs=3, max_tokens=10, ignore_eos=False
     )
@@ -582,6 +582,9 @@ def test_spec_decode_logprobs(
         seed=42,
         logprobs_mode=logprobs_mode,
         gpu_memory_utilization=0.4,
+        # Force prefill chunking
+        enable_chunked_prefill=True,
+        max_num_batched_tokens=32,
     )
     spec_results = spec_llm.generate([prompt], sampling_params)
     # Collect logprobs outputs from spec decode LLM.
@@ -597,6 +600,8 @@ def test_spec_decode_logprobs(
     # Per-token logprobs are expected to be the same.
     assert len(ref_logprobs) == len(spec_logprobs)
     for ref_logprob, spec_logprob in zip(ref_logprobs, spec_logprobs):
-        assert math.isclose(ref_logprob.logprob, spec_logprob.logprob, abs_tol=1e-3)
+        assert math.isclose(
+            ref_logprob.logprob, spec_logprob.logprob, rel_tol=5e-2, abs_tol=1e-1
+        )
         assert ref_logprob.rank == spec_logprob.rank
         assert ref_logprob.decoded_token == spec_logprob.decoded_token

vllm/v1/sample/sampler.py

@@ -81,7 +81,10 @@ class Sampler(nn.Module):
             if logprobs_mode == "raw_logprobs":
                 raw_logprobs = self.compute_logprobs(logits)
             elif logprobs_mode == "raw_logits":
-                raw_logprobs = logits.clone()
+                if logits.dtype == torch.float32:
+                    raw_logprobs = logits.clone()
+                else:
+                    raw_logprobs = logits.to(torch.float32)
 
         # Use float32 for the logits.
         logits = logits.to(torch.float32)

vllm/v1/worker/gpu_model_runner.py

@@ -2466,7 +2466,9 @@ class GPUModelRunner(
 
         num_sampled_tokens = sampler_output.sampled_token_ids.shape[0]
         sampled_token_ids = sampler_output.sampled_token_ids
+        logprobs_tensors = sampler_output.logprobs_tensors
         invalid_req_indices = []
+        cu_num_new_tokens: list[int] | None = None
         if not self.use_async_scheduling:
             # Get the valid generated tokens.
             max_gen_len = sampled_token_ids.shape[-1]
@@ -2479,6 +2481,12 @@ class GPUModelRunner(
                     sampled_token_ids,
                     self.input_batch.vocab_size,
                 )
+                if logprobs_tensors:
+                    # Needed for extracting logprobs when spec decoding.
+                    # This must be done prior to discarding sampled tokens.
+                    cu_num_new_tokens = [0]
+                    for toks in valid_sampled_token_ids:
+                        cu_num_new_tokens.append(cu_num_new_tokens[-1] + len(toks))
             # Mask out the sampled tokens that should not be sampled.
             for i in discard_sampled_tokens_req_indices:
                 valid_sampled_token_ids[int(i)].clear()
@@ -2506,10 +2514,6 @@ class GPUModelRunner(
         # the sampled tokens back, because there's no direct communication
         # between the first-stage worker and the last-stage worker.
         req_ids = self.input_batch.req_ids
-        logprobs_tensors = sampler_output.logprobs_tensors
-        cu_num_accepted_tokens = (
-            [0] if spec_decode_metadata and logprobs_tensors else None
-        )
         for req_idx in range(num_sampled_tokens):
             if self.use_async_scheduling:
                 sampled_ids = [-1] if req_idx not in invalid_req_indices_set else None
@@ -2518,11 +2522,6 @@ class GPUModelRunner(
 
             num_sampled_ids: int = len(sampled_ids) if sampled_ids else 0
 
-            if cu_num_accepted_tokens is not None:
-                cu_num_accepted_tokens.append(
-                    cu_num_accepted_tokens[-1] + num_sampled_ids
-                )
-
             if not sampled_ids:
                 continue
 
@@ -2544,7 +2543,7 @@ class GPUModelRunner(
             req_state.output_token_ids.extend(sampled_ids)
 
         logprobs_lists = (
-            logprobs_tensors.tolists(cu_num_accepted_tokens)
+            logprobs_tensors.tolists(cu_num_new_tokens)
             if not self.use_async_scheduling and logprobs_tensors is not None
             else None
         )