[V1][Spec Decode] Avoid logging useless nan metrics (#16023)

Signed-off-by: Mark McLoughlin <markmc@redhat.com>

tests/v1/core/test_scheduler.py

@@ -671,10 +671,7 @@ def test_schedule_spec_decoding_stats(spec_tokens, output_tokens, expected):
         assert running_req.num_tokens_with_spec == 2 + len(spec_tokens[i])
 
     # No draft or accepted tokens counted yet
-    assert engine_core_outputs.scheduler_stats.spec_decoding_stats is not None
-    stats = engine_core_outputs.scheduler_stats.spec_decoding_stats
-    assert stats.num_draft_tokens == 0
-    assert stats.num_accepted_tokens == 0
+    assert engine_core_outputs.scheduler_stats.spec_decoding_stats is None
 
     # Schedule the speculated tokens for validation
     output = scheduler.schedule()
@@ -702,7 +699,11 @@ def test_schedule_spec_decoding_stats(spec_tokens, output_tokens, expected):
     engine_core_outputs = scheduler.update_from_output(output,
                                                        model_runner_output)
 
-    assert engine_core_outputs.scheduler_stats.spec_decoding_stats is not None
-    stats = engine_core_outputs.scheduler_stats.spec_decoding_stats
+    scheduler_stats = engine_core_outputs.scheduler_stats
+    if expected[0] == 0:
+        assert scheduler_stats.spec_decoding_stats is None
+    else:
+        assert scheduler_stats.spec_decoding_stats is not None
+        stats = scheduler_stats.spec_decoding_stats
         assert stats.num_draft_tokens == expected[0]
         assert stats.num_accepted_tokens == expected[1]

vllm/v1/core/sched/scheduler.py

@@ -553,11 +553,11 @@ class Scheduler(SchedulerInterface):
         spec_token_ids = model_runner_output.spec_token_ids
         logprobs = model_runner_output.logprobs
         prompt_logprobs_dict = model_runner_output.prompt_logprobs_dict
-        spec_decoding_stats = SpecDecodingStats() if self.log_stats else None
         num_scheduled_tokens = scheduler_output.num_scheduled_tokens
 
         new_running: list[Request] = []
         outputs: list[EngineCoreOutput] = []
+        spec_decoding_stats: Optional[SpecDecodingStats] = None
 
         # NOTE(woosuk): As len(self.running) can be up to 1K or more, the below
         # loop can be a performance bottleneck. We should do our best to avoid
@@ -585,9 +585,8 @@ class Scheduler(SchedulerInterface):
                 num_tokens_rejected = (len(scheduled_spec_token_ids) + 1 -
                                        len(generated_token_ids))
                 request.num_computed_tokens -= num_tokens_rejected
-
-                if spec_decoding_stats is not None:
-                    spec_decoding_stats.observe(
+                spec_decoding_stats = self.make_spec_decoding_stats(
+                    spec_decoding_stats,
                     num_draft_tokens=len(scheduled_spec_token_ids),
                     num_accepted_tokens=len(generated_token_ids) - 1)
 
@@ -744,3 +743,17 @@ class Scheduler(SchedulerInterface):
             prefix_cache_stats=self.kv_cache_manager.make_prefix_cache_stats(),
             spec_decoding_stats=spec_decoding_stats,
         )
+
+    def make_spec_decoding_stats(
+        self,
+        spec_decoding_stats: Optional[SpecDecodingStats],
+        num_draft_tokens: int,
+        num_accepted_tokens: int,
+    ) -> Optional[SpecDecodingStats]:
+        if not self.log_stats:
+            return None
+        if spec_decoding_stats is None:
+            spec_decoding_stats = SpecDecodingStats()
+        spec_decoding_stats.observe(num_draft_tokens=num_draft_tokens,
+                                    num_accepted_tokens=num_accepted_tokens)
+        return spec_decoding_stats