[Core][Metrics] Remove `vllm:prompt_tokens_recomputed` metric (#38709)

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

tests/v1/metrics/test_stats.py

@@ -190,8 +190,7 @@ def test_prompt_token_stats_full_local_cache_recompute():
     )
 
     assert stats.computed == 1
-    assert stats.local_cache_hit == 1000
-    assert stats.recomputed_tokens == 1
+    assert stats.local_cache_hit == 999
 
 
 def test_prompt_token_stats_full_external_transfer_recompute():
@@ -201,11 +200,10 @@ def test_prompt_token_stats_full_external_transfer_recompute():
     # Case 6: Full external transfer (999 cached after reduction, 1 recomputed)
     stats.update_from_output(
         num_cached_tokens=999,
-        num_external_computed_tokens=1000,
+        num_external_computed_tokens=999,
         prompt_len=1000,
     )
 
     assert stats.computed == 1
     assert stats.local_cache_hit == 0
-    assert stats.external_kv_transfer == 1000
-    assert stats.recomputed_tokens == 1
+    assert stats.external_kv_transfer == 999

vllm/v1/metrics/loggers.py

@@ -622,16 +622,6 @@ class PrometheusStatLogger(AggregateStatLoggerBase):
             counter_prompt_tokens_cached, per_engine_labelvalues
         )
 
-        # Recomputed tokens (last token recomputed when entire prompt is cached)
-        counter_prompt_tokens_recomputed = self._counter_cls(
-            name="vllm:prompt_tokens_recomputed",
-            documentation="Number of cached tokens recomputed for forward pass.",
-            labelnames=labelnames,
-        )
-        self.counter_prompt_tokens_recomputed = create_metric_per_engine(
-            counter_prompt_tokens_recomputed, per_engine_labelvalues
-        )
-
         counter_generation_tokens = self._counter_cls(
             name="vllm:generation_tokens",
             documentation="Number of generation tokens processed.",
@@ -1122,7 +1112,6 @@ class PrometheusStatLogger(AggregateStatLoggerBase):
                 pts.get_by_source(source)
             )
         self.counter_prompt_tokens_cached[engine_idx].inc(pts.cached_tokens)
-        self.counter_prompt_tokens_recomputed[engine_idx].inc(pts.recomputed_tokens)
         self.counter_generation_tokens[engine_idx].inc(
             iteration_stats.num_generation_tokens
         )

vllm/v1/metrics/stats.py

@@ -246,12 +246,11 @@ class PromptTokenStats:
         local_cache_hit: Tokens from local prefix cache.
         external_kv_transfer: Tokens from external KV transfer.
         cached_tokens: Tokens skipped during prefill (from scheduler).
-        recomputed_tokens: Cached tokens that were recomputed (see below).
         total: Total prompt tokens.
 
     Invariants:
-        computed + local_cache_hit + external_kv_transfer - recomputed_tokens = total
-        local_cache_hit + external_kv_transfer - recomputed_tokens = cached_tokens
+        computed + local_cache_hit + external_kv_transfer = total
+        local_cache_hit + external_kv_transfer = cached_tokens
     """
 
     ALL_SOURCES: tuple[str, ...] = (
@@ -264,7 +263,6 @@ class PromptTokenStats:
     local_cache_hit: int = 0
     external_kv_transfer: int = 0
     cached_tokens: int = 0
-    recomputed_tokens: int = 0
     total: int = 0
 
     def update_from_output(
@@ -274,11 +272,6 @@ class PromptTokenStats:
         prompt_len: int,
     ) -> None:
         """Update stats from a prefill output."""
-        # When all tokens are cached, the scheduler reduces num_cached_tokens
-        # by 1 to force the model to recompute the last token, since the model
-        # needs at least one input token to run a forward pass.
-        recomputed = 1 if (num_cached_tokens + 1 == prompt_len) else 0
-
         self.computed += prompt_len - num_cached_tokens
         self.external_kv_transfer += num_external_computed_tokens
         # FIXME(yifan): local_cache_hit can go negative after preemption.
@@ -290,10 +283,9 @@ class PromptTokenStats:
         # as a separate metric rather than reusing num_external_computed_tokens
         # for metric directly.
         self.local_cache_hit += max(
-            0, (num_cached_tokens + recomputed - num_external_computed_tokens)
+            0, (num_cached_tokens - num_external_computed_tokens)
         )
         self.cached_tokens += num_cached_tokens
-        self.recomputed_tokens += recomputed
         self.total += prompt_len
 
     def get_by_source(self, source: str) -> int: