[Bugfix][Core] Fix negative prompt token counter increments with external KV cache accounting

[chenminghua8]

Purpose

Fix a crash in Prometheus metrics reporting:
ValueError: Counters can only be incremented by non-negative amounts
This can happen when external KV cache is enabled (e.g., LMCache), especially when requests are preempted and later rescheduled.
Closes #37354

Root Cause
The crash comes from mixing two values with different update semantics:
num_cached_tokens: snapshot-style value (set once when < 0), representing total cached/skippable tokens.
num_external_computed_tokens: dynamic value, refreshed when prefill restarts (request.num_computed_tokens == 0).
In PromptTokenStats.update_from_output, local_cache_hit was computed as:
This implicitly requires:
num_cached_tokens >= num_external_computed_tokens
After preemption/rescheduling, this may be violated because num_cached_tokens remains a stale snapshot while num_external_computed_tokens is refreshed and may increase.
That can produce a negative delta and eventually trigger a Prometheus counter increment with a negative amount.

Fix
This PR introduces num_external_cached_tokens with the same snapshot semantics as num_cached_tokens:
Set once together with num_cached_tokens (first successful scheduling, or async KV completion path in _update_waiting_for_remote_kv).
Carries the external-cache snapshot used for metrics accounting.
PromptTokenStats.update_from_output now uses this snapshot value for external-cache contribution.
num_external_computed_tokens remains unchanged as a dynamic scheduler-internal field for scheduling and recovery logic (e.g., invalid block handling).
After this fix, both values passed to PromptTokenStats.update_from_output are first-scheduling snapshots, guaranteeing num_cached_tokens >= num_external_cached_tokens at all times and making local_cache_hit always non-negative.

Why not PR #36859?
PR #36859 moves toward the right goal, but its approach (gating assignment with request.num_preemptions == 0) is risky for two reasons.

1. It weakens the existing one-time initialization contract
   In current scheduler code, one-time snapshot fields rely on:
   if request.num_cached_tokens < 0: ...
   This sentinel-based pattern is idempotent and independent of scheduling history.
   Using num_preemptions == 0 as the gate ties correctness to runtime history, not field state. That can miss initialization in edge timing/order cases where preemption happens before the first stable initialization point.
2. It can record optimistic values on async KV load path
   On load_kv_async=True, the scheduler may set num_computed_tokens optimistically (including external tokens) before remote KV load is actually confirmed, and then move the request to WAITING_FOR_REMOTE_KVS.
   If num_cached_tokens is snapshotted at that point, it may include unconfirmed external tokens.
   When async load later fails and _update_requests_with_invalid_blocks rolls back computed state, the one-time snapshot cannot be safely corrected, which can permanently skew metrics.
   Why this PR’s approach is safer
   This PR keeps existing field semantics intact and introduces a dedicated snapshot field (num_external_cached_tokens) for metrics accounting.
   That separates:
   dynamic scheduler state (num_external_computed_tokens), and
   one-time metrics snapshot state (num_external_cached_tokens),
   which avoids the above coupling and preserves recovery behavior.

Test Plan

•Run existing unit tests: pytest tests/v1/test_scheduler.py tests/v1/metrics/
•Integration test: run vLLM with LMCache enabled under concurrent load sufficient to trigger preemptions; verify no ValueError crash occurs and Prometheus metrics remain non-negative.

Test Result

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[gemini-code-assist]

Code Review

This pull request renames num_external_computed_tokens to num_external_cached_tokens within the scheduler and test suite to align with external KV cache terminology. While the logic was updated in several places, the refactoring is incomplete; missing updates to the EngineCoreOutput definition and metrics processing logic will likely result in AttributeError or TypeError at runtime.

[gemini-code-assist]

The field name passed to EngineCoreOutput has been changed from num_external_computed_tokens to num_external_cached_tokens. However, the corresponding updates to the EngineCoreOutput class definition (likely in vllm/v1/engine.py) and the metrics processing logic in vllm/v1/metrics/stats.py are missing from the provided patches. As seen in the provided file content for vllm/v1/metrics/stats.py (line 355), the code still attempts to access output.num_external_computed_tokens, which will result in an AttributeError at runtime. Additionally, the PromptTokenStats.update_from_output method signature (line 270) and its calls in the tests (which are updated in this PR) will be inconsistent, leading to TypeError during test execution. Please ensure that the refactoring is completed across all affected files.

[chenminghua8]

fixed

[markmc]

Thanks. The issue is patched over in #37160 and will be fixed properly by #38709

[chenminghua8]

@markmc

Thanks for the pointer.
I agree #37160 mitigates the crash path, but I don’t think #38709 is a full root-cause fix for the accounting issue.

The root cause is a semantic mismatch in PromptTokenStats.update_from_output inputs:

• num_cached_tokens is snapshot-like (one-time),
• external input has been dynamic across reschedules (num_external_computed_tokens path).

In preempt/reschedule flows, mixing these two can make local_cache_hit incorrect (or negative before clamping).

So from my perspective, #38709 is a metric-definition simplification + mitigation, not a semantic fix for input consistency.

My concern is that removing prompt_tokens_recomputed and adjusting/clamping metrics behavior can avoid crashes, but can still mask biased local_cache_hit accounting if the source-level mismatch remains.

In #38712, I addressed this at the source by separating dynamic scheduler state from snapshot metrics input (so PromptTokenStats consumes snapshot-consistent values).
I think that class of source-level fix is still needed (either by taking #38712 directly, or porting the same approach into #38709).

[markmc]

The issue has been fixed on main since #37160 introduced this band-aid:

        self.local_cache_hit += max(
            0, (num_cached_tokens + recomputed - num_external_computed_tokens)

#37460 is the current candidate for a long-term fix. See #37460 (comment)