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# Copyright (c) 2026, NVIDIA CORPORATION. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Runtime monkey-patches making vLLM v1 scheduler CFG-pair-aware.
Call ``apply_cfg_patches()`` **before** creating the ``LLM`` instance.
Post-sampling token synchronization is handled separately by
``CFGLogitsProcessor._ensure_sample_patched()`` (runs in each worker).
"""
from __future__ import annotations
import logging
from typing import Optional
logger = logging.getLogger(__name__)
_applied = False
def _get_cfg_pair_id(request) -> Optional[str]:
sp = request.sampling_params
if sp and sp.extra_args:
return sp.extra_args.get("cfg_pair_id")
return None
def _get_cfg_role(request) -> Optional[str]:
sp = request.sampling_params
if sp and sp.extra_args:
return sp.extra_args.get("cfg_role")
return None
def _hold_incomplete_pairs(scheduler) -> list:
"""Remove from the waiting queue any CFG request whose pair partner has not
yet been admitted to the engine.
vLLM v1's EngineCore runs a schedule step as soon as the first request of a
pair arrives over IPC; for small/fast models the partner often arrives only
after that step, so the lone member prefills one step early and the pair is
permanently offset by one token (breaking CFG). Holding the lone member
until both are present guarantees they prefill in the same step.
"""
held: list = []
keep: list = []
for req in list(scheduler.waiting):
pair_id = scheduler._cfg_req_to_pair.get(req.request_id)
complete = True
if pair_id is not None:
roles = scheduler._cfg_pairs.get(pair_id, {})
complete = len(roles) == 2 and all(
rid in scheduler.requests for rid in roles.values()
)
(keep if complete else held).append(req)
if held:
scheduler.waiting.clear()
scheduler.waiting.extend(keep)
return held
def _reorder_waiting_for_cfg(scheduler) -> None:
"""Move CFG pair partners adjacent in the waiting queue."""
waiting = scheduler.waiting
if len(waiting) < 2:
return
requests = list(waiting)
waiting.clear()
seen: set[str] = set()
result: list = []
for req in requests:
rid = req.request_id
if rid in seen:
continue
seen.add(rid)
result.append(req)
pair_id = scheduler._cfg_req_to_pair.get(rid)
if pair_id is None:
continue
roles = scheduler._cfg_pairs.get(pair_id, {})
for _role, partner_id in roles.items():
if partner_id != rid and partner_id not in seen:
for r in requests:
if r.request_id == partner_id:
seen.add(partner_id)
result.append(r)
break
waiting.extend(result)
def _equalize_cfg_pair_progress(scheduler, scheduler_output) -> None:
"""Ensure both members of every CFG pair reach the same num_computed_tokens.
After ``_orig_schedule`` (which calls ``_update_after_schedule``),
``num_computed_tokens`` is already advanced. Prefix-cache hits or unequal
chunked-prefill budget can leave one member ahead of the other, causing it
to finish prefill a step earlier and emit a decode token the partner misses.
Fix: reduce the faster member's allocation so both land on the same
``num_computed_tokens``. The over-allocated KV-cache blocks remain and are
consumed in the next step — no blocks are leaked.
"""
for _pair_id, roles in scheduler._cfg_pairs.items():
cond_id = roles.get("cond")
uncond_id = roles.get("uncond")
if not cond_id or not uncond_id:
continue
cond_sched = scheduler_output.num_scheduled_tokens.get(cond_id, 0)
uncond_sched = scheduler_output.num_scheduled_tokens.get(uncond_id, 0)
if cond_sched == 0 or uncond_sched == 0:
continue
cond_req = scheduler.requests.get(cond_id)
uncond_req = scheduler.requests.get(uncond_id)
if not cond_req or not uncond_req:
continue
if cond_req.num_computed_tokens == uncond_req.num_computed_tokens:
continue
target = min(cond_req.num_computed_tokens,
uncond_req.num_computed_tokens)
feasible = True
for req, sched in [(cond_req, cond_sched), (uncond_req, uncond_sched)]:
if req.num_computed_tokens - target >= sched:
feasible = False
break
if not feasible:
continue
for req_id, req, orig_sched in [
(cond_id, cond_req, cond_sched),
(uncond_id, uncond_req, uncond_sched),
]:
diff = req.num_computed_tokens - target
if diff > 0:
req.num_computed_tokens = target
scheduler_output.num_scheduled_tokens[req_id] = orig_sched - diff
scheduler_output.total_num_scheduled_tokens -= diff
logger.debug(
"CFG equalize %s: %d -> %d scheduled, computed -> %d",
req_id, orig_sched, orig_sched - diff, target,
)
def _co_preempt_split_pairs(scheduler, scheduler_output) -> None:
"""Detect a split CFG pair (one running, one not / desynced).
On vLLM 0.20 the original manual re-preemption surgery is unsafe, so this is
a detection-and-log guard; ``_hold_incomplete_pairs`` prevents the split.
"""
if not scheduler._cfg_pairs:
return
running_ids = {r.request_id for r in scheduler.running}
to_preempt: list[str] = []
for _pair_id, roles in scheduler._cfg_pairs.items():
cond_id = roles.get("cond")
uncond_id = roles.get("uncond")
if cond_id is None or uncond_id is None:
continue
if cond_id not in scheduler.requests or uncond_id not in scheduler.requests:
continue
cond_running = cond_id in running_ids
uncond_running = uncond_id in running_ids
if cond_running != uncond_running:
to_preempt.append(cond_id if cond_running else uncond_id)
continue
if cond_running and uncond_running:
cond_req = scheduler.requests.get(cond_id)
uncond_req = scheduler.requests.get(uncond_id)
if (cond_req and uncond_req
and cond_req.num_computed_tokens != uncond_req.num_computed_tokens):
faster = cond_id if cond_req.num_computed_tokens > uncond_req.num_computed_tokens else uncond_id
to_preempt.append(faster)
if to_preempt:
logger.warning(
"CFG pair split detected (to_preempt=%s) but co-preempt surgery is "
"disabled on vLLM 0.20; relying on pair-hold + equalize.",
to_preempt,
)
return
def _patch_scheduler() -> None:
from vllm.v1.core.sched.scheduler import Scheduler
_orig_init = Scheduler.__init__
_orig_add = Scheduler.add_request
_orig_finish = Scheduler.finish_requests
_orig_schedule = Scheduler.schedule
def _init(self, *args, **kwargs):
_orig_init(self, *args, **kwargs)
self._cfg_pairs: dict[str, dict[str, str]] = {}
self._cfg_req_to_pair: dict[str, str] = {}
def _add(self, request):
_orig_add(self, request)
pair_id = _get_cfg_pair_id(request)
role = _get_cfg_role(request)
if pair_id and role:
self._cfg_pairs.setdefault(pair_id, {})[role] = request.request_id
self._cfg_req_to_pair[request.request_id] = pair_id
def _finish(self, request_ids, finished_status):
if request_ids is None:
result = _orig_finish(self, request_ids, finished_status)
self._cfg_pairs.clear()
self._cfg_req_to_pair.clear()
return result
if isinstance(request_ids, str):
request_ids = {request_ids}
else:
request_ids = set(request_ids)
partner_ids: set[str] = set()
for req_id in request_ids:
pair_id = self._cfg_req_to_pair.get(req_id)
if pair_id is None:
continue
for _role, rid in self._cfg_pairs.get(pair_id, {}).items():
if rid != req_id and rid in self.requests:
partner_ids.add(rid)
all_ids = request_ids | partner_ids
result = _orig_finish(self, all_ids, finished_status)
for req_id in all_ids:
pair_id = self._cfg_req_to_pair.pop(req_id, None)
if pair_id:
self._cfg_pairs.pop(pair_id, None)
return result
def _schedule(self):
if not self._cfg_pairs:
return _orig_schedule(self)
_reorder_waiting_for_cfg(self)
held = _hold_incomplete_pairs(self)
orig_threshold = self.scheduler_config.long_prefill_token_threshold
self.scheduler_config.long_prefill_token_threshold = (
self.max_num_scheduled_tokens // 2
)
scheduler_output = _orig_schedule(self)
self.scheduler_config.long_prefill_token_threshold = orig_threshold
for req in reversed(held):
self.waiting.prepend_request(req)
_equalize_cfg_pair_progress(self, scheduler_output)
_co_preempt_split_pairs(self, scheduler_output)
return scheduler_output
Scheduler.__init__ = _init
Scheduler.add_request = _add
Scheduler.finish_requests = _finish
Scheduler.schedule = _schedule
def apply_cfg_patches() -> None:
"""Apply CFG scheduler monkey-patches to vLLM v1. Call before ``LLM()``.
Token synchronization (post-sampling copy of cond token to uncond slot)
is handled by ``CFGLogitsProcessor._ensure_sample_patched()`` which runs
inside each worker process automatically.
"""
global _applied
if _applied:
return
_applied = True
logger.info("Applying CFG scheduler patches to vLLM v1")
_patch_scheduler()
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