[https://nvbugs/6625851][fix] Fail guided-decoding requests reaching a dead-end grammar state - #18896
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Included review availability: Your plan provides up to 12 included reviews per hour; 8 remain after this review. WalkthroughThe guided decoder now validates only vocabulary bits in grammar bitmask rows. Draft requests stop at empty grammar states and preserve matcher rollback state. Regular requests reset advancement state and raise an error. Tests cover boundaries, padding, sign bits, dead ends, and batch isolation. ChangesGuided decoder validation
Priority: ⬆️ High — Prioritize the guided-decoding fix because dead-end grammar states can produce NaNs and a device-side sampler assert that terminates all ranks. Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to Guided decoding now fails only requests that reach an empty grammar state instead of allowing invalid logits to affect the batch. The implementation and targeted coverage support merge readiness, with no active merge-blocking risk identified. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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✨ Finishing Touches 💡 1🛠️ Fix failing CI checks 💡
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In `@tensorrt_llm/_torch/pyexecutor/guided_decoder.py`:
- Around line 314-318: Add a fake-matcher test covering the guided-decoder path
where one draft token is accepted and the next-row bitmask is empty. Verify that
remaining draft positions stay unguided, the request completes without failure,
and rollback state remains unchanged.
- Around line 284-296: Add focused _build coverage in the guided decoder bitmask
tests using a fake matcher whose fill_next_token_bitmask produces an empty row.
Verify regular requests return the existing error response and reset
num_advanced_tokens, while draft requests mark is_draft_terminated and
subsequently restore matcher state.
- Line 290: Before the continue following matcher.accept_token(req.new_token) in
the guided decoding loop, record the accepted draft-token advance so
_rollback_draft_tokens can undo it; preserve the existing continuation flow
while ensuring the matcher state is not left one token ahead.
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Inline comments:
In `@tests/unittest/_torch/misc/test_guided_decoder_bitmask.py`:
- Line 160: Extend the test around _build to include a second valid request in a
separate batch slot, while retaining request 7 as the dead-end case. Assert that
failed_requests contains only request 7 and verify the valid request’s guided
token-mask row remains correct, covering per-request isolation without changing
unrelated behavior.
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…a dead-end grammar state A grammar state with no valid next token fills an all-zero bitmask row. The apply kernel then masks the whole logits row to -inf, softmax returns NaN for it, and the sampler's device-side NaN assert fires. Because that assert is a global device assert, a single request's dead-end grammar state hard-kills every rank of the deployment (MPI_Abort). Detect the condition on the host, right after fill_next_token_bitmask, and fail that one request through the existing guided-decoding error path. This matches how vLLM and SGLang handle a stuck grammar matcher: both terminate only the affected request (RequestStatus.FINISHED_ERROR and FINISH_ABORT respectively) and never take down the engine. Detecting it at fill time rather than at apply time keeps the check off the GPU critical path and, unlike skipping the mask for that row, avoids emitting a token that violates the grammar. Only the bits below vocab_size_padded are counted: the trailing bits of a partial last word are never read by the apply kernel and are not guaranteed to be cleared by the backends. For draft requests the dead end terminates drafting instead of failing the request, consistent with the existing unacceptable-draft-token path. Signed-off-by: ZhaoyangWang <zhaoyangw@nvidia.com>
…a dead-end row Addresses review feedback on the dead-end handling: - A draft request hitting a dead end skipped the num_advanced_draft_tokens accumulation, so _rollback_draft_tokens did not undo the accept_token(new_token) that had already advanced the matcher, leaving the target model one token ahead. Unlike the unacceptable-token path, the matcher does advance here, so record it. - Add _build coverage for the three changed paths: a regular request failing on an immediate dead end (and clearing its rollback accounting), a draft request terminating drafting and rolling the advance back, and a dead end at a later draft position leaving the remaining positions unguided without failing the request. Signed-off-by: ZhaoyangWang <zhaoyangw@nvidia.com>
…t its script Without this, removing the dead-end check makes the draft-position test fail with an IndexError from the test helper rather than on the assertion it is meant to prove. Rows past the end of the script now produce a valid bitmask row. Signed-off-by: ZhaoyangWang <zhaoyangw@nvidia.com>
… batch The dead-end tests used single-request batches, so they could not tell a per-request failure apart from one that also disrupts the rest of the batch - which is the property this fix exists for. Add a two-request batch and assert the healthy request keeps its guided row. Signed-off-by: ZhaoyangWang <zhaoyangw@nvidia.com>
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5 participants
Description
Under sustained concurrent grammar-constrained load with
guided_decoding_backend: xgrammar, a single request can hard-kill an entire deployment (nvbugs/6625851: 16-rank DEP16 job,MPI_Aborterrorcode 137).When a grammar matcher reaches a state with no valid next token,
fill_next_token_bitmaskproduces an all-zero row.logits_bitmaskthen masks that entire logits row to-inf,softmaxreturns NaN for it, and the sampler's device-side NaN assert (_flashinfer_check_nansinsampler_strategy.py) fires. That assert is a global device assert, so one request's grammar dead-end escalates into a peer-kill of every rank. Because_assert_asyncis asynchronous, the reported stack sits downstream of the faulting kernel, which is why the crash surfaces atsampler.update_requests->sampler_event.synchronize()with no apparent link to guided decoding.This PR detects the all-zero row on the host, immediately after
fill_next_token_bitmask, and fails that one request through the guided decoder's existingfailed_requestserror path.Why detect at fill time on the host
vLLM and vSGLang both treat a stuck grammar matcher as a per-request failure and never let it reach a fatal assert. vLLM sets
RequestStatus.FINISHED_ERRORfor that request only whengrammar.accept_tokens()returns False (v1/core/sched/scheduler.py); SGLang setsFINISH_ABORT()for that request only whenaccept_tokenraises (managers/scheduler_components/batch_result_processor.py), and its NaN async assert is env-gated and off in production. Neither scans the bitmask for emptiness, on host or device.Detecting at fill time keeps the check where the row is already being written, so there is no GPU work and no added kernel launches on the decode critical path. It also means no grammar-violating token is ever emitted: a row with no valid continuation cannot produce valid output, so the request fails explicitly instead of silently returning a token that breaks the schema.
Only bits below
vocab_size_paddedare counted. The trailing bits of a partial last word are never read by the apply kernel and are not guaranteed to be cleared by the backends, so counting them would report a dead-end row as valid.For draft requests the dead end terminates drafting instead of failing the request, consistent with the existing unacceptable-draft-token handling. The matcher has already advanced past
new_tokenon that path, so the advance is recorded innum_advanced_draft_tokensand_rollback_draft_tokensstill undoes it.Scope and follow-ups
This PR fixes the producer of the NaN and deliberately does not touch the sampler. The underlying hazard remains:
_flashinfer_check_nansissues an unconditional global device assert, so any other NaN source (fp16 activation overflow, custom logit processors, draft logits) can still take down a whole deployment. Gating that assert behind an env var, as SGLang does, is left to a separate PR.Re-enabling the Kimi K3 strict-tool grammar by default (
TRTLLM_KIMI_K3_STRICT_TOOL_GRAMMAR, added in #17845 as a mitigation for this same bug) is also out of scope here: it needs validation on the original 16xGB300 workload first.Test Coverage
tests/unittest/_torch/misc/test_guided_decoder_bitmask.py(22 tests, all passing locally on B200).Unit coverage of
row_has_valid_token:test_dead_end_row_has_no_valid_token-- an all-zero row is reported dead, for word-aligned and partial-last-word vocab sizestest_single_valid_token_is_detected-- a single set bit anywhere belowvocab_size_paddedkeeps the row alivetest_trailing_padding_bits_do_not_count-- padding bits abovevocab_size_paddeddo not mark a dead row valid, while the highest in-range bit of that same partial word still doestest_sign_bit_counts_as_valid_token-- bit 31, which makes theint32word negative, counts as a valid tokenBehavior coverage driving the real
_buildwith a scripted fake matcher:test_build_fails_request_on_dead_end_row-- the request lands infailed_requests, its row stays unguided, andnum_advanced_tokensis cleared so_rollback_rejected_tokensskips ittest_build_draft_dead_end_is_rolled_back-- drafting terminates and_rollback_draft_tokensactually issuesrollback(1)test_build_draft_position_dead_end_stops_guiding-- a dead end at a later draft position leaves the remaining positions unguided without failing the requesttest_build_dead_end_isolates_the_failing_request-- in a two-request batch only the dead-end request fails and the healthy request keeps its guided rowVerified non-vacuous by mutation: removing the
num_advanced_draft_tokensaccumulation fails exactlytest_build_draft_dead_end_is_rolled_back, and removing the dead-end check entirely fails all four_buildtests and no others.Not covered: the tests use a scripted fake matcher rather than a real xgrammar matcher, and the fix has not been reproduced against the original 16xGB300 DEP16 workload.
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