Replaces the interim ack_queue_len==0 band-aid with a structural fix so the perf skip is
SAFE rather than removed.
Root cause: the radix attach/defer/prune/split probes read `_node_host_write_pending`
(membership in ongoing_write_through/pending_host_backups, which are drained ASYNCHRONOUSLY
at per-rank ack completion), and `_cp_subtree_has_unprunable_state` also read the rank-local
`lock_ref`. So per-rank ack-drain timing leaked into the match_prefix truncation ->
cache_protected_len -> logical_len/len(value) -> the attach predicate. On some ranks the
prepared backup attached, on others it was dropped to the synchronous write_backup catch-up
(catch_up_all_layers draft=True), whose ack-append is per-rank. ack_write_queue then diverged
across CP ranks, so writing_check's ack_queue_len==0 early-return diverged: a strict subset
entered the writing_check_min all_reduce while the rest advanced to recv_requests' broadcast
on the same tp_cpu_group (8 ranks; dp_size==1 resets enable_dp_attention to False) -> NCCL
deadlock (prod hang: node_id=50713, all 8 GPUs 0% util).
Fix: introduce TreeNode.cp_backup_pending, a rank-replicated marker set at the rank-replicated
prepared-backup attach (_attach_prepared_cp_backup) and the guarded write_backup registration,
cleared only at the MIN-committed commit (_commit_pending_backup) / rollback. Rewire the CP
radix probes (_cp_node_split_still_pending, _split_node guard, _cp_subtree_has_unprunable_state,
_inc_hit_count) to read this marker instead of the drain-timed dicts, and DROP the lock_ref read
in the prune probe (its only cross-rank skew is the backup lock, now covered by the marker).
_node_host_write_pending stays the drain-timed accessor used only by writing_check / eviction /
the visibility check (_node_host_write_ready). With the radix-path reads replicated by
construction, len(value)==logical_len on every rank -> the prepared backup always attaches (or
rolls back) symmetrically -> the asymmetric catch-up never fires -> ack_write_queue is symmetric
-> the existing ack_queue_len==0 skip is all-or-none (no per-tick no-op MIN, best perf).
Correctness: the marker is cleared only at/after _commit_pending_backup, which runs only under
the MIN frontier, so the MIN remains the SOLE host-visibility gate (no KV-corruption class
reopened); the marker is strictly more conservative than the old dicts (stays True until commit).
Fail-fasts: double-attach raises; _split_node propagates the marker. hi_mamba unchanged (gates
on ongoing only, no async ack skip). Single-layer-draft (c3fc3ff752) preserved: the design
touches only the attach decision; the draft notifier already fires post-MoE.
Adds test_cp_hicache_symmetric_attach.py: the probes read the replicated marker not the dicts,
the prune probe ignores lock_ref, and the marker lifecycle (attach->commit->rollback) + the
double-attach guard. All pass on the fix; all fail on the pre-fix code.
Design: docs_internal/cp_hicache_symmetric_attach_design.md. Pending: ETE (GSM8K + no-hang
flood + TTFT) on the dev-cu13 container.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
(cherry picked from commit 876a98177cd749b5a63623ba2ae8b868289b8e59)
leavelet
85585fcf4b