Enumerated every DtoH/HtoD/memcpy call in the DQN hot path
(run_full_step → child graphs). Classified 55 call sites:
31 OK-pinned, 20 COLD-PATH, 4 MIGRATED.
Fix 1 (gpu_dqn_trainer.rs): removed dead cuMemcpyDtoHAsync_v2 in
run_causal_intervention_unconditional — result (causal_mean_scratch)
was never consumed; now stays on device.
Fix 2 (fused_training.rs + training_loop.rs): compute_iqr() was
called inside submit_aux_ops (captured aux_child graph). Its sync
cuMemcpyDtoH_v2 cannot be graph-captured — silently ran only during
capture, then HtoD replayed stale IQR data on every step. Removed
from submit_aux_ops; added refresh_iqn_iqr() called once per epoch
in process_epoch_boundary.
Fix 3 (gpu_iqn_head.rs): tau_buf (CudaSlice<f32>) + tau_host (f32) +
cuMemcpyHtoDAsync_v2 each step replaced by tau_pinned (*mut f32) +
tau_dev_ptr (u64) via cuMemAllocHost_v2(DEVICEMAP) +
cuMemHostGetDevicePointer_v2. CPU writes *tau_pinned = tau; EMA
kernel reads via tau_dev_ptr — zero PCIe overhead. Drop updated.
Audit table populated in docs/dqn-gpu-hot-path-audit.md.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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