notes: Patch C2 Phase 7 — N=3 ramp, no measurable throughput delta (#15)

2026-05-08 05:46:06 +00:00
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+# Patch C2 Phase 7 — N=3 ramp results
+
+**Date:** 2026-05-08
+**Module:** `bes2600.ko` srcversion `619A51E61BF5479AAC146E6` (cleanups + F + G + D + E + C2)
+**Rig:** ohm fresh boot, wired enu1 path for control, wlan0 for data probes
+**Stress:** netcat sender, `pv -L 4m`, 30 s per rep
+
+---
+
+## Results table
+
+| rep | uptime (s) | rate (MB/s) |
+|---:|---:|---:|
+| 1 | 544 | **2.289** |
+| 2 | 716 | **2.165** |
+| 3 | 750 | **2.376** |
+
+**N=3:** mean 2.277, median 2.289, min 2.165, max 2.376
+
+## Comparison to baselines
+
+| series | mean MB/s | Δ vs Patch B | Δ vs v3 |
+|---|---:|---:|---:|
+| Patch B (run-20260507-patchC-preflight, N=1) | 1.362 | — | -42% |
+| Patch C v3 N=3 (run-20260507-N3v3-rep*) | 2.352 | +73% | — |
+| Patch C v3 + F + G + D + E + C2 N=3 (this rep set) | 2.277 | +67% | -3% |
+
+Δ vs v3 is **within rep variance** (v3 N=3 had min 2.102, max 2.590 → spread ±20%; this set's spread is similar). Statistically indistinguishable.
+
+## Verdict: no measurable C2 throughput delta
+
+The tasklet hop in `ieee80211_rx_irqsafe` was apparently cheap on this kernel. Migrating 6 sites from `_irqsafe` to `_rx_ni` (synchronous-from-process-context, internal `local_bh_disable` wrap) preserves throughput but doesn't measurably improve it.
+
+**This was a predicted outcome.** The C2 Phase 4 plan §4.5 said:
+> "If <2%, Phase 7 says 'marginal but no regression' and we ship anyway for upstream-cleanliness."
+
+Observed: -3% (within noise) → falls into the "marginal but no regression" bucket. Ship for the kernel.org submission story (no `_irqsafe` from process context = upstream-idiomatic) even though performance is unchanged.
+
+## Receipts checklist
+
+- [x] N=3 reps captured at fresh-chip uptime (544/716/750 s — within first 13 min, before scan-failure-cadence onset)
+- [x] All reps under same conditions: same fresh boot, same nc listener, same AP (newton, BSSID c0:25:06:e6:61:b0 on chan 1)
+- [x] No WARN/BUG/oops on any rep
+- [x] dmesg pattern: only the pre-existing wsm_generic_confirm 0x0007 noise — same on Patch B / Patch F / Patch C v3 / D / E / C2 (firmware-side, independent of all our patches)
+- [x] Wired-rig telemetry collection — would have caught any wedge that wlan0 ate
+- [x] Rig-failure-is-finding: an early "0-throughput" set of reps was rig artifact (nc-loop race, port-binding state from a prior session) — caught and discounted per `feedback_rig_failure_is_finding`. The recovered N=3 reps used setsid-detached listener + post-reboot fresh state.
+
+## Phase 8 lesson
+
+**Drop-in replacements with the right kerneldoc reading still need Phase 7 measurement.** I expected +5-15% from removing the tasklet schedule. Got -3% (noise). The cost we were saving was already amortised by something else (NAPI infra? per-CPU softirq scheduling?). The kerneldoc-correctness story stands; the perf story does not.
+
+**Memory entry:** the perf-vs-correctness distinction is worth keeping. `_irqsafe → _rx_ni` is a CORRECTNESS / API-cleanliness move, not a performance optimization. Don't oversell predicted deltas without baseline measurement.
+
+## Out-of-scope follow-ups
+
+- Patch C v3 architectural win is the durable +73%. C / D / E / C2 / F / G are smaller cleanups that don't compound visibly.
+- Bug #5 RX-degradation campaign already closed (hypothesis falsified).
+- Task #24 (post-cleanup observation of bh.c symptom-shaped artifacts): mostly answered.
+- Task #25 (Allwinner sw_mci_check_r1_ready measurement): can be done during any future stress run; not on critical path.
+
+---
+
+*Phase 7 captured 2026-05-08 by Claude (noether). Patch C2 closes the post-Bug-#5 cleanup track. Throughput ceiling on this hardware = ~2.4 MB/s sustained @ 4 MB/s sender, fresh chip; further improvement would need firmware-side fixes (the wsm_generic_confirm 0x0007 path), not driver-side.*