Measurements only, no behavior change:
- vsync.rs: EW lines (every early-waker landing, EW_ON flag, off by default)
- lib.rs/stg.rs: IT lines gain dec (locked/saw_wake/pr_before/do_render) and
pcpu (CPU us inside poll_events — 2% = asleep in epoll, rules out CPU
spin/deschedule as the wake latency)
Findings (docs/BUGS/hypotheses-120hz.md, sltest baseline):
- Choreographer callback fires at edge+0.52ms (p90 0.65) — the 'late
Choreographer' model from earlier sessions was a mislabeled STG field
(poll block time, not callback latency). The pipeline actually works at
median: present at edge+~4ms, inside the 5.5ms deadline.
- sltest zero-jank definition measured: 98.8% of frames land in the next
display slot; residual = 0.5-0.7% drops + 0.7-0.9% doubles + 1.4ms
phase spread (p10-p90) = the visible micro-judder.
- Residual cost is VARIANCE, not median: per-frame render 3.5ms median
(SKD 1.07 + CLR 0.8 + PRE 1.0 + SKF/ORD 0.65) with spikes to 10.7ms
(SKD max 5.2, CLR max 3.2, PRE max 2.5). Spikes push presents past the
deadline (drops) and shake the phase (judder).
Next: classify spikes (CPU work vs GPU/driver block) via render-window
CPU splits, then kill the top source.
