ROOT CAUSE of the 8-12% slot drops (uniform dec=1/0/1/0: locked, no wake
seen, dirty, no render): AOSP Looper::pollInner (system/core/libutils/
Looper.cpp) drains the wake fd in the SAME epoll batch (up to 16 fds) as
any ready input fd, and pollOnce returns the input's ident — the POLL_WAKE
result is a local that gets dropped. ALooper_wake's byte is consumed
WITHOUT PollEvent::Wake ever reaching Slint, unrecoverable. With motion
events arriving 1-4 ms after the edge (always for a real 120-240 Hz
finger; ~1 in 5 frames for 20 Hz adb input) the wake is lost and the
locked decision (saw_wake && slot-guard) drops the dirty frame for that
slot.
Fix (vendor backend): a dedicated eventfd registered with the MAIN looper
under ALOOPER_POLL_CALLBACK. The Looper invokes the C trampoline INSIDE
pollInner (before pollOnce returns, regardless of which ident is handed
back), so delivery can never be swallowed by a co-batched input response.
- frame_callback: note the edge, write the eventfd, then ALooper_wake
(secondary). The fd is leaked: it belongs to the main looper for the
process lifetime.
- trampoline: DRAINS the eventfd (level-triggered — without the drain the
counter stays >0, the fd stays readable and the loop busy-spins on
POLL_CALLBACK at ~10 kHz: 1812 spin iterations measured) and publishes
the edge into delivered_edge.
- main loop: saw_wake |= (delivered_edge newer than last seen).
- DIRTY BACKSTOP: the 6 ms poll cap was never a backstop — timeout
iterations have saw_wake=false so the locked decision never rendered
(the 20-40 ms silences). Now: dirty && last render >= 2 slots old
renders regardless of wake state.
Measured on the phone (Pixel 9 Pro, 120 Hz, sltest, identical capture):
buildC (before): 127 drops + 12 stalls in 1142 frames
buildE (after): 8 drops + 0 stalls in 1266 frames
Phase (p10-p90): 0.72-1.00 ms in all windows. Remaining 8/1266 (0.63%)
are slip-and-recover pairs where the UI thread is DESCHEDULED 6-8 ms
while sleeping in epoll (wakes arrive on time, cb-lat 0.5 ms) — EAS
scheduling variance, no app-side contention in sltest.
Referee fixes found while judging this data (tools/jank_eval.py):
- NEAREST-OBSERVED-EDGE slot grid: the 120 Hz panel clock phase-slips
after idle blanking (re-locks at arbitrary phase), invalidating any
grid anchored at ftns[0]. An entire drag window read late=230/
dropped=24 while per-cycle tracing showed presents at edge+2.0 ms.
slot_of_ftn assigns each present to the nearest observed hardware
edge in a time-continuous slot domain (slip-immune: a constant delta
shifts all slots, differences preserved; stall-accurate: ftn times
are CLOCK_MONOTONIC).
- tools/jank_capture.py: capture is INVALID unless the screen is Awake
AND the keyguard is down (a run on a sleeping phone silently injects
no input: 49 IT lines, 1 present).
Greg Pomerantz
0a50f9b832