Chinese version: 慢速从屏撕裂问题分析：显示背压与 NO_FENCE 释放

Purpose and audience

What this document covers

• How debug.sf.follower_display_backpressure in Scheduler::onFrameSignal can skip follower work for a frame.
• How CompositionEngine::postComposition can still fulfill release-fence promises on that path, often with Fence::NO_FENCE.
• How that combination can relate to tearing on a slower follower display.

Stance and scope

• Root cause is treated as confirmed from Trace on release fence.
• Author-verified repro: pacesetter VSync 90 Hz, follower display 45 Hz, and the app on the follower producing 90 FPSpaced work — tearing observed. Other skews (e.g. same nominal 90 Hz / 90 FPS with follower present fences still trailing pacesetter onFrameSignal) are noted in §5 as additional context, not a substitute for that verified setup.

Source references

• Code excerpts: AOSP platform/frameworks/native, branch android16-qpr2-release.
• Browser links: android-latest-release on Android Code Search (cs.android.com) (public snapshot; use those URLs to open the exact revision and range in the viewer).

1. Executive summary

Scheduler / follower skip (§2 — Scheduler.cpp, onFrameSignal)

• In multi-display mode, the pacesetter runs beginFrame first; onFrameSignal then walks follower displays.
• With follower_display_backpressure, each follower consults countPresentFencesPendingAt (unfinished present-related fences).
• Backpressure skip is driven only by that count: if countPresentFencesPendingAt is zero, this path does not skip—the follower still gets beginFrame.
• If the pending count is greater than zero, the code by default continues for that follower for this frame (no targeter.beginFrame).
• Single tolerance (exception): if exactly one fence is pending and the next follower vsync deadline (from vsyncDeadlineAfter(frameBeginTime)) is before the pacesetter’s expectedVsyncTime for this signal—i.e. the follower’s next vsync arrives sooner than the pacesetter’s next vsync—then the scheduler does not continue and still calls beginFrame. Otherwise, any non-zero pending count triggers the skip.