perf(latency): tier-0 attribution + tier-1 send-path levers from the latency plan

design/latency-reduction-2026-07.md T0.1/T0.2/T1.2/T1.3:

- T1.2 rate-capped front-loaded pacing: the paced overflow's budget is now
  min(0.9x slack, overflow wire time at ~3x the live encoder bitrate)
  (PUNKTFUNK_PACE_FACTOR, 0 = legacy deadline-only spread). A 300 KB-1 MB
  frame's tail leaves in ~2-5 ms instead of smearing across ~15 ms at 60 fps;
  GameStream schedule byte-identical (pins unchanged).
- T1.3 data-first wire order: packetize emits every block's data shards before
  any parity (per-block parity pools keep all blocks' parity alive for the
  second pass), so lossless completion stops waiting behind the parity tail.
  EOF = last emitted packet; receiver already order-agnostic.
- T0.1 staged 0xCF: HostTiming gains an append-extensible per-stage tail
  (queue/encode/pace us; seal+channel-wait derived as residual) - no cap bit
  needed, old peers read the 13-byte prefix. Joined client-side into
  Stats::host_{queue,encode,xfer,pace}_ms, the OSD detailed tier, and the
  probe's report.
- T0.2 true on-glass present timing: VK_KHR_present_id/present_wait enabled
  when supported; a PresentTimer waiter thread resolves each present id to
  real visibility, replacing the submit-time display stamp (which undercounts
  by up to a refresh and hides a silent-FIFO standing queue).

Validated on .21: core 185 + host 185 tests, pf-presenter 19, clippy
-D warnings across all five touched crates; loss-harness recovery curve
unchanged; C ABI harness round-trips.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-presenter/src/vk/mod.rs

@@ -28,6 +28,7 @@ use pf_client_core::video::{CpuFrame, VkVideoFrame};
 mod gpu;
 mod overlay_pipe;
 mod present;
+mod present_timing;
 mod reconfig;
 mod resources;
 mod setup;
@@ -187,6 +188,16 @@ pub struct Presenter {
     /// The submit fence has a submission pending (wait before recording again — also
     /// what makes the single staging buffer safe to overwrite).
     submitted: bool,
+    /// `VK_KHR_present_wait` on-glass timing (latency plan T0.2) — `None` when the
+    /// device lacks the present-id/present-wait pair; the run loop then keeps its
+    /// submit-time display stamp.
+    present_timer: Option<present_timing::PresentTimer>,
+    /// Monotonic present id (global counter — strictly increasing per swapchain, which
+    /// is all the spec asks). 0 = nothing presented with an id yet.
+    next_present_id: u64,
+    /// The last successful id-carrying present, awaiting its [`Presenter::note_presented`]
+    /// claim from the run loop (which owns the frame's pts/decode stamps).
+    last_presented: Option<(vk::SwapchainKHR, u64)>,
 }
 
 impl Presenter {
@@ -220,6 +231,30 @@ impl Presenter {
         unsafe { self.device.device_wait_idle() }.ok();
     }
 
+    /// True when `VK_KHR_present_wait` drives the display stamp — the run loop then
+    /// defers its e2e/display windows to [`Presenter::take_presented_samples`] instead
+    /// of stamping at `present()` return.
+    pub(crate) fn present_timing_active(&self) -> bool {
+        self.present_timer.is_some()
+    }
+
+    /// Claim the just-submitted present for on-glass timing. Call right after a
+    /// `present()` that returned `true`, with that frame's capture + decode stamps
+    /// (the presenter itself never sees them). No-op when timing is inactive.
+    pub(crate) fn note_presented(&mut self, pts_ns: u64, decoded_ns: u64) {
+        if let (Some(t), Some((sc, id))) = (&self.present_timer, self.last_presented.take()) {
+            t.enqueue(sc, id, pts_ns, decoded_ns);
+        }
+    }
+
+    /// Take the window's completed on-glass samples (empty when timing is inactive).
+    pub(crate) fn take_presented_samples(&self) -> Vec<present_timing::PresentedSample> {
+        self.present_timer
+            .as_ref()
+            .map(|t| t.take_samples())
+            .unwrap_or_default()
+    }
+
     /// The device handles the console-UI overlay renders on (§6.1). Valid for the
     /// presenter's lifetime; the run loop drops the overlay first.
     pub fn shared_device(&self) -> SharedDevice {
@@ -237,6 +272,10 @@ impl Presenter {
 
 impl Drop for Presenter {
     fn drop(&mut self) {
+        // The present-wait waiter references the swapchain — stop it (its Drop joins
+        // after in-flight waits complete, bounded by their 250 ms cap) BEFORE the
+        // swapchain teardown below.
+        self.present_timer.take();
         unsafe {
             {
                 // Insurance against a straggling submitter (the run loop joins the

crates/pf-presenter/src/vk/present.rs

@@ -569,20 +569,32 @@ impl Presenter {
             let swapchains = [self.swapchain];
             let indices = [index];
             let present_sems = [render_sem];
+            // On-glass timing (T0.2): attach a monotonically increasing present id the
+            // PresentTimer's `vkWaitForPresentKHR` resolves to real visibility.
+            let ids = [self.next_present_id + 1];
+            let mut pid_info = vk::PresentIdKHR::default().present_ids(&ids);
+            let mut present_info = vk::PresentInfoKHR::default()
+                .wait_semaphores(&present_sems)
+                .swapchains(&swapchains)
+                .image_indices(&indices);
+            if self.present_timer.is_some() {
+                self.next_present_id += 1;
+                present_info = present_info.push_next(&mut pid_info);
+            }
             // Same queue external-sync rule as the submit above. Scoped tightly: the
             // OUT_OF_DATE arm re-enters the lock via recreate_swapchain's queue drain.
             let present_res = {
                 let _q = self.queue_lock.guard();
-                self.swap_d.queue_present(
-                    self.queue,
-                    &vk::PresentInfoKHR::default()
-                        .wait_semaphores(&present_sems)
-                        .swapchains(&swapchains)
-                        .image_indices(&indices),
-                )
+                self.swap_d.queue_present(self.queue, &present_info)
             };
             match present_res {
-                Ok(_) => Ok(true),
+                Ok(_) => {
+                    // A failed present's id may never signal — claimable only on Ok.
+                    if self.present_timer.is_some() {
+                        self.last_presented = Some((self.swapchain, self.next_present_id));
+                    }
+                    Ok(true)
+                }
                 Err(vk::Result::ERROR_OUT_OF_DATE_KHR) => {
                     self.recreate_swapchain(window)?;
                     Ok(false)

crates/pf-presenter/src/vk/present_timing.rs

@@ -0,0 +1,132 @@
+//! True on-glass present timing via `VK_KHR_present_wait` (latency plan T0.2).
+//!
+//! The render loop's `displayed` stamp is taken when `vkQueuePresentKHR` *returns* — CPU
+//! submit time, excluding the presentation engine's queue and the vblank latch, so the
+//! HUD's `display` stage under-reports by up to a refresh (and hides a silent-FIFO
+//! standing queue entirely). When the device offers `VK_KHR_present_id` +
+//! `VK_KHR_present_wait`, each present carries a monotonically increasing id and a
+//! dedicated waiter thread blocks in `vkWaitForPresentKHR` — which completes when the
+//! image is actually visible — stamping the REAL on-glass time off the render loop.
+//!
+//! Lifecycle: `vkWaitForPresentKHR` requires the swapchain to stay alive for the call's
+//! duration, so [`PresentTimer::drain`] must run before any `vkDestroySwapchainKHR`
+//! (recreate and teardown both do). Waits carry a 250 ms cap: presentation ids complete
+//! in submission order (a MAILBOX-replaced image's id completes with the present that
+//! replaced it), so a wait only outlives that cap when the pipeline is already wedged —
+//! the timeout keeps the drain bounded rather than wedging a resize with it.
+
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::{mpsc, Arc, Mutex};
+
+use ash::vk;
+
+/// One presented frame's identity + the true on-glass stamp the waiter filled in.
+pub(crate) struct PresentedSample {
+    /// The frame's capture stamp (host clock) — the e2e anchor.
+    pub pts_ns: u64,
+    /// Decode-complete stamp (client clock) — the display-stage anchor.
+    pub decoded_ns: u64,
+    /// `vkWaitForPresentKHR` completion = the image is visible (client clock).
+    pub displayed_ns: u64,
+}
+
+struct Job {
+    swapchain: vk::SwapchainKHR,
+    present_id: u64,
+    pts_ns: u64,
+    decoded_ns: u64,
+}
+
+/// The waiter: a channel-fed thread turning (swapchain, present-id) pairs into
+/// [`PresentedSample`]s. One frame in flight upstream keeps the queue depth ~1.
+pub(crate) struct PresentTimer {
+    tx: Option<mpsc::Sender<Job>>,
+    /// Jobs enqueued but not yet finished — the drain barrier for swapchain teardown.
+    pending: Arc<AtomicUsize>,
+    results: Arc<Mutex<Vec<PresentedSample>>>,
+    join: Option<std::thread::JoinHandle<()>>,
+}
+
+impl PresentTimer {
+    pub(crate) fn spawn(wait_d: ash::khr::present_wait::Device) -> Self {
+        let (tx, rx) = mpsc::channel::<Job>();
+        let pending = Arc::new(AtomicUsize::new(0));
+        let results = Arc::new(Mutex::new(Vec::with_capacity(256)));
+        let (pending_t, results_t) = (pending.clone(), results.clone());
+        let join = std::thread::Builder::new()
+            .name("pf-present-wait".into())
+            .spawn(move || {
+                while let Ok(job) = rx.recv() {
+                    // 250 ms cap — see the module doc's lifecycle note.
+                    let r = unsafe {
+                        wait_d.wait_for_present(job.swapchain, job.present_id, 250_000_000)
+                    };
+                    if r.is_ok() {
+                        let displayed_ns = pf_client_core::session::now_ns();
+                        results_t.lock().unwrap().push(PresentedSample {
+                            pts_ns: job.pts_ns,
+                            decoded_ns: job.decoded_ns,
+                            displayed_ns,
+                        });
+                    }
+                    // SUBOPTIMAL/TIMEOUT/DEVICE_LOST: no sample; the frame still showed
+                    // (or the loop is about to find out) — never poison the window.
+                    pending_t.fetch_sub(1, Ordering::AcqRel);
+                }
+            })
+            .expect("spawn pf-present-wait");
+        PresentTimer {
+            tx: Some(tx),
+            pending,
+            results,
+            join: Some(join),
+        }
+    }
+
+    /// Hand a successfully submitted present to the waiter.
+    pub(crate) fn enqueue(
+        &self,
+        swapchain: vk::SwapchainKHR,
+        present_id: u64,
+        pts_ns: u64,
+        decoded_ns: u64,
+    ) {
+        if let Some(tx) = &self.tx {
+            self.pending.fetch_add(1, Ordering::AcqRel);
+            if tx
+                .send(Job {
+                    swapchain,
+                    present_id,
+                    pts_ns,
+                    decoded_ns,
+                })
+                .is_err()
+            {
+                self.pending.fetch_sub(1, Ordering::AcqRel);
+            }
+        }
+    }
+
+    /// Block until no wait references any swapchain — REQUIRED before
+    /// `vkDestroySwapchainKHR`. Bounded by the waiter's own 250 ms wait cap.
+    pub(crate) fn drain(&self) {
+        while self.pending.load(Ordering::Acquire) > 0 {
+            std::thread::sleep(std::time::Duration::from_millis(1));
+        }
+    }
+
+    /// Take the window's completed samples (called at the 1 s stats fold).
+    pub(crate) fn take_samples(&self) -> Vec<PresentedSample> {
+        std::mem::take(&mut *self.results.lock().unwrap())
+    }
+}
+
+impl Drop for PresentTimer {
+    fn drop(&mut self) {
+        // Close the channel, let in-flight waits finish (bounded), then join.
+        self.tx.take();
+        if let Some(j) = self.join.take() {
+            let _ = j.join();
+        }
+    }
+}

crates/pf-presenter/src/vk/reconfig.rs

@@ -71,6 +71,14 @@ impl Presenter {
         // The old swapchain and everything tied to its images dies NOW: the fence
         // quiesce covered our own command buffers, the queue drain above covered the
         // presentation engine's semaphore waits — nothing can still reference them.
+        // The present-wait waiter is the one remaining referent: `vkWaitForPresentKHR`
+        // requires the swapchain alive for the call, so drain it first (bounded by the
+        // waiter's 250 ms cap; ids complete in order so this is normally instant).
+        if let Some(t) = &self.present_timer {
+            t.drain();
+        }
+        // An unclaimed last present belonged to the dying swapchain — drop the claim.
+        self.last_presented = None;
         let (overlay_views, overlay_framebuffers) = self.overlay_pipe.take_targets();
         unsafe {
             for fb in overlay_framebuffers {

crates/pf-presenter/src/vk/setup.rs

@@ -129,17 +129,30 @@ impl Presenter {
         let dev_props = unsafe { instance.get_physical_device_properties(pdev) };
         let dev_is_13 = vk::api_version_major(dev_props.api_version) > 1
             || vk::api_version_minor(dev_props.api_version) >= 3;
+        let mut have_pid = vk::PhysicalDevicePresentIdFeaturesKHR::default();
+        let mut have_pwait = vk::PhysicalDevicePresentWaitFeaturesKHR::default();
         let mut have_f11 = vk::PhysicalDeviceVulkan11Features::default();
         let mut have_f12 = vk::PhysicalDeviceVulkan12Features::default();
         let mut have_f13 = vk::PhysicalDeviceVulkan13Features::default();
+        // Present-id/present-wait (on-glass timing, latency plan T0.2): query the feature
+        // structs only when the device lists both extensions.
+        let present_wait_exts =
+            has(ash::khr::present_id::NAME) && has(ash::khr::present_wait::NAME);
         let mut have_f2 = vk::PhysicalDeviceFeatures2::default()
             .push_next(&mut have_f11)
             .push_next(&mut have_f12)
             .push_next(&mut have_f13);
+        if present_wait_exts {
+            have_f2 = have_f2.push_next(&mut have_pid).push_next(&mut have_pwait);
+        }
         unsafe { instance.get_physical_device_features2(pdev, &mut have_f2) };
-        // Copy the one base-features fact out NOW: `have_f2` mutably borrows the 11/12/13
-        // structs through its pNext chain, so any later use of it would pin those borrows.
+        // Copy the one base-features fact out NOW: `have_f2` mutably borrows the chained
+        // structs through its pNext chain, so any later use of it would pin those borrows —
+        // every read of a chained struct below must come after this, have_f2's last use.
         let have_shader_int16 = have_f2.features.shader_int16;
+        let present_wait_ok = present_wait_exts
+            && have_pid.present_id == vk::TRUE
+            && have_pwait.present_wait == vk::TRUE;
         let features_ok = have_f11.sampler_ycbcr_conversion == vk::TRUE
             && have_f12.timeline_semaphore == vk::TRUE
             && have_f13.synchronization2 == vk::TRUE;
@@ -224,6 +237,15 @@ impl Presenter {
             );
         }
 
+        // Present-id/present-wait: enable when fully supported — the presenter then runs
+        // the on-glass PresentTimer; otherwise the display stamp stays submit-time.
+        if present_wait_ok {
+            dev_exts.push(ash::khr::present_id::NAME.as_ptr());
+            dev_exts.push(ash::khr::present_wait::NAME.as_ptr());
+        }
+        let mut en_pid = vk::PhysicalDevicePresentIdFeaturesKHR::default().present_id(true);
+        let mut en_pwait = vk::PhysicalDevicePresentWaitFeaturesKHR::default().present_wait(true);
+
         // Enable only the features the video path needs, and only where supported
         // (harmless when the path is off; reported to FFmpeg via device_features).
         let mut en_f11 = vk::PhysicalDeviceVulkan11Features::default()
@@ -240,6 +262,9 @@ impl Presenter {
             .push_next(&mut en_f11)
             .push_next(&mut en_f12)
             .push_next(&mut en_f13);
+        if present_wait_ok {
+            en_f2 = en_f2.push_next(&mut en_pid).push_next(&mut en_pwait);
+        }
         en_f2.features.shader_int16 = if pyrowave_ok { vk::TRUE } else { vk::FALSE };
 
         let priorities = [1.0f32];
@@ -265,6 +290,15 @@ impl Presenter {
         }
         .context("vkCreateDevice")?;
         let swap_d = ash::khr::swapchain::Device::new(&instance, &device);
+        let present_timer = present_wait_ok.then(|| {
+            super::present_timing::PresentTimer::spawn(ash::khr::present_wait::Device::new(
+                &instance, &device,
+            ))
+        });
+        tracing::info!(
+            present_wait = present_wait_ok,
+            "on-glass present timing (VK_KHR_present_wait)"
+        );
         let hdr_metadata_d =
             has_hdr_metadata.then(|| ash::ext::hdr_metadata::Device::new(&instance, &device));
         let queue = unsafe { device.get_device_queue(qfi, 0) };
@@ -441,6 +475,9 @@ impl Presenter {
             staging: None,
             video: None,
             submitted: false,
+            present_timer,
+            next_present_id: 0,
+            last_presented: None,
         };
         p.recreate_swapchain(window)?;
         Ok(p)