tmp: enable trace by default

.github/workflows/build.yml

@@ -5,7 +5,6 @@ on:
     branches: [ "master" ]
   pull_request:
     branches: [ "master" ]
-  workflow_dispatch:
 
 env:
   CARGO_TERM_COLOR: always
@@ -19,8 +18,5 @@ jobs:
     - uses: actions/checkout@v7
     - name: Install Mold
       run: sudo apt install mold
-    - name: Install Rust toolchain
-      uses: actions-rust-lang/setup-rust-toolchain@166cdcfd11aee3cb47222f9ddb555ce30ddb9659 # v1
-    
     - name: Build
       run: cargo build --verbose

.github/workflows/osv-scanner.yml

@@ -34,7 +34,7 @@ jobs:
       actions: read
     if: ${{ github.event_name == 'push' || github.event_name == 'schedule' }}
     # If you want to copy this config, highly suggest pinning this version to a release rather than tracking the main branch
-    uses: "google/osv-scanner-action/.github/workflows/osv-scanner-reusable.yml@v2.5.1"
+    uses: "google/osv-scanner-action/.github/workflows/osv-scanner-reusable.yml@v2.5.0"
     with:
       # Just scan the root directory and docs, since everything else is fixtures
       scan-args: |-
@@ -47,7 +47,7 @@ jobs:
       actions: read
     if: ${{ github.event_name == 'pull_request' || github.event_name == 'merge_group' }}
     # If you want to copy this config, highly suggest pinning this version to a release rather than tracking the main branch
-    uses: "google/osv-scanner-action/.github/workflows/osv-scanner-reusable-pr.yml@v2.5.1"
+    uses: "google/osv-scanner-action/.github/workflows/osv-scanner-reusable-pr.yml@v2.5.0"
     with:
       # Just scan the root directory and docs, since everything else is fixtures
       scan-args: |-

Cargo.lock

@@ -286,9 +286,9 @@ dependencies = [
 
 [[package]]
 name = "log"
-version = "0.4.34"
+version = "0.4.33"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f9f8bd3e56ce4dfc153cf470fffbfa98c7620958b312ca5c3a4b8d5181fd13c6"
+checksum = "0ceec5bc11778974d1bcb055b18002eba7f4b3518b6a0081b3af5f21666da9ad"
 
 [[package]]
 name = "maybe-async"
@@ -315,15 +315,15 @@ checksum = "68354c5c6bd36d73ff3feceb05efa59b6acb7626617f4962be322a825e61f79a"
 
 [[package]]
 name = "netlink-bindings"
-version = "0.3.4"
+version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2a2a0dd8beef757878b977fd3d662c82a93dac4bf7632f42577ca8f8f4d701a0"
+checksum = "114816d03a633cef7cfa951721dafbda16bb1ff6af83393ee568ec2d4e4ded2d"
 
 [[package]]
 name = "netlink-socket2"
-version = "0.3.4"
+version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5c7ddb0ea7aa692947d5105160acb7c75c17bc5ec6d5a6263440ca7db4be9a34"
+checksum = "11193969f046efafdc9d8670d395a6ddcf9b806169f504e520a35dc95cab3241"
 dependencies = [
  "libc",
  "maybe-async",
@@ -490,7 +490,7 @@ checksum = "e7a5d71263a5a7d47b41f6b3f06ba276f10cc18b0931f1799f710578e2309348"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 3.0.6",
+ "syn 3.0.3",
 ]
 
 [[package]]
@@ -563,6 +563,7 @@ dependencies = [
  "rust-uci",
  "rustix",
  "serde",
+ "serde_json",
  "thiserror",
  "uci-derive",
 ]
@@ -580,9 +581,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "3.0.6"
+version = "3.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8593e8e72159ed2257d083c7a454a85cbf854f37a0966d8d483aff8c8a3ebcee"
+checksum = "53e9bae58849f64dfa4f5d5ae372c8341f7305f82a3868709269343628b659a3"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -606,22 +607,22 @@ dependencies = [
 
 [[package]]
 name = "thiserror"
-version = "2.0.20"
+version = "2.0.19"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ec86235f5fcc2a73650310756d2ac5b138a5780bbbdfae3eeccec992c435ba4f"
+checksum = "09a43598840e33d5b0331f38c5e30d13bb11c11210a4b58f0d9b18a5a5eefcd9"
 dependencies = [
  "thiserror-impl",
 ]
 
 [[package]]
 name = "thiserror-impl"
-version = "2.0.20"
+version = "2.0.19"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bc04cd3e1236dd4a98afca4569f2deb3f120e5422a4023be2cb683f8486292af"
+checksum = "43cbfe0cf76104d42a574802844187e84a305e531ed54455f11fbde0f10541cd"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 3.0.6",
+ "syn 3.0.3",
 ]
 
 [[package]]
@@ -684,7 +685,7 @@ version = "0.1.0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 3.0.6",
+ "syn 3.0.3",
  "trybuild",
 ]
 

Cargo.toml

@@ -29,6 +29,7 @@ flume = { version = "0.12.0",  default-features = false, features = ["eventual-f
 config = { version = "0.15.25", default-features = false }
 csv = "1.4.0"
 serde = { version = "1", features = ["derive"] }
+serde_json = { version = "1", optional = true }
 uci-derive = { path = "crates/uci-derive" }
 
 [dependencies.log]
@@ -43,10 +44,11 @@ netlink-socket2 = { version = "0.3.3" }
 rust-uci = { git = "https://github.com/Lochnair/rust-uci.git", rev = "fcfedd204fb76928ef617bf9631b041c816e03d4", optional = true, default-features = false, }
 
 [features]
-default = ["toml", "uci-vendored"]
+default = ["toml", "trace", "uci-vendored"]
 toml = ["config/toml"]
 uci = ["dep:rust-uci"]
 uci-vendored = ["uci", "rust-uci/vendored"]
+trace = ["dep:serde_json"]
 
 [workspace]
 members = ["crates/uci-derive"]

README.md

@@ -10,6 +10,14 @@
 * **Configurable via environment or UCI**: reads settings from environment variables or OpenWRT UCI, with sensible defaults and type‑safe parsing .
 * **Minimal dependencies**: relies on Rust crates only (anyhow, socket2, neli, rustix), no external scripts .
 
+## Legacy control trace v1
+
+The additive legacy characterization recorder is excluded from default builds. Build with
+`--features trace` and set `SQMA_TRACE_V1_PATH` to a JSON Lines output path to enable it at
+runtime. The format is versioned as trace v1 and is intended only for replaying the current
+baseliner and rate-controller boundary during the `SqmControlLaw` extraction; it is not the
+final runtime event schema. Existing CSV and metrics outputs remain unchanged.
+
 ## Installation
 
 You can install **sqm‑autorate‑rust** in two ways:

renovate.json

@@ -2,18 +2,5 @@
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": [
     "config:recommended"
-  ],
-  "packageRules": [
-    {
-      "description": "Keep netlink-bindings and netlink-socket2 updates together",
-      "matchManagers": [
-        "cargo"
-      ],
-      "matchPackageNames": [
-        "netlink-bindings",
-        "netlink-socket2"
-      ],
-      "groupName": "netlink dependencies"
-    }
   ]
-}
+}

src/baseliner.rs

@@ -9,6 +9,7 @@ use crate::SHUTDOWN;
 use crate::metrics::{Metric, MetricsSender};
 use crate::pinger::PingReply;
 use crate::settings::Settings;
+use crate::trace::{Recorder, v1};
 use flume::{Receiver, Sender};
 use log::{debug, info};
 use std::collections::HashMap;
@@ -78,6 +79,7 @@ pub struct Baseliner {
     pub selection_tx: Option<Sender<ControlSnapshot>>,
     pub baseline_metrics: MetricsSender,
     pub event_metrics: MetricsSender,
+    pub trace: Recorder,
 }
 
 fn ewma_factor(tick: f64, dur: f64) -> f64 {
@@ -207,10 +209,23 @@ impl Baseliner {
             );
 
             let snapshot = control_snapshot(&reflectors, Instant::now());
-            if let Some(selection_tx) = &self.selection_tx {
+            self.trace.linearize(|trace| {
-                let _ = selection_tx.send(snapshot.clone());
+                if trace.is_enabled() {
-            }
+                    trace.record_at(
-            self.control_tx.send(snapshot)?;
+                        reply.last_receive_time_s,
+                        v1::Event::PingReply {
+                            reflector: reply.reflector,
+                            down_time_ms: reply.down_time,
+                            up_time_ms: reply.up_time,
+                        },
+                    );
+                }
+
+                if let Some(selection_tx) = &self.selection_tx {
+                    let _ = selection_tx.send(snapshot.clone());
+                }
+                self.control_tx.send(snapshot)
+            })?;
         }
     }
 }

src/main.rs

@@ -18,6 +18,7 @@ mod ratecontroller;
 mod reflector_selector;
 mod settings;
 mod time;
+mod trace;
 mod util;
 
 use crate::baseliner::Baseliner;
@@ -30,7 +31,7 @@ use crate::ratecontroller::Ratecontroller;
 use crate::reflector_selector::ReflectorSelector;
 use crate::settings::MeasurementType;
 use crate::settings::Settings;
-use ::log::{info, warn};
+use ::log::{debug, info, warn};
 use flume::RecvTimeoutError;
 use std::collections::HashMap;
 use std::net::IpAddr;
@@ -243,6 +244,7 @@ fn run(settings: &Settings) -> anyhow::Result<()> {
     }
 
     let start_time = Instant::now();
+    let trace = trace::Recorder::from_env(start_time, settings);
     let probe_id = probe_identifier();
 
     let ReflectorSetup {
@@ -334,6 +336,7 @@ fn run(settings: &Settings) -> anyhow::Result<()> {
         selection_tx: selection_snapshot_tx,
         baseline_metrics,
         event_metrics: event_metrics.clone(),
+        trace: trace.clone(),
     };
 
     let mut main_traffic_control = traffic_control_backend();
@@ -417,6 +420,7 @@ fn run(settings: &Settings) -> anyhow::Result<()> {
         reflector_peers,
         reselect_tx,
         rate_metrics,
+        trace.clone(),
     )?;
 
     let err_tx = error_tx.clone();
@@ -460,6 +464,8 @@ fn run(settings: &Settings) -> anyhow::Result<()> {
         &up_shaper,
     );
 
+    trace.finish();
+
     result
 }
 

src/ratecontroller.rs

@@ -14,6 +14,7 @@ use crate::platform::{
 };
 use crate::settings::Settings;
 use crate::time::Time;
+use crate::trace::{Recorder, v1};
 use crate::util::{ArcRwLock, RwLockExt};
 use flume::{Receiver, Sender};
 use log::{debug, info, warn};
@@ -145,9 +146,11 @@ pub struct Ratecontroller<S: InterfaceStatsProvider, T: TrafficControlBackend> {
     state_dl: State<T::Handle>,
     state_ul: State<T::Handle>,
     rng: fastrand::Rng,
+    next_evaluation_id: u64,
     stats_provider: S,
     traffic_control: T,
     metrics: MetricsSender,
+    trace: Recorder,
 }
 
 impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
@@ -160,6 +163,7 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
         mut stats_provider: S,
         mut traffic_control: T,
         mut rng: fastrand::Rng,
+        trace: Recorder,
     ) -> anyhow::Result<Self> {
         let dl_shaper =
             traffic_control.find_shaper(settings.network.download_interface.as_str())?;
@@ -178,7 +182,7 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
         let (cur_rx, cur_tx) =
             get_interface_stats(&mut stats_provider, &settings.network.upload_interface)?;
 
-        Ok(Self {
+        let controller = Self {
             settings,
             control_snapshot: None,
             control_snapshot_rx,
@@ -187,10 +191,32 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
             state_dl: State::new(dl_shaper, cur_rx, dl_safe_rates),
             state_ul: State::new(ul_shaper, cur_tx, ul_safe_rates),
             rng,
+            next_evaluation_id: 0,
             stats_provider,
             traffic_control,
             metrics,
-        })
+            trace,
+        };
+
+        if controller.trace.is_enabled() {
+            controller.trace.record_at(
+                Instant::now(),
+                v1::Event::ControllerInitialized {
+                    previous_rx_bytes: controller.state_dl.previous_bytes,
+                    previous_tx_bytes: controller.state_ul.previous_bytes,
+                    download_prev_mono_ns: controller
+                        .trace
+                        .monotonic_ns(controller.state_dl.prev_t),
+                    upload_prev_mono_ns: controller.trace.monotonic_ns(controller.state_ul.prev_t),
+                    download_history_index: controller.state_dl.nrate as u64,
+                    upload_history_index: controller.state_ul.nrate as u64,
+                    download_safe_rates_kbit: controller.state_dl.safe_rates.clone(),
+                    upload_safe_rates_kbit: controller.state_ul.safe_rates.clone(),
+                },
+            );
+        }
+
+        Ok(controller)
     }
 
     fn request_initial_rates(&mut self) -> anyhow::Result<()> {
@@ -198,6 +224,20 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
         self.state_dl.current_rate = self.settings.network.download_base_kbits * 0.6;
         self.state_ul.current_rate = self.settings.network.upload_base_kbits * 0.6;
 
+        if self.trace.is_enabled() {
+            self.trace.record_at(
+                Instant::now(),
+                v1::Event::RequestedRates {
+                    evaluation_id: None,
+                    reason: v1::RateRequestReason::Startup,
+                    download_kbit: self.state_dl.current_rate.round() as u64,
+                    upload_kbit: self.state_ul.current_rate.round() as u64,
+                    download_requested: true,
+                    upload_requested: true,
+                },
+            );
+        }
+
         self.traffic_control.set_rate(
             &self.state_dl.shaper,
             self.state_dl.current_rate.round() as u64,
@@ -212,7 +252,8 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
         Ok(())
     }
 
-    fn calculate_rate(&mut self, direction: Direction) -> anyhow::Result<()> {
+    fn calculate_rate(&mut self, direction: Direction, evaluation_id: u64) -> anyhow::Result<()> {
+        let trace = self.trace.clone();
         let (base_rate, delay_ms, min_rate, state) = if direction == Direction::Down {
             (
                 self.settings.network.download_base_kbits,
@@ -231,6 +272,20 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
 
         let now_t = Instant::now();
         let dur = now_t.duration_since(state.prev_t);
+        let trace_enabled = trace.is_enabled();
+        let trace_direction = match direction {
+            Direction::Down => v1::Direction::Download,
+            Direction::Up => v1::Direction::Upload,
+        };
+        if trace_enabled {
+            trace.record_at(
+                now_t,
+                v1::Event::RateCalculation {
+                    evaluation_id,
+                    direction: trace_direction,
+                },
+            );
+        }
 
         if !state.deltas.is_empty() {
             state.next_rate = state.current_rate;
@@ -269,11 +324,20 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
                 }
 
                 if state.delta_stat > delay_ms {
-                    match state
+                    let selected_index = self.rng.usize(..state.safe_rates.len());
-                        .safe_rates
+                    match state.safe_rates.get(selected_index) {
-                        .get(self.rng.usize(..state.safe_rates.len()))
-                    {
                         Some(rnd_rate) => {
+                            if trace_enabled {
+                                trace.record_at(
+                                    Instant::now(),
+                                    v1::Event::RandomSafeRateChoice {
+                                        evaluation_id,
+                                        direction: trace_direction,
+                                        index: selected_index as u64,
+                                        rate_kbit: *rnd_rate,
+                                    },
+                                );
+                            }
                             state.next_rate = rnd_rate.min(0.9 * state.current_rate * state.load);
                         }
                         None => {
@@ -349,7 +413,9 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
                     &mut self.stats_provider,
                     &self.settings.network.upload_interface,
                 )?;
-                self.update_deltas()?;
+                let evaluation_id = self.next_evaluation_id;
+                self.next_evaluation_id = self.next_evaluation_id.wrapping_add(1);
+                self.update_deltas(evaluation_id, now_t)?;
 
                 if self.state_dl.deltas.is_empty() || self.state_ul.deltas.is_empty() {
                     warn!("No reflector data available, dropping to minimum rates");
@@ -362,6 +428,20 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
                     self.state_dl.next_rate = self.settings.network.download_min_kbits();
                     self.state_ul.next_rate = self.settings.network.upload_min_kbits();
 
+                    if self.trace.is_enabled() {
+                        self.trace.record_at(
+                            Instant::now(),
+                            v1::Event::RequestedRates {
+                                evaluation_id: Some(evaluation_id),
+                                reason: v1::RateRequestReason::NoReflectorData,
+                                download_kbit: self.state_dl.next_rate as u64,
+                                upload_kbit: self.state_ul.next_rate as u64,
+                                download_requested: true,
+                                upload_requested: true,
+                            },
+                        );
+                    }
+
                     self.traffic_control.set_rate(
                         &self.state_dl.shaper,
                         self.state_dl.next_rate as u64,
@@ -378,8 +458,24 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
                     continue;
                 }
 
-                self.calculate_rate(Direction::Down)?;
+                self.calculate_rate(Direction::Down, evaluation_id)?;
-                self.calculate_rate(Direction::Up)?;
+                self.calculate_rate(Direction::Up, evaluation_id)?;
+
+                if self.trace.is_enabled() {
+                    let download_requested = self.state_dl.next_rate != self.state_dl.current_rate;
+                    let upload_requested = self.state_ul.next_rate != self.state_ul.current_rate;
+                    self.trace.record_at(
+                        Instant::now(),
+                        v1::Event::RequestedRates {
+                            evaluation_id: Some(evaluation_id),
+                            reason: v1::RateRequestReason::Control,
+                            download_kbit: self.state_dl.next_rate as u64,
+                            upload_kbit: self.state_ul.next_rate as u64,
+                            download_requested,
+                            upload_requested,
+                        },
+                    );
+                }
 
                 if self.state_dl.next_rate != self.state_dl.current_rate
                     || self.state_ul.next_rate != self.state_ul.current_rate
@@ -479,18 +575,36 @@ impl<S: InterfaceStatsProvider, T: TrafficControlBackend> Ratecontroller<S, T> {
         }
     }
 
-    fn update_deltas(&mut self) -> anyhow::Result<()> {
+    fn update_deltas(&mut self, evaluation_id: u64, loop_time: Instant) -> anyhow::Result<()> {
-        drain_latest_snapshot(&self.control_snapshot_rx, &mut self.control_snapshot);
+        let recorder = self.trace.clone();
-        let now_t = Instant::now();
+        recorder.linearize(|trace| -> anyhow::Result<()> {
-        let reflectors = self.reflectors_lock.read_anyhow()?;
+            drain_latest_snapshot(&self.control_snapshot_rx, &mut self.control_snapshot);
-        let (down_deltas, up_deltas) = deltas_from_snapshot(
+            let now_t = Instant::now();
-            self.control_snapshot.as_ref(),
+            let reflectors = self.reflectors_lock.read_anyhow()?;
-            &reflectors,
+            let (down_deltas, up_deltas) = deltas_from_snapshot(
-            now_t,
+                self.control_snapshot.as_ref(),
-            self.settings.advanced_settings.tick_interval,
+                &reflectors,
-        );
+                now_t,
-        self.state_dl.deltas = down_deltas;
+                self.settings.advanced_settings.tick_interval,
-        self.state_ul.deltas = up_deltas;
+            );
+            self.state_dl.deltas = down_deltas;
+            self.state_ul.deltas = up_deltas;
+
+            if trace.is_enabled() {
+                trace.record_at(
+                    now_t,
+                    v1::Event::ControlEvaluation {
+                        evaluation_id,
+                        loop_mono_ns: self.trace.monotonic_ns(loop_time),
+                        rx_bytes: self.state_dl.current_bytes,
+                        tx_bytes: self.state_ul.current_bytes,
+                        active_reflectors: reflectors.clone(),
+                    },
+                );
+            }
+
+            Ok(())
+        })?;
 
         if self.state_dl.deltas.len() < 5 || self.state_ul.deltas.len() < 5 {
             // trigger reselection
@@ -513,6 +627,7 @@ impl Ratecontroller<PlatformInterfaceStats, PlatformTrafficControl> {
         reflectors_lock: ArcRwLock<Vec<IpAddr>>,
         reselect_trigger: Sender<bool>,
         metrics: MetricsSender,
+        trace: Recorder,
     ) -> anyhow::Result<Self> {
         Self::new_with_backends(
             settings,
@@ -523,6 +638,7 @@ impl Ratecontroller<PlatformInterfaceStats, PlatformTrafficControl> {
             interface_stats_provider(),
             traffic_control_backend(),
             fastrand::Rng::new(),
+            trace,
         )
     }
 }

src/trace/mod.rs

@@ -0,0 +1,417 @@
+// SPDX-FileCopyrightText: 2026-Present Nils Andreas Svee mailto:contact@lochnair.net (github @Lochnair)
+//
+// SPDX-License-Identifier: MPL-2.0
+
+pub(crate) mod v1;
+
+#[cfg(feature = "trace")]
+mod writer;
+
+use crate::settings::Settings;
+use std::time::Instant;
+
+#[cfg(feature = "trace")]
+use std::time::SystemTime;
+
+#[cfg(feature = "trace")]
+use flume::Sender;
+#[cfg(feature = "trace")]
+use log::warn;
+#[cfg(feature = "trace")]
+use std::fs::File;
+#[cfg(feature = "trace")]
+use std::io::{BufWriter, Write};
+#[cfg(feature = "trace")]
+use std::path::Path;
+#[cfg(feature = "trace")]
+use std::sync::atomic::{AtomicBool, Ordering};
+#[cfg(feature = "trace")]
+use std::sync::{Arc, Mutex};
+#[cfg(feature = "trace")]
+use std::thread::JoinHandle;
+
+const TRACE_PATH_ENV: &str = "SQMA_TRACE_V1_PATH";
+#[cfg(feature = "trace")]
+const QUEUE_CAPACITY: usize = 4_096;
+
+#[derive(Clone, Default)]
+pub(crate) struct Recorder {
+    #[cfg(feature = "trace")]
+    inner: Option<Arc<Inner>>,
+}
+
+#[cfg(feature = "trace")]
+struct Inner {
+    origin: Instant,
+    records: Sender<v1::Record>,
+    next_seq: Mutex<u64>,
+    healthy: Arc<AtomicBool>,
+    writer: Mutex<Option<JoinHandle<()>>>,
+}
+
+pub(crate) struct OrderedRecorder<'a> {
+    #[cfg(feature = "trace")]
+    inner: Option<&'a Inner>,
+    #[cfg(feature = "trace")]
+    next_seq: Option<&'a mut u64>,
+    #[cfg(not(feature = "trace"))]
+    marker: std::marker::PhantomData<&'a ()>,
+}
+
+impl Recorder {
+    pub(crate) fn from_env(origin: Instant, settings: &Settings) -> Self {
+        let Some(path) = std::env::var_os(TRACE_PATH_ENV) else {
+            return Self::disabled();
+        };
+
+        #[cfg(feature = "trace")]
+        {
+            let header = v1::Event::Header {
+                format: v1::FORMAT_NAME.to_owned(),
+                version: v1::FORMAT_VERSION,
+                program_version: env!("CARGO_PKG_VERSION").to_owned(),
+                control: v1::ControlConfig::from_settings(settings),
+            };
+            Self::start_path(Path::new(&path), origin, header)
+        }
+
+        #[cfg(not(feature = "trace"))]
+        {
+            let _ = (origin, settings, path);
+            log::warn!(
+                "{TRACE_PATH_ENV} is set, but this binary was built without the `trace` feature"
+            );
+            Self::disabled()
+        }
+    }
+
+    pub(crate) fn disabled() -> Self {
+        Self::default()
+    }
+
+    pub(crate) fn is_enabled(&self) -> bool {
+        #[cfg(feature = "trace")]
+        {
+            self.inner
+                .as_ref()
+                .is_some_and(|inner| inner.healthy.load(Ordering::Acquire))
+        }
+
+        #[cfg(not(feature = "trace"))]
+        {
+            false
+        }
+    }
+
+    pub(crate) fn monotonic_ns(&self, instant: Instant) -> u64 {
+        #[cfg(feature = "trace")]
+        {
+            self.inner
+                .as_ref()
+                .map_or(0, |inner| v1::monotonic_ns(inner.origin, instant))
+        }
+
+        #[cfg(not(feature = "trace"))]
+        {
+            let _ = instant;
+            0
+        }
+    }
+
+    pub(crate) fn record_at(&self, instant: Instant, event: v1::Event) -> Option<u64> {
+        self.linearize(|recorder| recorder.record_at(instant, event))
+    }
+
+    pub(crate) fn linearize<R>(&self, callback: impl FnOnce(&mut OrderedRecorder<'_>) -> R) -> R {
+        #[cfg(feature = "trace")]
+        {
+            if self
+                .inner
+                .as_ref()
+                .is_some_and(|inner| inner.healthy.load(Ordering::Acquire))
+            {
+                let inner = self
+                    .inner
+                    .as_ref()
+                    .expect("healthy trace recorder must have inner state");
+                match inner.next_seq.lock() {
+                    Ok(mut next_seq) => {
+                        if inner.healthy.load(Ordering::Acquire) {
+                            let mut recorder = OrderedRecorder {
+                                inner: Some(inner),
+                                next_seq: Some(&mut next_seq),
+                            };
+                            return callback(&mut recorder);
+                        }
+                    }
+                    Err(_) => {
+                        if inner.healthy.swap(false, Ordering::AcqRel) {
+                            warn!("Legacy control trace sequencing lock failed; tracing disabled");
+                        }
+                    }
+                }
+            }
+
+            let mut recorder = OrderedRecorder {
+                inner: None,
+                next_seq: None,
+            };
+            callback(&mut recorder)
+        }
+
+        #[cfg(not(feature = "trace"))]
+        {
+            let mut recorder = OrderedRecorder {
+                marker: std::marker::PhantomData,
+            };
+            callback(&mut recorder)
+        }
+    }
+
+    pub(crate) fn finish(&self) {
+        #[cfg(feature = "trace")]
+        if let Some(inner) = self.inner.as_ref() {
+            self.linearize(|recorder| {
+                recorder.record_at(Instant::now(), v1::Event::End { clean: true });
+                inner.healthy.store(false, Ordering::Release);
+            });
+
+            let writer = match inner.writer.lock() {
+                Ok(mut writer) => writer.take(),
+                Err(_) => {
+                    warn!("Legacy control trace writer lock failed during shutdown");
+                    None
+                }
+            };
+            let writer_panicked = writer.map(|writer| writer.join().is_err()).unwrap_or(false);
+            if writer_panicked {
+                warn!("Legacy control trace writer panicked during shutdown");
+            }
+        }
+    }
+
+    #[cfg(feature = "trace")]
+    fn start_path(path: &Path, origin: Instant, header: v1::Event) -> Self {
+        let file = match File::options()
+            .create(true)
+            .write(true)
+            .truncate(true)
+            .open(path)
+        {
+            Ok(file) => file,
+            Err(error) => {
+                warn!(
+                    "Failed to open legacy control trace at {}: {error}; tracing disabled",
+                    path.display()
+                );
+                return Self::disabled();
+            }
+        };
+
+        Self::start_writer(BufWriter::new(file), origin, header)
+    }
+
+    #[cfg(feature = "trace")]
+    fn start_writer<W>(writer: W, origin: Instant, header: v1::Event) -> Self
+    where
+        W: Write + Send + 'static,
+    {
+        let (records, record_rx) = flume::bounded(QUEUE_CAPACITY);
+        let healthy = Arc::new(AtomicBool::new(true));
+        let header = v1::Record {
+            seq: 0,
+            timestamp: v1::timestamp(origin, origin, SystemTime::now()),
+            event: header,
+        };
+        let writer = match writer::spawn(writer, record_rx, header, Arc::clone(&healthy)) {
+            Ok(writer) => writer,
+            Err(error) => {
+                warn!("Failed to start legacy control trace writer: {error}; tracing disabled");
+                return Self::disabled();
+            }
+        };
+
+        Self {
+            inner: Some(Arc::new(Inner {
+                origin,
+                records,
+                next_seq: Mutex::new(1),
+                healthy,
+                writer: Mutex::new(Some(writer)),
+            })),
+        }
+    }
+}
+
+impl OrderedRecorder<'_> {
+    pub(crate) fn is_enabled(&self) -> bool {
+        #[cfg(feature = "trace")]
+        {
+            self.inner.is_some()
+        }
+
+        #[cfg(not(feature = "trace"))]
+        {
+            false
+        }
+    }
+
+    pub(crate) fn record_at(&mut self, instant: Instant, event: v1::Event) -> Option<u64> {
+        #[cfg(feature = "trace")]
+        {
+            let inner = self.inner?;
+            let next_seq = self.next_seq.as_deref_mut()?;
+            let seq = *next_seq;
+            let record = v1::Record {
+                seq,
+                timestamp: v1::timestamp(inner.origin, instant, SystemTime::now()),
+                event,
+            };
+
+            if inner.records.send(record).is_err() {
+                if inner.healthy.swap(false, Ordering::AcqRel) {
+                    warn!("Legacy control trace channel closed; tracing disabled");
+                }
+                return None;
+            }
+
+            *next_seq = (*next_seq).saturating_add(1);
+            Some(seq)
+        }
+
+        #[cfg(not(feature = "trace"))]
+        {
+            let _ = (instant, event);
+            None
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn disabled_recorder_is_a_no_op() {
+        let recorder = Recorder::disabled();
+
+        assert!(!recorder.is_enabled());
+        assert_eq!(
+            recorder.record_at(Instant::now(), v1::Event::End { clean: true }),
+            None
+        );
+        recorder.finish();
+    }
+
+    #[cfg(feature = "trace")]
+    mod enabled {
+        use super::*;
+        use std::io;
+        use std::net::{IpAddr, Ipv4Addr};
+        use std::path::PathBuf;
+        use std::sync::Barrier;
+        use std::sync::atomic::{AtomicU64, Ordering};
+        use std::thread;
+
+        static NEXT_PATH_ID: AtomicU64 = AtomicU64::new(0);
+
+        struct FailingWriter;
+
+        impl Write for FailingWriter {
+            fn write(&mut self, _buffer: &[u8]) -> io::Result<usize> {
+                Err(io::Error::other("injected writer failure"))
+            }
+
+            fn flush(&mut self) -> io::Result<()> {
+                Err(io::Error::other("injected writer failure"))
+            }
+        }
+
+        fn temp_path() -> PathBuf {
+            let id = NEXT_PATH_ID.fetch_add(1, Ordering::Relaxed);
+            std::env::temp_dir().join(format!(
+                "sqm-autorate-trace-v1-{}-{id}.jsonl",
+                std::process::id()
+            ))
+        }
+
+        fn ping_event(reflector: Ipv4Addr, value: f64) -> v1::Event {
+            v1::Event::PingReply {
+                reflector: IpAddr::V4(reflector),
+                down_time_ms: value,
+                up_time_ms: value,
+            }
+        }
+
+        #[test]
+        fn writer_preserves_global_sequence_order_from_multiple_producers() {
+            let path = temp_path();
+            let origin = Instant::now();
+            let recorder = Recorder::start_path(&path, origin, v1::tests::header_event());
+            let barrier = Arc::new(Barrier::new(3));
+            let mut producers = Vec::new();
+
+            for producer in 1..=2 {
+                let recorder = recorder.clone();
+                let barrier = Arc::clone(&barrier);
+                producers.push(thread::spawn(move || {
+                    barrier.wait();
+                    for value in 0..50 {
+                        recorder.record_at(
+                            origin,
+                            ping_event(Ipv4Addr::new(192, 0, 2, producer), value as f64),
+                        );
+                    }
+                }));
+            }
+
+            barrier.wait();
+            for producer in producers {
+                producer.join().unwrap();
+            }
+            recorder.finish();
+
+            let contents = std::fs::read_to_string(&path).unwrap();
+            let records = contents
+                .lines()
+                .map(|line| serde_json::from_str::<v1::Record>(line).unwrap())
+                .collect::<Vec<_>>();
+
+            assert_eq!(records.len(), 102);
+            assert!(matches!(
+                &records.first().unwrap().event,
+                v1::Event::Header { .. }
+            ));
+            assert!(matches!(
+                &records.last().unwrap().event,
+                v1::Event::End { clean: true }
+            ));
+            for (expected, record) in records.iter().enumerate() {
+                assert_eq!(record.seq, expected as u64);
+            }
+
+            std::fs::remove_file(path).unwrap();
+        }
+
+        #[test]
+        fn writer_failure_disables_recorder_without_propagating() {
+            let recorder =
+                Recorder::start_writer(FailingWriter, Instant::now(), v1::tests::header_event());
+
+            recorder.record_at(Instant::now(), ping_event(Ipv4Addr::new(192, 0, 2, 1), 1.0));
+            recorder.finish();
+
+            assert!(!recorder.is_enabled());
+        }
+
+        #[test]
+        fn open_failure_returns_disabled_recorder() {
+            let path = temp_path().join("missing-parent").join("trace.jsonl");
+            let recorder = Recorder::start_path(&path, Instant::now(), v1::tests::header_event());
+
+            assert!(!recorder.is_enabled());
+            recorder.record_at(Instant::now(), v1::Event::End { clean: true });
+            recorder.finish();
+        }
+    }
+}

src/trace/v1.rs

@@ -0,0 +1,234 @@
+// SPDX-FileCopyrightText: 2026-Present Nils Andreas Svee mailto:contact@lochnair.net (github @Lochnair)
+//
+// SPDX-License-Identifier: MPL-2.0
+
+use serde::{Deserialize, Serialize};
+use std::net::IpAddr;
+
+#[cfg(feature = "trace")]
+use crate::settings::Settings;
+#[cfg(any(feature = "trace", test))]
+use std::time::Duration;
+#[cfg(any(feature = "trace", test))]
+use std::time::Instant;
+#[cfg(feature = "trace")]
+use std::time::{SystemTime, UNIX_EPOCH};
+
+#[cfg(feature = "trace")]
+pub(crate) const FORMAT_VERSION: u16 = 1;
+#[cfg(feature = "trace")]
+pub(crate) const FORMAT_NAME: &str = "legacy-control-trace-v1";
+
+#[cfg(feature = "trace")]
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub(crate) struct Timestamp {
+    pub(crate) mono_ns: u64,
+    pub(crate) unix_us: i64,
+}
+
+#[cfg(feature = "trace")]
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub(crate) struct Record {
+    pub(crate) seq: u64,
+    pub(crate) timestamp: Timestamp,
+    #[serde(flatten)]
+    pub(crate) event: Event,
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub(crate) enum Event {
+    #[cfg(feature = "trace")]
+    Header {
+        format: String,
+        version: u16,
+        program_version: String,
+        control: ControlConfig,
+    },
+    PingReply {
+        reflector: IpAddr,
+        down_time_ms: f64,
+        up_time_ms: f64,
+    },
+    ControllerInitialized {
+        previous_rx_bytes: i128,
+        previous_tx_bytes: i128,
+        download_prev_mono_ns: u64,
+        upload_prev_mono_ns: u64,
+        download_history_index: u64,
+        upload_history_index: u64,
+        download_safe_rates_kbit: Vec<f64>,
+        upload_safe_rates_kbit: Vec<f64>,
+    },
+    ControlEvaluation {
+        evaluation_id: u64,
+        loop_mono_ns: u64,
+        rx_bytes: i128,
+        tx_bytes: i128,
+        active_reflectors: Vec<IpAddr>,
+    },
+    RateCalculation {
+        evaluation_id: u64,
+        direction: Direction,
+    },
+    RandomSafeRateChoice {
+        evaluation_id: u64,
+        direction: Direction,
+        index: u64,
+        rate_kbit: f64,
+    },
+    RequestedRates {
+        evaluation_id: Option<u64>,
+        reason: RateRequestReason,
+        download_kbit: u64,
+        upload_kbit: u64,
+        download_requested: bool,
+        upload_requested: bool,
+    },
+    End {
+        clean: bool,
+    },
+}
+
+#[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub(crate) enum Direction {
+    Download,
+    Upload,
+}
+
+#[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub(crate) enum RateRequestReason {
+    Startup,
+    Control,
+    NoReflectorData,
+}
+
+#[cfg(feature = "trace")]
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub(crate) struct ControlConfig {
+    pub(crate) tick_interval_secs: f64,
+    pub(crate) min_change_interval_secs: f64,
+    pub(crate) download_base_kbit: f64,
+    pub(crate) upload_base_kbit: f64,
+    pub(crate) download_min_kbit: f64,
+    pub(crate) upload_min_kbit: f64,
+    pub(crate) download_delay_ms: f64,
+    pub(crate) upload_delay_ms: f64,
+    pub(crate) high_load_level: f64,
+    pub(crate) speed_history_size: u32,
+}
+
+#[cfg(feature = "trace")]
+impl ControlConfig {
+    pub(crate) fn from_settings(settings: &Settings) -> Self {
+        Self {
+            tick_interval_secs: settings.advanced_settings.tick_interval,
+            min_change_interval_secs: settings.advanced_settings.min_change_interval,
+            download_base_kbit: settings.network.download_base_kbits,
+            upload_base_kbit: settings.network.upload_base_kbits,
+            download_min_kbit: settings.network.download_min_kbits(),
+            upload_min_kbit: settings.network.upload_min_kbits(),
+            download_delay_ms: settings.advanced_settings.download_delay_ms,
+            upload_delay_ms: settings.advanced_settings.upload_delay_ms,
+            high_load_level: settings.advanced_settings.high_load_level,
+            speed_history_size: settings.advanced_settings.speed_hist_size,
+        }
+    }
+}
+
+#[cfg(feature = "trace")]
+pub(crate) fn timestamp(origin: Instant, monotonic: Instant, realtime: SystemTime) -> Timestamp {
+    Timestamp {
+        mono_ns: monotonic_ns(origin, monotonic),
+        unix_us: unix_us(realtime),
+    }
+}
+
+#[cfg(any(feature = "trace", test))]
+pub(crate) fn monotonic_ns(origin: Instant, monotonic: Instant) -> u64 {
+    duration_ns(monotonic.saturating_duration_since(origin))
+}
+
+#[cfg(any(feature = "trace", test))]
+fn duration_ns(duration: Duration) -> u64 {
+    duration.as_nanos().min(u64::MAX as u128) as u64
+}
+
+#[cfg(feature = "trace")]
+fn unix_us(realtime: SystemTime) -> i64 {
+    match realtime.duration_since(UNIX_EPOCH) {
+        Ok(duration) => duration
+            .as_micros()
+            .min(i64::MAX as u128)
+            .try_into()
+            .unwrap_or(i64::MAX),
+        Err(error) => {
+            let micros = error.duration().as_micros().min(i64::MAX as u128) as i64;
+            -micros
+        }
+    }
+}
+
+#[cfg(test)]
+pub(super) mod tests {
+    use super::*;
+
+    #[cfg(feature = "trace")]
+    fn control_config() -> ControlConfig {
+        ControlConfig {
+            tick_interval_secs: 0.5,
+            min_change_interval_secs: 0.5,
+            download_base_kbit: 100_000.0,
+            upload_base_kbit: 20_000.0,
+            download_min_kbit: 20_000.0,
+            upload_min_kbit: 4_000.0,
+            download_delay_ms: 15.0,
+            upload_delay_ms: 15.0,
+            high_load_level: 0.8,
+            speed_history_size: 100,
+        }
+    }
+
+    #[cfg(feature = "trace")]
+    pub(crate) fn header_event() -> Event {
+        Event::Header {
+            format: FORMAT_NAME.to_owned(),
+            version: FORMAT_VERSION,
+            program_version: "test".to_owned(),
+            control: control_config(),
+        }
+    }
+
+    #[cfg(feature = "trace")]
+    #[test]
+    fn serializes_versioned_header() {
+        let origin = Instant::now();
+        let record = Record {
+            seq: 0,
+            timestamp: timestamp(origin, origin, UNIX_EPOCH + Duration::from_secs(1)),
+            event: header_event(),
+        };
+
+        let json = serde_json::to_value(&record).unwrap();
+
+        assert_eq!(json["seq"], 0);
+        assert_eq!(json["type"], "header");
+        assert_eq!(json["format"], FORMAT_NAME);
+        assert_eq!(json["version"], FORMAT_VERSION);
+        assert_eq!(json["timestamp"]["mono_ns"], 0);
+        assert_eq!(serde_json::from_value::<Record>(json).unwrap(), record);
+    }
+
+    #[test]
+    fn converts_existing_instants_relative_to_origin() {
+        let origin = Instant::now();
+
+        assert_eq!(
+            monotonic_ns(origin, origin + Duration::from_nanos(123_456)),
+            123_456
+        );
+        assert_eq!(monotonic_ns(origin, origin - Duration::from_nanos(1)), 0);
+    }
+}

src/trace/writer.rs

@@ -0,0 +1,54 @@
+// SPDX-FileCopyrightText: 2026-Present Nils Andreas Svee mailto:contact@lochnair.net (github @Lochnair)
+//
+// SPDX-License-Identifier: MPL-2.0
+
+use super::v1::{Event, Record};
+use flume::Receiver;
+use log::warn;
+use std::io::{self, Write};
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::thread::{self, JoinHandle};
+
+pub(super) fn spawn<W>(
+    writer: W,
+    records: Receiver<Record>,
+    header: Record,
+    healthy: Arc<AtomicBool>,
+) -> io::Result<JoinHandle<()>>
+where
+    W: Write + Send + 'static,
+{
+    thread::Builder::new()
+        .name("trace-v1-writer".to_owned())
+        .spawn(move || {
+            if let Err(error) = write_records(writer, records, header) {
+                warn!("Legacy control trace writer failed; tracing disabled: {error}");
+                healthy.store(false, Ordering::Release);
+            }
+        })
+}
+
+fn write_records<W>(mut writer: W, records: Receiver<Record>, header: Record) -> io::Result<()>
+where
+    W: Write,
+{
+    write_record(&mut writer, &header)?;
+
+    while let Ok(record) = records.recv() {
+        let clean_end = matches!(record.event, Event::End { clean: true });
+        write_record(&mut writer, &record)?;
+
+        if clean_end {
+            writer.flush()?;
+            return Ok(());
+        }
+    }
+
+    writer.flush()
+}
+
+fn write_record(writer: &mut impl Write, record: &Record) -> io::Result<()> {
+    serde_json::to_writer(&mut *writer, record).map_err(io::Error::other)?;
+    writer.write_all(b"\n")
+}