RFC 016 Chunk 2c: approximate per-actor time-budget (reductions-like)

Accumulate on-CPU cycles per actor as ActorInfo.budget_cycles, behind
the off-by-default budget-accounting feature (D6) — a reductions-style
work metric for relative comparison across runs.

- Approximate by design (per Mark): charge now - slice-start once at the
  yield point, reusing the timestamp reset_timeslice already sets, so
  one RDTSC per resume not two. Wake-slot resumes inherit the slice and
  so slightly over-attribute the chain's time to the woken actor — noise
  that averages out; we trade exactness for half the hot-path cost.
- Field/ActorInfo member are unconditional (keeps the snapshot shape
  stable across the feature flag, D1); only the accumulation is gated,
  so default builds are byte-identical and pay nothing. Reads return 0
  when off. Single-writer Relaxed like the other counters; reset in
  reset_counters (D7).

Matrix: default + feature-on + rq-mpmc + rq-striped + release + trace all
green; loom unaffected (feature off under --cfg loom).

src/introspect.rs

@@ -91,6 +91,11 @@ pub struct ActorInfo {
     /// Chunk 2) — answers "is this actor a hotspot / draining slower than its
     /// mailbox fills." Counts received, not sent (D4). Per-incarnation (D7).
     pub messages_received: u64,
+    /// Approximate on-CPU cycles this incarnation has consumed (RFC 016 Chunk 2)
+    /// — a reductions-like work metric for relative comparison. Always 0 unless
+    /// the `budget-accounting` feature is enabled (it costs an RDTSC per resume,
+    /// D6). Per-incarnation (D7).
+    pub budget_cycles: u64,
 }
 
 /// A whole-runtime snapshot. See the module docs for the D2 tearing model.
@@ -170,6 +175,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
     // Counters are hot-region atomics, read lock-free (RFC 016 Chunk 2).
     let overruns = slot.overruns();
     let messages_received = slot.messages_received();
+    let budget_cycles = slot.budget_cycles();
 
     // Names + depth belong to this incarnation only if the registry mailbox's
     // pid matches the slab generation; a stale registry entry (dead prior
@@ -191,6 +197,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
         mailbox_depth,
         overruns,
         messages_received,
+        budget_cycles,
     })
 }
 

src/preempt.rs

@@ -166,6 +166,16 @@ pub fn reset_timeslice() {
     TIMESLICE_START.with(|c| c.set(rdtsc()));
 }
 
+/// Cycles elapsed since the current slice started (RFC 016 Chunk 2,
+/// `budget-accounting`). Read by the scheduler right after an actor yields back,
+/// on the same thread that armed `TIMESLICE_START`. Approximate for wake-slot
+/// resumes, which inherit the slice (see `Slot::add_budget`).
+#[cfg(feature = "budget-accounting")]
+#[inline]
+pub(crate) fn elapsed_slice_cycles() -> u64 {
+    rdtsc().saturating_sub(TIMESLICE_START.with(|c| c.get()))
+}
+
 #[inline(always)]
 pub fn rdtsc() -> u64 {
     unsafe {

src/runtime.rs

@@ -439,6 +439,12 @@ pub(crate) struct Slot {
     /// own thread, increments it on the receive path (D4/D5), Relaxed load+store
     /// with no RMW; snapshot reads Relaxed.
     messages_received: AtomicU64,
+    /// RFC 016 Chunk 2 — cumulative on-CPU cycles this incarnation has consumed
+    /// (the cycle-accurate "budget used", an analogue of OTP reductions).
+    /// Written only by the scheduler thread that ran the actor, once per resume,
+    /// and only when the `budget-accounting` feature is on (it costs two RDTSC
+    /// per resume); stays 0 otherwise. Same single-writer Relaxed discipline.
+    budget_cycles: AtomicU64,
     /// Cold lifecycle data. See [`SlotCold`].
     pub(crate) cold: RawMutex<SlotCold>,
 }
@@ -452,6 +458,7 @@ impl Slot {
             closure: AtomicPtr::new(std::ptr::null_mut()),
             overruns: AtomicU64::new(0),
             messages_received: AtomicU64::new(0),
+            budget_cycles: AtomicU64::new(0),
             cold: RawMutex::new(SlotCold {
                 actor: None,
                 waiters: Vec::new(),
@@ -511,6 +518,26 @@ impl Slot {
         self.messages_received.load(Ordering::Relaxed)
     }
 
+    /// Accumulate on-CPU cycles consumed in one resume (RFC 016 Chunk 2,
+    /// `budget-accounting`). Single-writer (the scheduler thread that ran the
+    /// actor), Relaxed load+store. Approximate by design: the figure is
+    /// `now − slice-start`, and a wake-slot resume inherits the waker's slice,
+    /// so a handed-off actor is charged a little of the chain's time — noise
+    /// that averages out across runs, traded for one RDTSC instead of two.
+    #[cfg(feature = "budget-accounting")]
+    #[inline]
+    pub(crate) fn add_budget(&self, cycles: u64) {
+        let v = self.budget_cycles.load(Ordering::Relaxed);
+        self.budget_cycles.store(v.wrapping_add(cycles), Ordering::Relaxed);
+    }
+
+    /// Read the accumulated budget cycles (Relaxed). Always 0 unless the
+    /// `budget-accounting` feature is enabled.
+    #[inline]
+    pub(crate) fn budget_cycles(&self) -> u64 {
+        self.budget_cycles.load(Ordering::Relaxed)
+    }
+
     /// Zero the per-actor introspection counters. Called at every point a slot
     /// is recycled or freshly occupied (`reclaim_slot`, `install_actor`) so a
     /// reused slot never carries a previous incarnation's counts — the standing
@@ -519,6 +546,7 @@ impl Slot {
     pub(crate) fn reset_counters(&self) {
         self.overruns.store(0, Ordering::Relaxed);
         self.messages_received.store(0, Ordering::Relaxed);
+        self.budget_cycles.store(0, Ordering::Relaxed);
     }
 
     /// A pid's-eye snapshot of the slot. Cold paths re-read this under the
@@ -1530,6 +1558,11 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
         unsafe { switch_to_actor() };
 
         PREEMPTION_ENABLED.with(|c| c.set(false));
+        // RFC 016 Chunk 2: charge the cycles this resume consumed to the actor
+        // (approximate; reuses the slice-start timestamp — one RDTSC). Read
+        // before the next resume re-arms TIMESLICE_START.
+        #[cfg(feature = "budget-accounting")]
+        slot.add_budget(crate::preempt::elapsed_slice_cycles());
         stats.current_pid_index.store(u32::MAX, Ordering::Relaxed);
         clear_current_pid();
         crate::preempt::clear_current_stop();