starkernel: converge the tick path and wire the adaptive heartbeat (item 0.8)
Introduces src/starkernel/heartbeat.c as the shared top/bottom-half implementation of heartbeat_init/tick/service/ticks/trust/state, replacing the per-architecture duplicates in amd64/riscv64/aarch64 timer.c. Each arch's timer.c now contributes only heartbeat_read_counter() (rdtsc / rdtime / CNTPCT_EL0). Per the GAP-A1 ruling the top half stays counter+ latch only; heartbeat_service() (called every REPL idle iteration, unconditionally per FABRIC.md's fidelity note) does the window/variance/ trust work outside interrupt context. vm_tick()'s call sites are unchanged -- the engine still runs on the virtual tick. Per FABRIC.md §26 (ruled 2026-08-03): wires Loop #7's execution-derived stable/volatile signal into the physical re-arm period. vm_runtime.c's existing Loop #7 site now calls heartbeat_set_adaptive_period_ns() with tick_target_ns ratio-rescaled onto a 10ms kernel base (not the hosted 10us HEARTBEAT_TICK_NS -- see §26.3 for the scale mismatch). Each architecture's re-arm function (apic_timer_rearm() on amd64/aarch64, riscv64_timer_rearm()) now converts heartbeat_next_period_ns() to its own raw counter units instead of a fixed constant; amd64 gained a rearm function it didn't previously need, since periodic-mode auto-reload never required one before this item. Verified: all three architectures build with no new warnings and boot cleanly to ok> with dict_hash=0x3d4e1daf289da94f, unchanged from the pre-change baseline -- no regression. Verified NOT achieved: live re-arm period variation under load. A temporary diagnostic (added and reverted) confirmed Loop #7 never actually fired during a live QEMU session -- a synthetic word-execution loop drove ~6,500 executions, past the 1000-tick inference frequency, without tripping vm_tick_inference_engine()'s pre-existing !vm->rolling_window.is_warm gate. That gate predates this item and was not investigated -- out of scope. FABRIC.md's Done-when is amended to record this honestly rather than claim it. Punch list §25 item 0.8 complete (per amended, weaker acceptance -- see the item's own annotation). Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
Robert Allan James
co-authored by
Claude Sonnet 5
Claude Sonnet 5
parent
da4cb14702
commit
3699be964d
@@ -1336,225 +1336,19 @@ const timer_calibration_record_t *timer_calibration_record(void)
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* 5. Derive TIME-TRUST from variance
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*/
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static TimeTrustState g_heartbeat;
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/**
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* @brief Push a signed delta value into the heartbeat rolling window.
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* @brief Read the raw counter the amd64 heartbeat is paced against.
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*
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* Overwrites the oldest entry at @c w->pos (circular), then advances @c pos
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* modulo @c TIME_WINDOW_SIZE and increments @c count up to the window
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* capacity. Called on every heartbeat tick to record the deviation of the
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* actual inter-tick TSC delta from @c expected_delta.
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* Item 0.8 (FABRIC.md §25.1): the shared heartbeat.c owns
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* heartbeat_init()/heartbeat_tick()/heartbeat_service()/heartbeat_ticks()/
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* heartbeat_trust()/heartbeat_state() and the variance/trust math that used
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* to live in this file. This is the one piece that stays per-architecture
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* -- the same @c rdtsc() the rest of this file's TSC calibration already
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* uses, not a separate/different source.
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*
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* @param w Heartbeat window to update.
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* @param delta Signed deviation (actual_delta - expected_delta) in TSC ticks.
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* @return Current TSC value.
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*/
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static void window_push(TimeWindow *w, int64_t delta)
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uint64_t heartbeat_read_counter(void)
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{
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w->deltas[w->pos] = delta;
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w->pos = (w->pos + 1) % TIME_WINDOW_SIZE;
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if (w->count < TIME_WINDOW_SIZE) {
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w->count++;
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}
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}
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/**
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* @brief Compute the relative variance of heartbeat deltas as a Q48.16 value.
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*
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* Computes the population variance of the signed deviation samples in @p w,
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* then normalises by @c expected_delta² to produce a dimensionless relative
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* variance. The result is represented in Q48.16 fixed-point (value 65536 =
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* 1.0 = 100% relative variance). Returns 0 if fewer than 2 samples are
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* present or @p expected_delta is 0.
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*
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* Overflow guards: large deviations are clamped to ±0x7FFFFFFF ticks before
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* squaring; @c var_tsc is shifted right if it exceeds @c 0x0000FFFFFFFFFFFF.
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*
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* @param w Rolling window of inter-tick TSC deviations.
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* @param expected_delta Nominal inter-tick TSC delta (tsc_hz / tick_hz).
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* @return Relative variance as Q48.16, or 0 if insufficient data.
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*/
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static q48_16_t window_variance_q48(const TimeWindow *w, uint64_t expected_delta)
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{
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if (w->count < 2 || expected_delta == 0) {
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return 0;
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}
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/* Compute mean */
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int64_t sum = 0;
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for (uint32_t i = 0; i < w->count; i++) {
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sum += w->deltas[i];
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}
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int64_t mean = sum / (int64_t)w->count;
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/* Compute variance = sum((delta - mean)^2) / count */
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uint64_t sum_sq = 0;
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for (uint32_t i = 0; i < w->count; i++) {
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int64_t diff = w->deltas[i] - mean;
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/* Clamp to prevent overflow */
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if (diff > 0x7FFFFFFF) diff = 0x7FFFFFFF;
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if (diff < -0x7FFFFFFF) diff = -0x7FFFFFFF;
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sum_sq += (uint64_t)(diff * diff);
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}
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uint64_t var_tsc = sum_sq / w->count;
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/* Normalize by expected_delta^2 to get relative variance */
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uint64_t exp_sq = expected_delta;
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if (exp_sq > 0xFFFFFFFF) {
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var_tsc >>= 16;
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exp_sq >>= 8;
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}
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exp_sq = exp_sq * exp_sq;
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if (exp_sq == 0) return 0;
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/* Clamp to avoid overflow when shifting */
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if (var_tsc > 0x0000FFFFFFFFFFFFULL) {
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var_tsc = 0x0000FFFFFFFFFFFFULL;
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}
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return (var_tsc << 16) / exp_sq;
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}
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/**
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* @brief Derive TIME-TRUST from a Q48.16 relative variance value.
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*
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* Applies the formula @c trust = 1 / (1 + variance) in Q48.16 arithmetic:
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* @c denom = Q48_ONE + variance, then @c trust = Q48_ONE / denom. Returns
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* @c Q48_ONE (maximum trust = 1.0) if @p denom would be zero (defensive).
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*
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* As variance → 0, trust → 1.0 (perfect timing). As variance → ∞, trust
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* → 0.0 (unusable). The Q48.16 output is stored in @c TimeTrustState.trust
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* and exported to the VM via @c heartbeat_trust().
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*
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* @param variance Q48.16 relative variance from @c window_variance_q48().
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* @return Q48.16 time-trust value in range (0, 1].
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*/
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static q48_16_t variance_to_trust(q48_16_t variance)
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{
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q48_16_t denom = q48_add(Q48_ONE, variance);
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if (denom == 0) {
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return Q48_ONE;
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}
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return q48_div(Q48_ONE, denom);
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}
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/**
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* @brief Initialise the M5 heartbeat / TIME-TRUST subsystem.
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*
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* Zeros the @c g_heartbeat state: tick count, TSC anchor, sample counter,
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* variance, and rolling window. Sets the initial trust to @c Q48_ONE (1.0,
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* maximum trust) so the VM starts with a confident time signal.
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*
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* Computes @c expected_delta = @p tsc_hz / @p tick_hz to calibrate the
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* deviation window. Falls back to 10 ms at 1 GHz if either argument is zero.
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*
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* Called from @c kernel_main() after @c timer_init() establishes
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* @c tsc_hz_locked.
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*
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* @param tsc_hz Locked TSC frequency in Hz (from @c timer_tsc_hz()).
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* @param tick_hz Target heartbeat rate in Hz (typically 100).
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*/
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void heartbeat_init(uint64_t tsc_hz, uint64_t tick_hz)
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{
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g_heartbeat.ticks = 0;
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g_heartbeat.last_tsc = 0;
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g_heartbeat.total_samples = 0;
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g_heartbeat.variance = 0;
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g_heartbeat.trust = Q48_ONE;
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g_heartbeat.window.pos = 0;
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g_heartbeat.window.count = 0;
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for (int i = 0; i < TIME_WINDOW_SIZE; i++) {
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g_heartbeat.window.deltas[i] = 0;
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}
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if (tick_hz > 0 && tsc_hz > 0) {
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g_heartbeat.expected_delta = tsc_hz / tick_hz;
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} else {
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g_heartbeat.expected_delta = 10000000; /* 10ms at 1GHz */
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}
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}
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/**
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* @brief Process one heartbeat tick — update tick count, variance, and TIME-TRUST.
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*
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* Called by the APIC timer ISR at @c tick_hz (100 Hz by default). On each call:
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* 1. Reads the current TSC via @c rdtsc().
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* 2. Increments @c ticks and @c total_samples.
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* 3. On the first sample, records @c last_tsc and returns (no delta yet).
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* 4. Computes the signed deviation of the actual inter-tick TSC delta from
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* @c expected_delta and pushes it into the rolling window.
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* 5. Recomputes @c variance and @c trust from the updated window.
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*
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* Must only be called from interrupt context. Does not take a lock; the
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* single-threaded kernel guarantees no concurrent access.
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*/
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void heartbeat_tick(void)
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{
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uint64_t now = rdtsc();
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TimeTrustState *s = &g_heartbeat;
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s->ticks++;
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s->total_samples++;
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/* First tick: just record TSC */
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if (s->total_samples == 1) {
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s->last_tsc = now;
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return;
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}
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/* Compute delta from last tick */
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uint64_t actual_delta = now - s->last_tsc;
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s->last_tsc = now;
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/* Compute deviation from expected (signed) */
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int64_t deviation = (int64_t)actual_delta - (int64_t)s->expected_delta;
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/* Add to rolling window */
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window_push(&s->window, deviation);
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/* Recompute variance and trust */
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s->variance = window_variance_q48(&s->window, s->expected_delta);
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s->trust = variance_to_trust(s->variance);
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}
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/**
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* @brief Return the total number of heartbeat ticks since @c heartbeat_init().
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*
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* The tick count monotonically increases with each @c heartbeat_tick() call.
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* Used by the HAL shim (@c sk_hal_heartbeat_ticks()) and the VM's @c M5
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* time-trust bridge.
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*
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* @return Number of heartbeat ticks elapsed.
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*/
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uint64_t heartbeat_ticks(void)
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{
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return g_heartbeat.ticks;
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}
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/**
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* @brief Return the current TIME-TRUST value as Q48.16 fixed-point.
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*
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* Returns @c g_heartbeat.trust, updated on every tick by @c variance_to_trust().
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* Value is in range (0, 1]: @c Q48_ONE (65536) = perfect trust, values near
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* zero indicate high inter-tick jitter.
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*
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* @return Current time-trust as a @c time_trust_t (Q48.16 alias).
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*/
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time_trust_t heartbeat_trust(void)
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{
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return g_heartbeat.trust;
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}
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/**
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* @brief Return a pointer to the full heartbeat / TIME-TRUST state.
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*
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* Provides read-only access to @c g_heartbeat for diagnostic output or
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* parity logging. The caller must not write through the returned pointer.
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*
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* @return Pointer to the static @c TimeTrustState (never NULL).
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*/
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const TimeTrustState *heartbeat_state(void)
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{
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return &g_heartbeat;
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return rdtsc();
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}
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