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Stage C-b: compudynamic switch decision, idle point, timer path removed (FABRIC-3.8 §3.13.3-§3.13.5)

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Who runs next is now sk_vm_switch_decide(): among participants with work
(pending kernel-Hermes mail, parked mid-task, or Hera with console input),
the coolest by VM-physics heat; ties with the incumbent keep the incumbent,
ties among candidates go to the longest waiting (unique wait stamps). No
scheduler state, no timer: heartbeat_tick() no longer touches switching,
readiness/threshold/has_work/take_pending removed. Decided at owned word
checkpoints (colon words and, new, between top-level words) after a beat
or new mail, at the trampoline between messages, and at Hera's idle point
in the console poll loops. The idle pump skips participants (G3 closed for
them): the WIREBIND relay is delivered by a real switch to rajames~user.

Probe (restored away): relay via switch Hera->rajames~user->Hera on all
three ISAs, parked saved_sp inside own stack, walk-order disagreements 0
over tens of millions of decisions, 0 bytes idle log growth. Acceptance:
dict_hash unchanged, relay 3, no faults. Rulings: C-3, C-5 as proposed;
C-4 taken as (alpha), consequence recorded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Robert Allan James
2026-09-23 17:53:55 -04:00
co-authored by Claude Opus 5.5
parent 7c3736841d
commit 06bcab504d
27 changed files with 64636 additions and 218 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/starkernel/vm/switch.c
+86 -17
View File
@@ -32,6 +32,8 @@
#include "starkernel/capsule_birth.h" /* capsule_vm_set_state */
#include "starkernel/vm/kernel_hermes.h" /* FABRIC-3.8.md §3.2 -- sk_hermes_drain_checkpoint() */
#include "starkernel/hal/hal.h" /* sk_hal_panic() */
#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.8.md §3.13.3 -- the decision */
#include "starkernel/vm/bootstrap/sk_vm_bootstrap.h" /* sk_get_mama_vm() -- the idle root */
#include <stdint.h>
/* Arch-specific (switch.S in each arch dir). sk_vm_switch_prime() builds
@@ -121,39 +123,105 @@ void sk_vm_switch_set_current(VM *vm) { g_switch_current_vm = vm; }
* indistinguishable from a switch storm. A failed yield is a kernel
* invariant violation, not something to retry, so it panics loudly.
*
* Deliberately NOT here yet (FABRIC-3.8.md §3.11, a Stage C design
* question): this point between messages is also a clean switch point
* (own stack, depth 0), and the compudynamic switch decision should be
* consulted here too, so a VM with a queue of checkpoint-free payloads
* cannot hold the CPU indefinitely.
* Stage C-b (below) adds the compudynamic decision at this point between
* messages, so a VM with a queue of checkpoint-free payloads cannot hold
* the CPU indefinitely.
*/
/*
* FABRIC-3.8.md §3.13.3 (Stage C-b, 2026-09-23): the trampoline now asks
* the compudynamic decision at its between-messages point, instead of
* always draining to empty and then yielding to whoever switched in:
*
* - still has mail: keep draining unless some VM with work is strictly
* cooler (then switch to it; this VM stays eligible through its mail);
* - no mail: switch to the coolest VM with work, or -- nobody has any --
* to Hera, the console root, whose idle point polls for input.
*
* The no-work fallback is Hera rather than native_switch_back_to because
* back_to is "whoever switched in last": two idle trampolines that switched
* into each other would each yield to the other forever -- a switch storm.
* Hera is never waiting on a trampoline, so the chain always ends at her.
*/
void sk_vm_switch_entry(void) {
VM *self = g_switch_entry_vm;
for (;;) {
if (sk_hermes_drain_checkpoint(self) != 0) {
continue; /* consumed one message (1 = ok, -1 = payload erred) */
VM *next;
int drained, has_mail;
drained = sk_hermes_drain_checkpoint(self); /* at most one message */
has_mail = sk_hermes_pending_count(self->stadium_vm_id) > 0;
/* Stage B's rule, kept: never spin on mail the drain won't take --
* with no word checkpoint in this loop, that would be a silent storm. */
if (drained == 0 && has_mail) {
sk_hal_panic("switch trampoline: pending mail the drain did not consume");
}
/* Stage FR (§3.12): parked-idle is a VM_FRAME_IDLE frame on this VM's
* own stack, pushed only around the yield and popped (by ID) on
* resume, BEFORE any further drain -- so the drain's own frame is
* still the only frame and the ownership gate's exactly-one-frame
* rule holds. It records WHERE the VM is parked: an empty stack alone
* would also describe Hera parked at an idle point with her REPL call
* chain live, which is not "nothing in progress." */
sk_vm_switch_reap_pending();
next = sk_vm_switch_decide(self, has_mail);
if (!next) {
if (has_mail) continue; /* still coolest with work: keep going */
next = (VM *)sk_get_mama_vm(); /* nobody has work: back to the root */
if (!next || next == self) sk_hal_panic("switch trampoline: no root to idle to");
}
/* Stage FR (§3.12): parked is a VM_FRAME_IDLE frame on this VM's own
* stack, pushed only around the switch and popped (by ID) on resume,
* BEFORE any further drain -- so the drain's own frame is still the
* only frame and the ownership gate's exactly-one-frame rule holds.
* It records WHERE the VM is parked: at its trampoline, nothing in
* progress (any remaining mail keeps it eligible by itself). */
{
VMFrameID idle_id = vm_frame_push(&self->frames, self->stadium_vm_id, VM_FRAME_IDLE);
if (idle_id == 0) {
sk_hal_panic("switch trampoline: idle frame push failed");
}
if (!self->native_switch_back_to ||
sk_vm_context_switch(self, self->native_switch_back_to) != 0) {
sk_hal_panic("switch trampoline: idle yield failed");
sk_vm_switch_signal_note_switch_performed(next->stadium_vm_id);
if (sk_vm_context_switch(self, next) != 0) {
sk_hal_panic("switch trampoline: switch failed");
}
vm_frame_pop(&self->frames, idle_id); /* resumed: someone switched back in */
}
}
}
/* See switch.h. The caller has already established that `vm` owns its
* outermost frame (sk_vm_owns_outermost_frame(), vm_core.c). */
void sk_vm_switch_checkpoint(VM *vm) {
VM *next;
if (!sk_vm_switch_signal_take_decision_due()) return;
sk_vm_switch_reap_pending();
next = sk_vm_switch_decide(vm, 1); /* vm is mid-task: it still has work */
if (!next) return;
sk_vm_switch_signal_note_switch_performed(next->stadium_vm_id);
(void)sk_vm_context_switch(vm, next);
/* Resumes here once the decision, somewhere, switches back to vm. */
}
/* See switch.h. FABRIC-3.8.md §3.13.5 (ruling C-5): Hera between lines,
* polling for input, is the idle point. She has no work of her own there,
* so any VM with work goes -- the coolest one. She is parked with an IDLE
* frame (nothing in progress), so she becomes eligible again only through
* mail or pending console input, or is returned to by a trampoline with
* nothing left to do. */
void sk_vm_idle_point(void) {
VM *self = g_switch_current_vm;
VM *next;
if (!self || self != (VM *)sk_get_mama_vm()) return;
if (self->frames.count != 0) return; /* inside a word (KEY/EXPECT): not idle */
sk_vm_switch_reap_pending();
next = sk_vm_switch_decide(self, 0);
if (!next) return;
{
VMFrameID idle_id = vm_frame_push(&self->frames, self->stadium_vm_id, VM_FRAME_IDLE);
if (idle_id == 0) return; /* fail closed: stay put rather than park unmarked */
sk_vm_switch_signal_note_switch_performed(next->stadium_vm_id);
(void)sk_vm_context_switch(self, next);
vm_frame_pop(&self->frames, idle_id);
}
}
int sk_vm_context_switch(VM *from, VM *to) {
uint64_t new_sp;
@@ -177,6 +245,7 @@ int sk_vm_context_switch(VM *from, VM *to) {
capsule_vm_set_state(to->stadium_vm_id, VM_STATE_LIVE);
capsule_vm_set_state(from->stadium_vm_id, VM_STATE_SWITCHED_OUT);
sk_vm_switch_signal_note_parked(from->stadium_vm_id); /* §3.13.3 wait stamp */
g_switch_current_vm = to;
sk_vm_switch_to(&from->native_stack_saved_sp, new_sp);