Replace CFS/EEVDF with compact O(1) tiny scheduler

Linux 7.0's kernel/sched/fair.c carries no #ifdef CONFIG_SMP guard.
On a UP NOMMU image the SMP load-balancer (select_task_rq_fair 1,484,
sched_balance_rq 1,460, update_sd_lb_stats 912,
sched_balance_find_*_group 1,262, _nohz_idle_balance 424,
can_migrate_task 324, active_load_balance_cpu_stop 316, ~7.8KB total)
gets pinned by the sched_class callback table; --gc-sections cannot
reach it through the table.  Add the same kind of out-of-tree gate
0012/0013 used for debug.c and deadline.c, but for the whole class.

CONFIG_SCHED_FAIR_TINY (default n) wraps fair.c body in #ifndef and
provides a three-priority O(1) class in the #else branch:

  - per-CPU bitmap + per-priority FIFO (HIGH/NORMAL/LOW)
  - O(1) pick:    find_first_bit(active) + list_first_entry
  - O(1) enqueue: list_add_tail + __set_bit
  - O(1) dequeue: list_del_init + __clear_bit when queue empties
  - cross-priority preemption at wakeup; round-robin within a
    priority via a fixed jiffies time-slice reset on set_next_task

Priority is a pure function of nice value: nice<0 -> HIGH,
nice==0 -> NORMAL, nice>0 -> LOW; SCHED_IDLE collapses to LOW;
SCHED_BATCH uses nice normally.  Tasks chain through the existing
&p->se.group_node (dead under !FAIR_GROUP_SCHED) so task_struct
stays unchanged.  The bucket index is recomputed from p->static_prio
on every callback; core.c's dequeue-modify-enqueue protocol (verified
across all four static_prio mutation sites: sched_fork at 4650/4653,
syscalls.c set_user_nice at 84 for RT/DL and at 89 for fair via
scoped_guard(sched_change, ...)) keeps the value stable across the
removal/insertion bracket.  RT preemption is unchanged: rt_sched_class
still preempts fair via the existing class chain walk in
pick_next_task_balance.

Not the historical 2.6 O(1) scheduler -- no active/expired arrays,
no interactivity estimator (the gameable heuristic that motivated
CFS), no priority recalculation.  Just the priority bitmap + FIFO
data structure that O(1) got right, without the policy machinery that
O(1) got wrong.

The #else branch re-exports every symbol other TUs depend on:
update_curr_common (rt.c / deadline-class stub / stop_task.c / ext.c
runtime accounting), init_cfs_rq, fair_server_init,
init_sched_fair_class, sched_init_granularity, update_max_interval,
init_entity_runnable_average, post_init_entity_util_avg,
sched_balance_trigger, nohz_balance_{enter,exit}_idle,
nohz_run_idle_balance, update_group_capacity, __setparam_fair,
arch_asym_cpu_priority, plus sysctl_sched_base_slice and
sysctl_sched_migration_cost storage.  switched_to_fair and
prio_changed_fair filter on rq->donor->sched_class != fair_sched_class
to avoid spurious resched_curr when the runner is RT (mirrors mainline
behavior; the kernel dispatches both hooks unconditionally).

pelt.c is left untouched.  With fair.c gated, its CFS-side entry
points lose their callers; rt.c keeps update_rt_rq_load_avg live.
The remaining PELT symbols are non-static so LTO largely cannot strip
them, but the cost is small (1.7KB).

Build wiring: build.sh adds the 0014 patch to the apply glob, sets
CONFIG_SCHED_FAIR_TINY=y in the inline kernel .config block, and
extends the post-olddefconfig verifier with a positive presence check.

Result: linux.axf 1,204,768 -> 1,188,352 bytes (-16,416 / -1.36%);
vmlinux .text 729,380 -> 713,412 (-15,968), .rodata -96, .init.text
-116, .bss -36, .data -32; kernel/sched/fair.c collapses 16,782 / 97
syms -> ~1,160 bytes / 22 syms; pick_task_fair compiles to 24 bytes
of pure O(1) machine code (find_first_bit + list head deref +
container_of, no loops or rb-tree walks).

QEMU MPS2-AN386 boots clean to the BusyBox shell across three
back-to-back validate-qemu.sh runs against the full PGO workload
(17 fork/exec/wait sequences exercising hush spawn, cp /bin/busybox,
mv, ln, mkdir, rm, test pipelines).

Author: jserv

build.sh

@@ -666,7 +666,7 @@ build_linux() {
     cd linux-${LINUX_VERSION}
 
     # Apply linux-tiny patches for reduced memory footprint and LTO support
-    for p in ../patches/0002-*.patch ../patches/0003-*.patch ../patches/0004-*.patch ../patches/0005-*.patch ../patches/0006-*.patch ../patches/0010-*.patch ../patches/0011-*.patch ../patches/0012-*.patch ../patches/0013-*.patch; do
+    for p in ../patches/0002-*.patch ../patches/0003-*.patch ../patches/0004-*.patch ../patches/0005-*.patch ../patches/0006-*.patch ../patches/0010-*.patch ../patches/0011-*.patch ../patches/0012-*.patch ../patches/0013-*.patch ../patches/0014-*.patch; do
         [ -f "${p}" ] || continue
         apply_patch_once "${p}"
     done
@@ -938,6 +938,20 @@ build_linux() {
     echo "# CONFIG_CGROUPS is not set" >>.config
     echo "# CONFIG_SCHED_AUTOGROUP is not set" >>.config
 
+    # Patch 0014 replaces fair.c (CFS/EEVDF) with a compact O(1) priority
+    # round-robin SCHED_NORMAL class under CONFIG_SCHED_FAIR_TINY.  fair.c
+    # body is wrapped in #ifndef; the #else branch provides a per-CPU
+    # bitmap + per-priority FIFO (HIGH/NORMAL/LOW) plus stubs for every
+    # symbol other TUs (rt.c, deadline.c stub, syscalls.c, topology.c,
+    # idle.c, build_utility.c) reference.  Linux 7.0 has no CONFIG_SMP
+    # guard inside fair.c so balance code (select_task_rq, sched_balance_*,
+    # _nohz_idle_balance, ~7.8KB total) stays linked under UP via the
+    # sched_class table; this knob is the only way to remove it.  nice
+    # values quantise to the three buckets (nice<0 -> HIGH, nice==0 ->
+    # NORMAL, nice>0 -> LOW); SCHED_IDLE collapses to LOW; rt_sched_class
+    # still pre-empts via the existing class chain walk.
+    echo "CONFIG_SCHED_FAIR_TINY=y" >>.config
+
     run_logged "olddefconfig" kernel_make olddefconfig
 
     # Verify critical config options survived olddefconfig resolution.
@@ -1003,7 +1017,8 @@ build_linux() {
         "# CONFIG_SCHED_DEADLINE_CLASS is not set" \
         "# CONFIG_PSI is not set" \
         "# CONFIG_CGROUPS is not set" \
-        "# CONFIG_SCHED_AUTOGROUP is not set"; do
+        "# CONFIG_SCHED_AUTOGROUP is not set" \
+        "CONFIG_SCHED_FAIR_TINY=y"; do
         if ! grep -q "^${opt}\$" .config; then
             echo "ERROR: expected '${opt}' in .config after olddefconfig"
             exit 1