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

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
@@ -854,13 +854,20 @@ build_linux() {
     # defaults to y under EXPERT and pulls a full decompressor library into
     # the image -- olddefconfig silently restores them after defconfig.
     # Sub-bucket rollup measured RD_ZSTD = 36,942 bytes (lib/zstd),
-    # RD_LZ4 = 10,972 bytes (lib/lz4), RD_XZ = 6,598 bytes (lib/xz) of
-    # dead .text in the production vmlinux. RD_ZSTD also pulls
-    # lib/xxhash.c (~3KB). Keep RD_GZIP=y as the boot-path requirement;
-    # explicitly disable the rest.
+    # RD_LZ4 = 10,972 bytes (lib/lz4), RD_XZ = 6,598 bytes (lib/xz),
+    # RD_LZO = 728 bytes (lib/lzo) of dead .text in the production vmlinux;
+    # RD_LZMA / RD_BZIP2 do not currently land any .text but their default-y
+    # status lets olddefconfig drift them back on the next time a fs/ or
+    # net/ symbol gets enabled. RD_ZSTD also pulls lib/xxhash.c (~3KB).
+    # Keep RD_GZIP=y as the boot-path requirement; explicitly disable the
+    # rest, including the residuals, so olddefconfig cannot silently
+    # re-enable them.
     echo "# CONFIG_RD_ZSTD is not set" >>.config
     echo "# CONFIG_RD_LZ4 is not set" >>.config
     echo "# CONFIG_RD_XZ is not set" >>.config
+    echo "# CONFIG_RD_LZMA is not set" >>.config
+    echo "# CONFIG_RD_BZIP2 is not set" >>.config
+    echo "# CONFIG_RD_LZO is not set" >>.config
 
     # Serial-only target: drop the VT terminal layer and accessibility
     # console support.  CONFIG_TTY stays on -- the AMBA PL011 console
@@ -931,6 +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.
@@ -989,11 +1010,15 @@ build_linux() {
         "# CONFIG_RD_ZSTD is not set" \
         "# CONFIG_RD_LZ4 is not set" \
         "# CONFIG_RD_XZ is not set" \
+        "# CONFIG_RD_LZMA is not set" \
+        "# CONFIG_RD_BZIP2 is not set" \
+        "# CONFIG_RD_LZO is not set" \
         "# CONFIG_SCHED_DEBUG_OUTPUT is not set" \
         "# 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
@@ -1056,16 +1081,22 @@ build_linux() {
         fi
     done
 
-    # Decompressor library guard. RD_ZSTD/RD_LZ4/RD_XZ disabled above
-    # must cascade to ZSTD_DECOMPRESS / LZ4_DECOMPRESS / XZ_DEC, the
-    # umbrella DECOMPRESS_* hidden bools, and XXHASH (selected by
-    # ZSTD_DECOMPRESS, also pulled by BCACHE / BTRFS but those need
-    # BLOCK=y which this target lacks). If anything else still
-    # selects them (a future fs/ or net/ enable, e.g. squashfs+zstd),
-    # we must catch that drift loudly so the size win does not
-    # silently regress.
+    # Decompressor library guard. RD_ZSTD/RD_LZ4/RD_XZ/RD_LZMA/RD_BZIP2/
+    # RD_LZO disabled above must cascade to ZSTD_DECOMPRESS /
+    # LZ4_DECOMPRESS / XZ_DEC / LZO_DECOMPRESS, the umbrella DECOMPRESS_*
+    # hidden bools, and XXHASH (selected by ZSTD_DECOMPRESS, also pulled
+    # by BCACHE / BTRFS but those need BLOCK=y which this target lacks).
+    # LZO_DECOMPRESS has more upstream selectors than the others
+    # (squashfs / btrfs / jffs2 / f2fs / zram / crypto / lib), all of
+    # which depend on BLOCK=y or NET=y / crypto knobs that are off here;
+    # the negative guard catches any future drift that re-enables one of
+    # them. If anything else still selects these symbols (a future fs/ or
+    # net/ enable, e.g. squashfs+zstd), we must catch that drift loudly
+    # so the size win does not silently regress.
     for sym in ZSTD_DECOMPRESS ZSTD_COMMON LZ4_DECOMPRESS XZ_DEC \
-               XXHASH DECOMPRESS_ZSTD DECOMPRESS_LZ4 DECOMPRESS_XZ; do
+               LZO_DECOMPRESS \
+               XXHASH DECOMPRESS_ZSTD DECOMPRESS_LZ4 DECOMPRESS_XZ \
+               DECOMPRESS_LZMA DECOMPRESS_BZIP2 DECOMPRESS_LZO; do
         if grep -q "^CONFIG_${sym}=y\$" .config; then
             echo "ERROR: CONFIG_${sym}=y survived olddefconfig (decompressor guard tripped)"
             exit 1