sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 578 insertions, 0 deletions

diff --git a/scripts/check-uapi.sh b/scripts/check-uapi.sh
new file mode 100755
index 000000000..c8beec588
--- /dev/null
+++ b/scripts/check-uapi.sh
@@ -0,0 +1,578 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0-only
+# Script to check commits for UAPI backwards compatibility
+
+set -o errexit
+set -o pipefail
+
+print_usage() {
+	name=$(basename "$0")
+	cat << EOF
+$name - check for UAPI header stability across Git commits
+
+By default, the script will check to make sure the latest commit (or current
+dirty changes) did not introduce ABI changes when compared to HEAD^1. You can
+check against additional commit ranges with the -b and -p options.
+
+The script will not check UAPI headers for architectures other than the one
+defined in ARCH.
+
+Usage: $name [-b BASE_REF] [-p PAST_REF] [-j N] [-l ERROR_LOG] [-i] [-q] [-v]
+
+Options:
+    -b BASE_REF    Base git reference to use for comparison. If unspecified or empty,
+                   will use any dirty changes in tree to UAPI files. If there are no
+                   dirty changes, HEAD will be used.
+    -p PAST_REF    Compare BASE_REF to PAST_REF (e.g. -p v6.1). If unspecified or empty,
+                   will use BASE_REF^1. Must be an ancestor of BASE_REF. Only headers
+                   that exist on PAST_REF will be checked for compatibility.
+    -j JOBS        Number of checks to run in parallel (default: number of CPU cores).
+    -l ERROR_LOG   Write error log to file (default: no error log is generated).
+    -i             Ignore ambiguous changes that may or may not break UAPI compatibility.
+    -q             Quiet operation.
+    -v             Verbose operation (print more information about each header being checked).
+
+Environmental args:
+    ABIDIFF        Custom path to abidiff binary
+    CROSS_COMPILE  Toolchain prefix for compiler
+    CC             C compiler (default is "\${CROSS_COMPILE}gcc")
+    ARCH           Target architecture for the UAPI check (default is host arch)
+
+Exit codes:
+    $SUCCESS) Success
+    $FAIL_ABI) ABI difference detected
+    $FAIL_PREREQ) Prerequisite not met
+EOF
+}
+
+readonly SUCCESS=0
+readonly FAIL_ABI=1
+readonly FAIL_PREREQ=2
+
+# Print to stderr
+eprintf() {
+	# shellcheck disable=SC2059
+	printf "$@" >&2
+}
+
+# Expand an array with a specific character (similar to Python string.join())
+join() {
+	local IFS="$1"
+	shift
+	printf "%s" "$*"
+}
+
+# Create abidiff suppressions
+gen_suppressions() {
+	# Common enum variant names which we don't want to worry about
+	# being shifted when new variants are added.
+	local -a enum_regex=(
+		".*_AFTER_LAST$"
+		".*_CNT$"
+		".*_COUNT$"
+		".*_END$"
+		".*_LAST$"
+		".*_MASK$"
+		".*_MAX$"
+		".*_MAX_BIT$"
+		".*_MAX_BPF_ATTACH_TYPE$"
+		".*_MAX_ID$"
+		".*_MAX_SHIFT$"
+		".*_NBITS$"
+		".*_NETDEV_NUMHOOKS$"
+		".*_NFT_META_IIFTYPE$"
+		".*_NL80211_ATTR$"
+		".*_NLDEV_NUM_OPS$"
+		".*_NUM$"
+		".*_NUM_ELEMS$"
+		".*_NUM_IRQS$"
+		".*_SIZE$"
+		".*_TLSMAX$"
+		"^MAX_.*"
+		"^NUM_.*"
+	)
+
+	# Common padding field names which can be expanded into
+	# without worrying about users.
+	local -a padding_regex=(
+		".*end$"
+		".*pad$"
+		".*pad[0-9]?$"
+		".*pad_[0-9]?$"
+		".*padding$"
+		".*padding[0-9]?$"
+		".*padding_[0-9]?$"
+		".*res$"
+		".*resv$"
+		".*resv[0-9]?$"
+		".*resv_[0-9]?$"
+		".*reserved$"
+		".*reserved[0-9]?$"
+		".*reserved_[0-9]?$"
+		".*rsvd[0-9]?$"
+		".*unused$"
+	)
+
+	cat << EOF
+[suppress_type]
+  type_kind = enum
+  changed_enumerators_regexp = $(join , "${enum_regex[@]}")
+EOF
+
+	for p in "${padding_regex[@]}"; do
+		cat << EOF
+[suppress_type]
+  type_kind = struct
+  has_data_member_inserted_at = offset_of_first_data_member_regexp(${p})
+EOF
+	done
+
+if [ "$IGNORE_AMBIGUOUS_CHANGES" = "true" ]; then
+	cat << EOF
+[suppress_type]
+  type_kind = struct
+  has_data_member_inserted_at = end
+  has_size_change = yes
+EOF
+fi
+}
+
+# Check if git tree is dirty
+tree_is_dirty() {
+	! git diff --quiet
+}
+
+# Get list of files installed in $ref
+get_file_list() {
+	local -r ref="$1"
+	local -r tree="$(get_header_tree "$ref")"
+
+	# Print all installed headers, filtering out ones that can't be compiled
+	find "$tree" -type f -name '*.h' -printf '%P\n' | grep -v -f "$INCOMPAT_LIST"
+}
+
+# Add to the list of incompatible headers
+add_to_incompat_list() {
+	local -r ref="$1"
+
+	# Start with the usr/include/Makefile to get a list of the headers
+	# that don't compile using this method.
+	if [ ! -f usr/include/Makefile ]; then
+		eprintf "error - no usr/include/Makefile present at %s\n" "$ref"
+		eprintf "Note: usr/include/Makefile was added in the v5.3 kernel release\n"
+		exit "$FAIL_PREREQ"
+	fi
+	{
+		# shellcheck disable=SC2016
+		printf 'all: ; @echo $(no-header-test)\n'
+		cat usr/include/Makefile
+	} | SRCARCH="$ARCH" make --always-make -f - | tr " " "\n" \
+	  | grep -v "asm-generic" >> "$INCOMPAT_LIST"
+
+	# The makefile also skips all asm-generic files, but prints "asm-generic/%"
+	# which won't work for our grep match. Instead, print something grep will match.
+	printf "asm-generic/.*\.h\n" >> "$INCOMPAT_LIST"
+}
+
+# Compile the simple test app
+do_compile() {
+	local -r inc_dir="$1"
+	local -r header="$2"
+	local -r out="$3"
+	printf "int f(void) { return 0; }\n" | \
+		"$CC" \
+		  -shared \
+		  -nostdlib \
+		  -fPIC \
+		  -o "$out" \
+		  -x c \
+		  -O0 \
+		  -std=c90 \
+		  -fno-eliminate-unused-debug-types \
+		  -g \
+		  "-I${inc_dir}" \
+		  "-Iusr/dummy-include" \
+		  -include "$header" \
+		  -
+}
+
+# Run make headers_install
+run_make_headers_install() {
+	local -r ref="$1"
+	local -r install_dir="$(get_header_tree "$ref")"
+	make -j "$MAX_THREADS" CROSS_COMPILE="${CROSS_COMPILE}" ARCH="$ARCH" INSTALL_HDR_PATH="$install_dir" \
+		headers_install > /dev/null
+}
+
+# Install headers for both git refs
+install_headers() {
+	local -r base_ref="$1"
+	local -r past_ref="$2"
+
+	for ref in "$base_ref" "$past_ref"; do
+		printf "Installing user-facing UAPI headers from %s... " "${ref:-dirty tree}"
+		if [ -n "$ref" ]; then
+			git archive --format=tar --prefix="${ref}-archive/" "$ref" \
+				| (cd "$TMP_DIR" && tar xf -)
+			(
+				cd "${TMP_DIR}/${ref}-archive"
+				run_make_headers_install "$ref"
+				add_to_incompat_list "$ref" "$INCOMPAT_LIST"
+			)
+		else
+			run_make_headers_install "$ref"
+			add_to_incompat_list "$ref" "$INCOMPAT_LIST"
+		fi
+		printf "OK\n"
+	done
+	sort -u -o "$INCOMPAT_LIST" "$INCOMPAT_LIST"
+	sed -i -e '/^$/d' "$INCOMPAT_LIST"
+}
+
+# Print the path to the headers_install tree for a given ref
+get_header_tree() {
+	local -r ref="$1"
+	printf "%s" "${TMP_DIR}/${ref}/usr"
+}
+
+# Check file list for UAPI compatibility
+check_uapi_files() {
+	local -r base_ref="$1"
+	local -r past_ref="$2"
+	local -r abi_error_log="$3"
+
+	local passed=0;
+	local failed=0;
+	local -a threads=()
+	set -o errexit
+
+	printf "Checking changes to UAPI headers between %s and %s...\n" "$past_ref" "${base_ref:-dirty tree}"
+	# Loop over all UAPI headers that were installed by $past_ref (if they only exist on $base_ref,
+	# there's no way they're broken and no way to compare anyway)
+	while read -r file; do
+		if [ "${#threads[@]}" -ge "$MAX_THREADS" ]; then
+			if wait "${threads[0]}"; then
+				passed=$((passed + 1))
+			else
+				failed=$((failed + 1))
+			fi
+			threads=("${threads[@]:1}")
+		fi
+
+		check_individual_file "$base_ref" "$past_ref" "$file" &
+		threads+=("$!")
+	done < <(get_file_list "$past_ref")
+
+	for t in "${threads[@]}"; do
+		if wait "$t"; then
+			passed=$((passed + 1))
+		else
+			failed=$((failed + 1))
+		fi
+	done
+
+	if [ -n "$abi_error_log" ]; then
+		printf 'Generated by "%s %s" from git ref %s\n\n' \
+			"$0" "$*" "$(git rev-parse HEAD)" > "$abi_error_log"
+	fi
+
+	while read -r error_file; do
+		{
+			cat "$error_file"
+			printf "\n\n"
+		} | tee -a "${abi_error_log:-/dev/null}" >&2
+	done < <(find "$TMP_DIR" -type f -name '*.error' | sort)
+
+	total="$((passed + failed))"
+	if [ "$failed" -gt 0 ]; then
+		eprintf "error - %d/%d UAPI headers compatible with %s appear _not_ to be backwards compatible\n" \
+			"$failed" "$total" "$ARCH"
+		if [ -n "$abi_error_log" ]; then
+			eprintf "Failure summary saved to %s\n" "$abi_error_log"
+		fi
+	else
+		printf "All %d UAPI headers compatible with %s appear to be backwards compatible\n" \
+			"$total" "$ARCH"
+	fi
+
+	return "$failed"
+}
+
+# Check an individual file for UAPI compatibility
+check_individual_file() {
+	local -r base_ref="$1"
+	local -r past_ref="$2"
+	local -r file="$3"
+
+	local -r base_header="$(get_header_tree "$base_ref")/${file}"
+	local -r past_header="$(get_header_tree "$past_ref")/${file}"
+
+	if [ ! -f "$base_header" ]; then
+		mkdir -p "$(dirname "$base_header")"
+		printf "==== UAPI header %s was removed between %s and %s ====" \
+			"$file" "$past_ref" "$base_ref" \
+				> "${base_header}.error"
+		return 1
+	fi
+
+	compare_abi "$file" "$base_header" "$past_header" "$base_ref" "$past_ref"
+}
+
+# Perform the A/B compilation and compare output ABI
+compare_abi() {
+	local -r file="$1"
+	local -r base_header="$2"
+	local -r past_header="$3"
+	local -r base_ref="$4"
+	local -r past_ref="$5"
+	local -r log="${TMP_DIR}/log/${file}.log"
+	local -r error_log="${TMP_DIR}/log/${file}.error"
+
+	mkdir -p "$(dirname "$log")"
+
+	if ! do_compile "$(get_header_tree "$base_ref")/include" "$base_header" "${base_header}.bin" 2> "$log"; then
+		{
+			warn_str=$(printf "==== Could not compile version of UAPI header %s at %s ====\n" \
+				"$file" "$base_ref")
+			printf "%s\n" "$warn_str"
+			cat "$log"
+			printf -- "=%.0s" $(seq 0 ${#warn_str})
+		} > "$error_log"
+		return 1
+	fi
+
+	if ! do_compile "$(get_header_tree "$past_ref")/include" "$past_header" "${past_header}.bin" 2> "$log"; then
+		{
+			warn_str=$(printf "==== Could not compile version of UAPI header %s at %s ====\n" \
+				"$file" "$past_ref")
+			printf "%s\n" "$warn_str"
+			cat "$log"
+			printf -- "=%.0s" $(seq 0 ${#warn_str})
+		} > "$error_log"
+		return 1
+	fi
+
+	local ret=0
+	"$ABIDIFF" --non-reachable-types \
+		--suppressions "$SUPPRESSIONS" \
+		"${past_header}.bin" "${base_header}.bin" > "$log" || ret="$?"
+	if [ "$ret" -eq 0 ]; then
+		if [ "$VERBOSE" = "true" ]; then
+			printf "No ABI differences detected in %s from %s -> %s\n" \
+				"$file" "$past_ref" "$base_ref"
+		fi
+	else
+		# Bits in abidiff's return code can be used to determine the type of error
+		if [ $((ret & 0x2)) -gt 0 ]; then
+			eprintf "error - abidiff did not run properly\n"
+			exit 1
+		fi
+
+		if [ "$IGNORE_AMBIGUOUS_CHANGES" = "true" ] && [ "$ret" -eq 4 ]; then
+			return 0
+		fi
+
+		# If the only changes were additions (not modifications to existing APIs), then
+		# there's no problem. Ignore these diffs.
+		if grep "Unreachable types summary" "$log" | grep -q "0 removed" &&
+		   grep "Unreachable types summary" "$log" | grep -q "0 changed"; then
+			return 0
+		fi
+
+		{
+			warn_str=$(printf "==== ABI differences detected in %s from %s -> %s ====" \
+				"$file" "$past_ref" "$base_ref")
+			printf "%s\n" "$warn_str"
+			sed  -e '/summary:/d' -e '/changed type/d' -e '/^$/d' -e 's/^/  /g' "$log"
+			printf -- "=%.0s" $(seq 0 ${#warn_str})
+			if cmp "$past_header" "$base_header" > /dev/null 2>&1; then
+				printf "\n%s did not change between %s and %s...\n" "$file" "$past_ref" "${base_ref:-dirty tree}"
+				printf "It's possible a change to one of the headers it includes caused this error:\n"
+				grep '^#include' "$base_header"
+				printf "\n"
+			fi
+		} > "$error_log"
+
+		return 1
+	fi
+}
+
+# Check that a minimum software version number is satisfied
+min_version_is_satisfied() {
+	local -r min_version="$1"
+	local -r version_installed="$2"
+
+	printf "%s\n%s\n" "$min_version" "$version_installed" \
+		| sort -Vc > /dev/null 2>&1
+}
+
+# Make sure we have the tools we need and the arguments make sense
+check_deps() {
+	ABIDIFF="${ABIDIFF:-abidiff}"
+	CC="${CC:-${CROSS_COMPILE}gcc}"
+	ARCH="${ARCH:-$(uname -m)}"
+	if [ "$ARCH" = "x86_64" ]; then
+		ARCH="x86"
+	fi
+
+	local -r abidiff_min_version="2.4"
+	local -r libdw_min_version_if_clang="0.171"
+
+	if ! command -v "$ABIDIFF" > /dev/null 2>&1; then
+		eprintf "error - abidiff not found!\n"
+		eprintf "Please install abigail-tools version %s or greater\n" "$abidiff_min_version"
+		eprintf "See: https://sourceware.org/libabigail/manual/libabigail-overview.html\n"
+		return 1
+	fi
+
+	local -r abidiff_version="$("$ABIDIFF" --version | cut -d ' ' -f 2)"
+	if ! min_version_is_satisfied "$abidiff_min_version" "$abidiff_version"; then
+		eprintf "error - abidiff version too old: %s\n" "$abidiff_version"
+		eprintf "Please install abigail-tools version %s or greater\n" "$abidiff_min_version"
+		eprintf "See: https://sourceware.org/libabigail/manual/libabigail-overview.html\n"
+		return 1
+	fi
+
+	if ! command -v "$CC" > /dev/null 2>&1; then
+		eprintf 'error - %s not found\n' "$CC"
+		return 1
+	fi
+
+	if "$CC" --version | grep -q clang; then
+		local -r libdw_version="$(ldconfig -v 2>/dev/null | grep -v SKIPPED | grep -m 1 -o 'libdw-[0-9]\+.[0-9]\+' | cut -c 7-)"
+		if ! min_version_is_satisfied "$libdw_min_version_if_clang" "$libdw_version"; then
+			eprintf "error - libdw version too old for use with clang: %s\n" "$libdw_version"
+			eprintf "Please install libdw from elfutils version %s or greater\n" "$libdw_min_version_if_clang"
+			eprintf "See: https://sourceware.org/elfutils/\n"
+			return 1
+		fi
+	fi
+
+	if [ ! -d "arch/${ARCH}" ]; then
+		eprintf 'error - ARCH "%s" is not a subdirectory under arch/\n' "$ARCH"
+		eprintf "Please set ARCH to one of:\n%s\n" "$(find arch -maxdepth 1 -mindepth 1 -type d -printf '%f ' | fmt)"
+		return 1
+	fi
+
+	if ! git rev-parse --is-inside-work-tree > /dev/null 2>&1; then
+		eprintf "error - this script requires the kernel tree to be initialized with Git\n"
+		return 1
+	fi
+
+	if ! git rev-parse --verify "$past_ref" > /dev/null 2>&1; then
+		printf 'error - invalid git reference "%s"\n' "$past_ref"
+		return 1
+	fi
+
+	if [ -n "$base_ref" ]; then
+		if ! git merge-base --is-ancestor "$past_ref" "$base_ref" > /dev/null 2>&1; then
+			printf 'error - "%s" is not an ancestor of base ref "%s"\n' "$past_ref" "$base_ref"
+			return 1
+		fi
+		if [ "$(git rev-parse "$base_ref")" = "$(git rev-parse "$past_ref")" ]; then
+			printf 'error - "%s" and "%s" are the same reference\n' "$past_ref" "$base_ref"
+			return 1
+		fi
+	fi
+}
+
+run() {
+	local base_ref="$1"
+	local past_ref="$2"
+	local abi_error_log="$3"
+	shift 3
+
+	if [ -z "$KERNEL_SRC" ]; then
+		KERNEL_SRC="$(realpath "$(dirname "$0")"/..)"
+	fi
+
+	cd "$KERNEL_SRC"
+
+	if [ -z "$base_ref" ] && ! tree_is_dirty; then
+		base_ref=HEAD
+	fi
+
+	if [ -z "$past_ref" ]; then
+		if [ -n "$base_ref" ]; then
+			past_ref="${base_ref}^1"
+		else
+			past_ref=HEAD
+		fi
+	fi
+
+	if ! check_deps; then
+		exit "$FAIL_PREREQ"
+	fi
+
+	TMP_DIR=$(mktemp -d)
+	readonly TMP_DIR
+	trap 'rm -rf "$TMP_DIR"' EXIT
+
+	readonly INCOMPAT_LIST="${TMP_DIR}/incompat_list.txt"
+	touch "$INCOMPAT_LIST"
+
+	readonly SUPPRESSIONS="${TMP_DIR}/suppressions.txt"
+	gen_suppressions > "$SUPPRESSIONS"
+
+	# Run make install_headers for both refs
+	install_headers "$base_ref" "$past_ref"
+
+	# Check for any differences in the installed header trees
+	if diff -r -q "$(get_header_tree "$base_ref")" "$(get_header_tree "$past_ref")" > /dev/null 2>&1; then
+		printf "No changes to UAPI headers were applied between %s and %s\n" "$past_ref" "${base_ref:-dirty tree}"
+		exit "$SUCCESS"
+	fi
+
+	if ! check_uapi_files "$base_ref" "$past_ref" "$abi_error_log"; then
+		exit "$FAIL_ABI"
+	fi
+}
+
+main() {
+	MAX_THREADS=$(nproc)
+	VERBOSE="false"
+	IGNORE_AMBIGUOUS_CHANGES="false"
+	quiet="false"
+	local base_ref=""
+	while getopts "hb:p:j:l:iqv" opt; do
+		case $opt in
+		h)
+			print_usage
+			exit "$SUCCESS"
+			;;
+		b)
+			base_ref="$OPTARG"
+			;;
+		p)
+			past_ref="$OPTARG"
+			;;
+		j)
+			MAX_THREADS="$OPTARG"
+			;;
+		l)
+			abi_error_log="$OPTARG"
+			;;
+		i)
+			IGNORE_AMBIGUOUS_CHANGES="true"
+			;;
+		q)
+			quiet="true"
+			VERBOSE="false"
+			;;
+		v)
+			VERBOSE="true"
+			quiet="false"
+			;;
+		*)
+			exit "$FAIL_PREREQ"
+		esac
+	done
+
+	if [ "$quiet" = "true" ]; then
+		exec > /dev/null 2>&1
+	fi
+
+	run "$base_ref" "$past_ref" "$abi_error_log" "$@"
+}
+
+main "$@"