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, 97 insertions, 0 deletions

diff --git a/scripts/package/mkspec b/scripts/package/mkspec
new file mode 100755
index 000000000..c604f8c17
--- /dev/null
+++ b/scripts/package/mkspec
@@ -0,0 +1,97 @@
+#!/bin/sh
+#
+#	Output a simple RPM spec file.
+#	This version assumes a minimum of RPM 4.13
+#
+#	The only gothic bit here is redefining install_post to avoid
+#	stripping the symbols from files in the kernel which we want
+#
+#	Patched for non-x86 by Opencon (L) 2002 <opencon@rio.skydome.net>
+#
+
+set -eu
+
+output=$1
+
+mkdir -p "$(dirname "${output}")"
+
+exec >"${output}"
+
+if grep -q CONFIG_MODULES=y include/config/auto.conf; then
+echo '%define with_devel %{?_without_devel: 0} %{?!_without_devel: 1}'
+else
+echo '%define with_devel 0'
+fi
+
+# use %{debug_package} machinery to generate -debuginfo
+with_debuginfo_rpm=0
+# manually generate -debuginfo package
+with_debuginfo_manual=0
+# debuginfo package generation uses find-debuginfo.sh under the hood,
+# which only works on uncompressed modules that contain debuginfo
+if grep -q CONFIG_DEBUG_INFO=y include/config/auto.conf &&
+   (! grep -q CONFIG_MODULE_COMPRESS=y include/config/auto.conf) &&
+   (! grep -q CONFIG_DEBUG_INFO_SPLIT=y include/config/auto.conf); then
+	# If module signing is enabled (which may be required to boot with
+	# lockdown enabled), the find-debuginfo.sh machinery cannot be used
+	# because the signatures will be stripped off the modules. However, due
+	# to an rpm bug in versions prior to 4.20.0
+	#
+	#     https://github.com/rpm-software-management/rpm/issues/3057
+	#     https://github.com/rpm-software-management/rpm/commit/49f906998f3cf1f4152162ca61ac0869251c380f
+	#
+	# We cannot provide our own debuginfo package because it does not listen
+	# to our custom files list, failing the build due to unpackaged files.
+	# Manually generate the debug info package if using rpm 4.20.0. If not
+	# using rpm 4.20.0, avoid generating a -debuginfo package altogether,
+	# as it is not safe.
+	if grep -q CONFIG_MODULE_SIG=y include/config/auto.conf; then
+		rpm_ver_str=$(rpm --version 2>/dev/null)
+		# Split the version on spaces
+		IFS=' '
+		set -- $rpm_ver_str
+		if [ "${1:-}" = RPM -a "${2:-}" = version ]; then
+			IFS=.
+			set -- $3
+			rpm_ver=$(( 1000000 * $1 + 10000 * $2 + 100 * $3 + ${4:-0} ))
+			if [ "$rpm_ver" -ge 4200000 ]; then
+				with_debuginfo_manual='%{?_without_debuginfo:0}%{?!_without_debuginfo:1}'
+			fi
+		fi
+	else
+		with_debuginfo_rpm='%{?_without_debuginfo:0}%{?!_without_debuginfo:1}'
+	fi
+fi
+echo "%define with_debuginfo_manual $with_debuginfo_manual"
+echo "%define with_debuginfo_rpm $with_debuginfo_rpm"
+
+cat<<EOF
+%define ARCH ${ARCH}
+%define KERNELRELEASE ${KERNELRELEASE}
+%define pkg_release $("${srctree}/scripts/build-version")
+EOF
+
+cat "${srctree}/scripts/package/kernel.spec"
+
+# collect the user's name and email address for the changelog entry
+if [ "$(command -v git)" ]; then
+	name=$(git config user.name) || true
+	email=$(git config user.email) || true
+fi
+
+if [ ! "${name:+set}" ]; then
+	name=${KBUILD_BUILD_USER:-$(id -nu)}
+fi
+
+if [ ! "${email:+set}" ]; then
+	buildhost=${KBUILD_BUILD_HOST:-$(hostname -f 2>/dev/null || hostname)}
+	builduser=${KBUILD_BUILD_USER:-$(id -nu)}
+	email="${builduser}@${buildhost}"
+fi
+
+cat << EOF
+
+%changelog
+* $(LC_ALL=C date +'%a %b %d %Y') ${name} <${email}>
+- Custom built Linux kernel.
+EOF