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, 226 insertions, 0 deletions

diff --git a/scripts/package/kernel.spec b/scripts/package/kernel.spec
new file mode 100644
index 000000000..46e80970f
--- /dev/null
+++ b/scripts/package/kernel.spec
@@ -0,0 +1,226 @@
+# _arch is undefined if /usr/lib/rpm/platform/*/macros was not included.
+%{!?_arch: %define _arch dummy}
+%{!?make: %define make make}
+%define makeflags %{?_smp_mflags} ARCH=%{ARCH}
+
+Name: kernel
+Summary: The Linux Kernel
+Version: %(echo %{KERNELRELEASE} | sed -e 's/-/_/g')
+Release: %{pkg_release}%{?dist}
+License: GPL
+Group: System Environment/Kernel
+Vendor: The Linux Community
+URL: https://www.kernel.org
+Source0: linux.tar.gz
+Source1: config
+Source2: diff.patch
+Provides: kernel-%{KERNELRELEASE}
+BuildRequires: bc binutils bison dwarves
+BuildRequires: (elfutils-devel or libdw-devel)
+BuildRequires: (elfutils-libelf-devel or libelf-devel) flex
+BuildRequires: gcc make openssl openssl-devel perl python3 rsync
+
+%description
+The Linux Kernel, the operating system core itself
+
+%package headers
+Summary: Header files for the Linux kernel for use by glibc
+Group: Development/System
+Obsoletes: kernel-headers < %{version}
+Provides: kernel-headers = %{version}
+%description headers
+Kernel-headers includes the C header files that specify the interface
+between the Linux kernel and userspace libraries and programs.  The
+header files define structures and constants that are needed for
+building most standard programs and are also needed for rebuilding the
+glibc package.
+
+%if %{with_devel}
+%package devel
+Summary: Development package for building kernel modules to match the %{version} kernel
+Group: System Environment/Kernel
+AutoReqProv: no
+%description -n kernel-devel
+This package provides kernel headers and makefiles sufficient to build modules
+against the %{version} kernel package.
+%endif
+
+%if %{with_debuginfo_manual}
+%package debuginfo
+Summary: Debug information package for the Linux kernel
+Group: Development/Debug
+AutoReq: 0
+AutoProv: 1
+%description debuginfo
+This package provides debug information for the kernel image and modules from the
+%{version} package.
+%define install_mod_strip 1
+%endif
+
+%if %{with_debuginfo_rpm}
+# list of debuginfo-related options taken from distribution kernel.spec
+# files
+%undefine _include_minidebuginfo
+%undefine _find_debuginfo_dwz_opts
+%undefine _unique_build_ids
+%undefine _unique_debug_names
+%undefine _unique_debug_srcs
+%undefine _debugsource_packages
+%undefine _debuginfo_subpackages
+
+# Preserve .BTF and .BTF.base sections in kernel modules during debuginfo
+# stripping. find-debuginfo.sh uses eu-strip which removes non-allocated ELF
+# sections like .BTF by default. .BTF.base is required for BTF distillation
+# support; without it, module BTF validation fails.
+%global with_keep_section %(%{__find_debuginfo} --help 2>&1 | grep -c keep-section)
+%if %{with_keep_section}
+%global _find_debuginfo_opts -r --keep-section .BTF --keep-section .BTF.base
+%else
+%global _find_debuginfo_opts -r
+%endif
+
+%global _missing_build_ids_terminate_build 1
+%global _no_recompute_build_ids 1
+%{debug_package}
+
+# later, we make all modules executable so that find-debuginfo.sh strips
+# them up. but they don't actually need to be executable, so remove the
+# executable bit, taking care to do it _after_ find-debuginfo.sh has run
+%define __spec_install_post \
+	%{?__debug_package:%{__debug_install_post}} \
+	%{__arch_install_post} \
+	%{__os_install_post} \
+	find %{buildroot}/lib/modules/%{KERNELRELEASE} -name "*.ko" -type f \\\
+		| xargs --no-run-if-empty chmod u-x
+%else
+%define __spec_install_post /usr/lib/rpm/brp-compress || :
+%endif
+# some (but not all) versions of rpmbuild emit %%debug_package with
+# %%install. since we've already emitted it manually, that would cause
+# a package redefinition error. ensure that doesn't happen
+%define debug_package %{nil}
+
+%prep
+%setup -q -n linux
+cp %{SOURCE1} .config
+patch -p1 < %{SOURCE2}
+
+%build
+%{make} %{makeflags} KERNELRELEASE=%{KERNELRELEASE} KBUILD_BUILD_VERSION=%{release}
+
+%install
+mkdir -p %{buildroot}/lib/modules/%{KERNELRELEASE}
+cp $(%{make} %{makeflags} -s image_name) %{buildroot}/lib/modules/%{KERNELRELEASE}/vmlinuz
+# DEPMOD=true makes depmod no-op. We do not package depmod-generated files.
+%{make} %{makeflags} INSTALL_MOD_PATH=%{buildroot} %{?install_mod_strip:INSTALL_MOD_STRIP=1} DEPMOD=true modules_install
+%{make} %{makeflags} INSTALL_HDR_PATH=%{buildroot}/usr headers_install
+cp System.map %{buildroot}/lib/modules/%{KERNELRELEASE}
+cp .config %{buildroot}/lib/modules/%{KERNELRELEASE}/config
+if %{make} %{makeflags} run-command KBUILD_RUN_COMMAND='test -d ${srctree}/arch/${SRCARCH}/boot/dts' 2>/dev/null; then
+	%{make} %{makeflags} INSTALL_DTBS_PATH=%{buildroot}/lib/modules/%{KERNELRELEASE}/dtb dtbs_install
+fi
+ln -fns /usr/src/kernels/%{KERNELRELEASE} %{buildroot}/lib/modules/%{KERNELRELEASE}/build
+%if %{with_devel}
+%{make} %{makeflags} run-command KBUILD_RUN_COMMAND='${srctree}/scripts/package/install-extmod-build %{buildroot}/usr/src/kernels/%{KERNELRELEASE}'
+%endif
+
+{
+	echo "/lib/modules/%{KERNELRELEASE}"
+
+	for x in alias alias.bin builtin.alias.bin builtin.bin dep dep.bin \
+				devname softdep symbols symbols.bin weakdep; do
+		echo "%ghost /lib/modules/%{KERNELRELEASE}/modules.${x}"
+	done
+
+	for x in System.map config vmlinuz; do
+		echo "%ghost /boot/${x}-%{KERNELRELEASE}"
+	done
+
+	if [ -d "%{buildroot}/lib/modules/%{KERNELRELEASE}/dtb" ];then
+		find "%{buildroot}/lib/modules/%{KERNELRELEASE}/dtb" -printf "%%%ghost /boot/dtb-%{KERNELRELEASE}/%%P\n"
+	fi
+
+	echo "%exclude /lib/modules/%{KERNELRELEASE}/build"
+} > %{buildroot}/kernel.list
+
+%if 0%{with_debuginfo_manual}%{with_debuginfo_rpm} > 0
+# copying vmlinux directly to the debug directory means it will not get
+# stripped (but its source paths will still be collected + fixed up)
+mkdir -p %{buildroot}/usr/lib/debug/lib/modules/%{KERNELRELEASE}
+cp vmlinux %{buildroot}/usr/lib/debug/lib/modules/%{KERNELRELEASE}
+%endif
+
+%if %{with_debuginfo_rpm}
+# make modules executable so that find-debuginfo.sh strips them. this
+# will be undone later in %%__spec_install_post
+find %{buildroot}/lib/modules/%{KERNELRELEASE} -name "*.ko" -type f \
+	| xargs --no-run-if-empty chmod u+x
+%endif
+
+%if %{with_debuginfo_manual}
+echo /usr/lib/debug/lib/modules/%{KERNELRELEASE}/vmlinux > %{buildroot}/debuginfo.list
+while read -r mod; do
+	mod="${mod%.o}.ko"
+	dbg="%{buildroot}/usr/lib/debug/lib/modules/%{KERNELRELEASE}/kernel/${mod}"
+	buildid=$("${READELF:-readelf}" -n "${mod}" | sed -n 's@^.*Build ID: \(..\)\(.*\)@\1/\2@p')
+	link="%{buildroot}/usr/lib/debug/.build-id/${buildid}.debug"
+
+	mkdir -p "${dbg%/*}" "${link%/*}"
+	"${OBJCOPY:-objcopy}" --only-keep-debug "${mod}" "${dbg}"
+	ln -sf --relative "${dbg}" "${link}"
+
+	echo "${dbg#%{buildroot}}" >> %{buildroot}/debuginfo.list
+	echo "${link#%{buildroot}}" >> %{buildroot}/debuginfo.list
+done < modules.order
+%endif
+
+%clean
+rm -rf %{buildroot}
+%if %{with_debuginfo_rpm}
+rm -f debugfiles.list debuglinks.list debugsourcefiles.list debugsources.list \
+	elfbins.list
+%endif
+
+%post
+if [ -x /usr/bin/kernel-install ]; then
+	/usr/bin/kernel-install add %{KERNELRELEASE} /lib/modules/%{KERNELRELEASE}/vmlinuz
+fi
+for file in vmlinuz System.map config; do
+	if ! cmp --silent "/lib/modules/%{KERNELRELEASE}/${file}" "/boot/${file}-%{KERNELRELEASE}"; then
+		cp "/lib/modules/%{KERNELRELEASE}/${file}" "/boot/${file}-%{KERNELRELEASE}"
+	fi
+done
+if [ -d "/lib/modules/%{KERNELRELEASE}/dtb" ] && \
+   ! diff -rq "/lib/modules/%{KERNELRELEASE}/dtb" "/boot/dtb-%{KERNELRELEASE}" >/dev/null 2>&1; then
+	rm -rf "/boot/dtb-%{KERNELRELEASE}"
+	cp -r "/lib/modules/%{KERNELRELEASE}/dtb" "/boot/dtb-%{KERNELRELEASE}"
+fi
+if [ ! -e "/lib/modules/%{KERNELRELEASE}/modules.dep" ]; then
+	/usr/sbin/depmod %{KERNELRELEASE}
+fi
+
+%preun
+if [ -x /usr/bin/kernel-install ]; then
+kernel-install remove %{KERNELRELEASE}
+fi
+
+%files -f %{buildroot}/kernel.list
+%defattr (-, root, root)
+%exclude /kernel.list
+
+%files headers
+%defattr (-, root, root)
+/usr/include
+
+%if %{with_devel}
+%files devel
+%defattr (-, root, root)
+/usr/src/kernels/%{KERNELRELEASE}
+/lib/modules/%{KERNELRELEASE}/build
+%endif
+
+%if %{with_debuginfo_manual}
+%files -f %{buildroot}/debuginfo.list debuginfo
+%defattr (-, root, root)
+%exclude /debuginfo.list
+%endif