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, 245 insertions, 0 deletions

diff --git a/scripts/rust_is_available.sh b/scripts/rust_is_available.sh
new file mode 100755
index 000000000..551f1ebd0
--- /dev/null
+++ b/scripts/rust_is_available.sh
@@ -0,0 +1,245 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+#
+# Tests whether a suitable Rust toolchain is available.
+
+set -e
+
+min_tool_version=$(dirname $0)/min-tool-version.sh
+
+# Convert the version string x.y.z to a canonical up-to-7-digits form.
+#
+# Note that this function uses one more digit (compared to other
+# instances in other version scripts) to give a bit more space to
+# `rustc` since it will reach 1.100.0 in late 2026.
+get_canonical_version()
+{
+	IFS=.
+	set -- $1
+	echo $((100000 * $1 + 100 * $2 + $3))
+}
+
+# Print a reference to the Quick Start guide in the documentation.
+print_docs_reference()
+{
+	echo >&2 "***"
+	echo >&2 "*** Please see Documentation/rust/quick-start.rst for details"
+	echo >&2 "*** on how to set up the Rust support."
+	echo >&2 "***"
+}
+
+# Print an explanation about the fact that the script is meant to be called from Kbuild.
+print_kbuild_explanation()
+{
+	echo >&2 "***"
+	echo >&2 "*** This script is intended to be called from Kbuild."
+	echo >&2 "*** Please use the 'rustavailable' target to call it instead."
+	echo >&2 "*** Otherwise, the results may not be meaningful."
+	exit 1
+}
+
+# If the script fails for any reason, or if there was any warning, then
+# print a reference to the documentation on exit.
+warning=0
+trap 'if [ $? -ne 0 ] || [ $warning -ne 0 ]; then print_docs_reference; fi' EXIT
+
+# Check that the expected environment variables are set.
+if [ -z "${RUSTC+x}" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Environment variable 'RUSTC' is not set."
+	print_kbuild_explanation
+fi
+
+if [ -z "${BINDGEN+x}" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Environment variable 'BINDGEN' is not set."
+	print_kbuild_explanation
+fi
+
+if [ -z "${CC+x}" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Environment variable 'CC' is not set."
+	print_kbuild_explanation
+fi
+
+# Check that the Rust compiler exists.
+if ! command -v "$RUSTC" >/dev/null; then
+	echo >&2 "***"
+	echo >&2 "*** Rust compiler '$RUSTC' could not be found."
+	echo >&2 "***"
+	exit 1
+fi
+
+# Check that the Rust bindings generator exists.
+if ! command -v "$BINDGEN" >/dev/null; then
+	echo >&2 "***"
+	echo >&2 "*** Rust bindings generator '$BINDGEN' could not be found."
+	echo >&2 "***"
+	exit 1
+fi
+
+# Check that the Rust compiler version is suitable.
+#
+# Non-stable and distributions' versions may have a version suffix, e.g. `-dev`.
+rust_compiler_output=$( \
+	LC_ALL=C "$RUSTC" --version 2>/dev/null
+) || rust_compiler_code=$?
+if [ -n "$rust_compiler_code" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$RUSTC' to check the Rust compiler version failed with"
+	echo >&2 "*** code $rust_compiler_code. See output and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$rust_compiler_output"
+	echo >&2 "***"
+	exit 1
+fi
+rust_compiler_version=$( \
+	echo "$rust_compiler_output" \
+		| sed -nE '1s:.*rustc ([0-9]+\.[0-9]+\.[0-9]+).*:\1:p'
+)
+if [ -z "$rust_compiler_version" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$RUSTC' to check the Rust compiler version did not return"
+	echo >&2 "*** an expected output. See output and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$rust_compiler_output"
+	echo >&2 "***"
+	exit 1
+fi
+rust_compiler_min_version=$($min_tool_version rustc)
+rust_compiler_cversion=$(get_canonical_version $rust_compiler_version)
+rust_compiler_min_cversion=$(get_canonical_version $rust_compiler_min_version)
+if [ "$rust_compiler_cversion" -lt "$rust_compiler_min_cversion" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Rust compiler '$RUSTC' is too old."
+	echo >&2 "***   Your version:    $rust_compiler_version"
+	echo >&2 "***   Minimum version: $rust_compiler_min_version"
+	echo >&2 "***"
+	exit 1
+fi
+
+# Check that the Rust bindings generator is suitable.
+#
+# Non-stable and distributions' versions may have a version suffix, e.g. `-dev`.
+rust_bindings_generator_output=$( \
+	LC_ALL=C "$BINDGEN" --version 2>/dev/null
+) || rust_bindings_generator_code=$?
+if [ -n "$rust_bindings_generator_code" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$BINDGEN' to check the Rust bindings generator version failed with"
+	echo >&2 "*** code $rust_bindings_generator_code. See output and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$rust_bindings_generator_output"
+	echo >&2 "***"
+	exit 1
+fi
+rust_bindings_generator_version=$( \
+	echo "$rust_bindings_generator_output" \
+		| sed -nE '1s:.*bindgen ([0-9]+\.[0-9]+\.[0-9]+).*:\1:p'
+)
+if [ -z "$rust_bindings_generator_version" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$BINDGEN' to check the bindings generator version did not return"
+	echo >&2 "*** an expected output. See output and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$rust_bindings_generator_output"
+	echo >&2 "***"
+	exit 1
+fi
+rust_bindings_generator_min_version=$($min_tool_version bindgen)
+rust_bindings_generator_cversion=$(get_canonical_version $rust_bindings_generator_version)
+rust_bindings_generator_min_cversion=$(get_canonical_version $rust_bindings_generator_min_version)
+if [ "$rust_bindings_generator_cversion" -lt "$rust_bindings_generator_min_cversion" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Rust bindings generator '$BINDGEN' is too old."
+	echo >&2 "***   Your version:    $rust_bindings_generator_version"
+	echo >&2 "***   Minimum version: $rust_bindings_generator_min_version"
+	echo >&2 "***"
+	exit 1
+fi
+
+# Check that the `libclang` used by the Rust bindings generator is suitable.
+#
+# In order to do that, first invoke `bindgen` to get the `libclang` version
+# found by `bindgen`. This step may already fail if, for instance, `libclang`
+# is not found, thus inform the user in such a case.
+bindgen_libclang_output=$( \
+	LC_ALL=C "$BINDGEN" $(dirname $0)/rust_is_available_bindgen_libclang.h 2>&1 >/dev/null
+) || bindgen_libclang_code=$?
+if [ -n "$bindgen_libclang_code" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$BINDGEN' to check the libclang version (used by the Rust"
+	echo >&2 "*** bindings generator) failed with code $bindgen_libclang_code. This may be caused by"
+	echo >&2 "*** a failure to locate libclang. See output and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$bindgen_libclang_output"
+	echo >&2 "***"
+	exit 1
+fi
+
+# `bindgen` returned successfully, thus use the output to check that the version
+# of the `libclang` found by the Rust bindings generator is suitable.
+#
+# Unlike other version checks, note that this one does not necessarily appear
+# in the first line of the output, thus no `sed` address is provided.
+bindgen_libclang_version=$( \
+	echo "$bindgen_libclang_output" \
+		| sed -nE 's:.*clang version ([0-9]+\.[0-9]+\.[0-9]+).*:\1:p'
+)
+if [ -z "$bindgen_libclang_version" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$BINDGEN' to check the libclang version (used by the Rust"
+	echo >&2 "*** bindings generator) did not return an expected output. See output"
+	echo >&2 "*** and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$bindgen_libclang_output"
+	echo >&2 "***"
+	exit 1
+fi
+bindgen_libclang_min_version=$($min_tool_version llvm)
+bindgen_libclang_cversion=$(get_canonical_version $bindgen_libclang_version)
+bindgen_libclang_min_cversion=$(get_canonical_version $bindgen_libclang_min_version)
+if [ "$bindgen_libclang_cversion" -lt "$bindgen_libclang_min_cversion" ]; then
+	echo >&2 "***"
+	echo >&2 "*** libclang (used by the Rust bindings generator '$BINDGEN') is too old."
+	echo >&2 "***   Your version:    $bindgen_libclang_version"
+	echo >&2 "***   Minimum version: $bindgen_libclang_min_version"
+	echo >&2 "***"
+	exit 1
+fi
+
+# If the C compiler is Clang, then we can also check whether its version
+# matches the `libclang` version used by the Rust bindings generator.
+#
+# In the future, we might be able to perform a full version check, see
+# https://github.com/rust-lang/rust-bindgen/issues/2138.
+cc_name=$($(dirname $0)/cc-version.sh $CC | cut -f1 -d' ')
+if [ "$cc_name" = Clang ]; then
+	clang_version=$( \
+		LC_ALL=C $CC --version 2>/dev/null \
+			| sed -nE '1s:.*version ([0-9]+\.[0-9]+\.[0-9]+).*:\1:p'
+	)
+	if [ "$clang_version" != "$bindgen_libclang_version" ]; then
+		echo >&2 "***"
+		echo >&2 "*** libclang (used by the Rust bindings generator '$BINDGEN')"
+		echo >&2 "*** version does not match Clang's. This may be a problem."
+		echo >&2 "***   libclang version: $bindgen_libclang_version"
+		echo >&2 "***   Clang version:    $clang_version"
+		echo >&2 "***"
+		warning=1
+	fi
+fi
+
+# Check that the source code for the `core` standard library exists.
+#
+# `$KRUSTFLAGS` is passed in case the user added `--sysroot`.
+rustc_sysroot=$("$RUSTC" $KRUSTFLAGS --print sysroot)
+rustc_src=${RUST_LIB_SRC:-"$rustc_sysroot/lib/rustlib/src/rust/library"}
+rustc_src_core="$rustc_src/core/src/lib.rs"
+if [ ! -e "$rustc_src_core" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Source code for the 'core' standard library could not be found"
+	echo >&2 "*** at '$rustc_src_core'."
+	echo >&2 "***"
+	exit 1
+fi