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, 232 insertions, 0 deletions

diff --git a/tools/testing/fault-injection/failcmd.sh b/tools/testing/fault-injection/failcmd.sh
new file mode 100755
index 000000000..c4f243275
--- /dev/null
+++ b/tools/testing/fault-injection/failcmd.sh
@@ -0,0 +1,232 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# NAME
+#	failcmd.sh - run a command with injecting slab/page allocation failures
+#
+# SYNOPSIS
+#	failcmd.sh --help
+#	failcmd.sh [<options>] command [arguments]
+#
+# DESCRIPTION
+#	Run command with injecting slab/page allocation failures by fault
+#	injection.
+#
+#	NOTE: you need to run this script as root.
+#
+
+usage()
+{
+	cat >&2 <<EOF
+Usage: $0 [options] command [arguments]
+
+OPTIONS
+	-p percent
+	--probability=percent
+		likelihood of failure injection, in percent.
+		Default value is 1
+
+	-t value
+	--times=value
+		specifies how many times failures may happen at most.
+		Default value is 1
+
+	--oom-kill-allocating-task=value
+		set /proc/sys/vm/oom_kill_allocating_task to specified value
+		before running the command.
+		Default value is 1
+
+	-h, --help
+		Display a usage message and exit
+
+	--interval=value, --space=value, --verbose=value, --task-filter=value,
+	--stacktrace-depth=value, --require-start=value, --require-end=value,
+	--reject-start=value, --reject-end=value, --ignore-gfp-wait=value
+		See Documentation/fault-injection/fault-injection.rst for more
+		information
+
+	failslab options:
+	--cache-filter=value
+
+	fail_page_alloc options:
+	--ignore-gfp-highmem=value, --min-order=value
+
+ENVIRONMENT
+	FAILCMD_TYPE
+		The following values for FAILCMD_TYPE are recognized:
+
+		failslab
+			inject slab allocation failures
+		fail_page_alloc
+			inject page allocation failures
+
+		If FAILCMD_TYPE is not defined, then failslab is used.
+EOF
+}
+
+exit_if_not_hex() {
+    local value="$1"
+    if ! [[ $value =~ ^0x[0-9a-fA-F]+$ ]]; then
+        echo "Error: The provided value '$value' is not a valid hexadecimal number." >&2
+        exit 1
+    fi
+}
+
+if [ $UID != 0 ]; then
+	echo must be run as root >&2
+	exit 1
+fi
+
+DEBUGFS=`mount -t debugfs | head -1 | awk '{ print $3}'`
+
+if [ ! -d "$DEBUGFS" ]; then
+	echo debugfs is not mounted >&2
+	exit 1
+fi
+
+FAILCMD_TYPE=${FAILCMD_TYPE:-failslab}
+FAULTATTR=$DEBUGFS/$FAILCMD_TYPE
+
+if [ ! -d $FAULTATTR ]; then
+	echo $FAILCMD_TYPE is not available >&2
+	exit 1
+fi
+
+LONGOPTS=probability:,interval:,times:,space:,verbose:,task-filter:
+LONGOPTS=$LONGOPTS,stacktrace-depth:,require-start:,require-end:
+LONGOPTS=$LONGOPTS,reject-start:,reject-end:,oom-kill-allocating-task:,help
+
+if [ $FAILCMD_TYPE = failslab ]; then
+	LONGOPTS=$LONGOPTS,ignore-gfp-wait:,cache-filter:
+elif [ $FAILCMD_TYPE = fail_page_alloc ]; then
+	LONGOPTS=$LONGOPTS,ignore-gfp-wait:,ignore-gfp-highmem:,min-order:
+fi
+
+TEMP=`getopt -o p:i:t:s:v:h --long $LONGOPTS -n 'failcmd.sh' -- "$@"`
+
+if [ $? != 0 ]; then
+	usage
+	exit 1
+fi
+
+eval set -- "$TEMP"
+
+fault_attr_default()
+{
+	echo N > $FAULTATTR/task-filter
+	echo 0 > $FAULTATTR/probability
+	echo 1 > $FAULTATTR/times
+}
+
+fault_attr_default
+
+oom_kill_allocating_task_saved=`cat /proc/sys/vm/oom_kill_allocating_task`
+
+restore_values()
+{
+	fault_attr_default
+	echo $oom_kill_allocating_task_saved \
+		> /proc/sys/vm/oom_kill_allocating_task
+}
+
+#
+# Default options
+#
+declare -i oom_kill_allocating_task=1
+declare task_filter=Y
+declare -i probability=1
+declare -i times=1
+
+while true; do
+	case "$1" in
+	-p|--probability)
+		probability=$2
+		shift 2
+		;;
+	-i|--interval)
+		echo $2 > $FAULTATTR/interval
+		shift 2
+		;;
+	-t|--times)
+		times=$2
+		shift 2
+		;;
+	-s|--space)
+		echo $2 > $FAULTATTR/space
+		shift 2
+		;;
+	-v|--verbose)
+		echo $2 > $FAULTATTR/verbose
+		shift 2
+		;;
+	--task-filter)
+		task_filter=$2
+		shift 2
+		;;
+	--stacktrace-depth)
+		echo $2 > $FAULTATTR/stacktrace-depth
+		shift 2
+		;;
+	--require-start)
+		exit_if_not_hex "$2"
+		echo $2 > $FAULTATTR/require-start
+		shift 2
+		;;
+	--require-end)
+		exit_if_not_hex "$2"
+		echo $2 > $FAULTATTR/require-end
+		shift 2
+		;;
+	--reject-start)
+		exit_if_not_hex "$2"
+		echo $2 > $FAULTATTR/reject-start
+		shift 2
+		;;
+	--reject-end)
+		exit_if_not_hex "$2"
+		echo $2 > $FAULTATTR/reject-end
+		shift 2
+		;;
+	--oom-kill-allocating-task)
+		oom_kill_allocating_task=$2
+		shift 2
+		;;
+	--ignore-gfp-wait)
+		echo $2 > $FAULTATTR/ignore-gfp-wait
+		shift 2
+		;;
+	--cache-filter)
+		echo $2 > $FAULTATTR/cache_filter
+		shift 2
+		;;
+	--ignore-gfp-highmem)
+		echo $2 > $FAULTATTR/ignore-gfp-highmem
+		shift 2
+		;;
+	--min-order)
+		echo $2 > $FAULTATTR/min-order
+		shift 2
+		;;
+	-h|--help)
+		usage
+		exit 0
+		shift
+		;;
+	--)
+		shift
+		break
+		;;
+	esac
+done
+
+[ -z "$1" ] && exit 0
+
+echo $oom_kill_allocating_task > /proc/sys/vm/oom_kill_allocating_task
+echo $task_filter > $FAULTATTR/task-filter
+echo $probability > $FAULTATTR/probability
+echo $times > $FAULTATTR/times
+
+trap "restore_values" SIGINT SIGTERM EXIT
+
+cmd="echo 1 > /proc/self/make-it-fail && exec $@"
+bash -c "$cmd"