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>

4 files changed, 143 insertions, 0 deletions

diff --git a/tools/testing/rbtree/Makefile b/tools/testing/rbtree/Makefile
new file mode 100644
index 000000000..d7bbae2af
--- /dev/null
+++ b/tools/testing/rbtree/Makefile
@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: GPL-2.0
+
+.PHONY: clean
+
+TARGETS = rbtree_test interval_tree_test
+OFILES = $(SHARED_OFILES) rbtree-shim.o interval_tree-shim.o maple-shim.o
+DEPS = ../../../include/linux/rbtree.h \
+	../../../include/linux/rbtree_types.h \
+	../../../include/linux/rbtree_augmented.h \
+	../../../include/linux/interval_tree.h \
+	../../../include/linux/interval_tree_generic.h \
+	../../../lib/rbtree.c \
+	../../../lib/interval_tree.c
+
+targets: $(TARGETS)
+
+include ../shared/shared.mk
+
+ifeq ($(DEBUG), 1)
+	CFLAGS += -g
+endif
+
+$(TARGETS):	$(OFILES)
+
+rbtree-shim.o: $(DEPS)
+rbtree_test.o:  ../../../lib/rbtree_test.c
+interval_tree-shim.o: $(DEPS)
+interval_tree-shim.o: CFLAGS += -DCONFIG_INTERVAL_TREE_SPAN_ITER
+interval_tree_test.o: 	../../../lib/interval_tree_test.c
+interval_tree_test.o: CFLAGS += -DCONFIG_INTERVAL_TREE_SPAN_ITER
+
+clean:
+	$(RM) $(TARGETS) *.o radix-tree.c idr.c generated/*
diff --git a/tools/testing/rbtree/interval_tree_test.c b/tools/testing/rbtree/interval_tree_test.c
new file mode 100644
index 000000000..49bc5b534
--- /dev/null
+++ b/tools/testing/rbtree/interval_tree_test.c
@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * interval_tree.c: Userspace Interval Tree test-suite
+ * Copyright (c) 2025 Wei Yang <richard.weiyang@gmail.com>
+ */
+#include <linux/math64.h>
+#include <linux/kern_levels.h>
+#include "shared.h"
+#include "maple-shared.h"
+
+#include "../../../lib/interval_tree_test.c"
+
+int usage(void)
+{
+	fprintf(stderr, "Userland interval tree test cases\n");
+	fprintf(stderr, "  -n: Number of nodes in the interval tree\n");
+	fprintf(stderr, "  -p: Number of iterations modifying the tree\n");
+	fprintf(stderr, "  -q: Number of searches to the interval tree\n");
+	fprintf(stderr, "  -s: Number of iterations searching the tree\n");
+	fprintf(stderr, "  -a: Searches will iterate all nodes in the tree\n");
+	fprintf(stderr, "  -m: Largest value for the interval's endpoint\n");
+	fprintf(stderr, "  -r: Random seed\n");
+	exit(-1);
+}
+
+void interval_tree_tests(void)
+{
+	interval_tree_test_init();
+	interval_tree_test_exit();
+}
+
+int main(int argc, char **argv)
+{
+	int opt;
+
+	while ((opt = getopt(argc, argv, "n:p:q:s:am:r:")) != -1) {
+		if (opt == 'n')
+			nnodes = strtoul(optarg, NULL, 0);
+		else if (opt == 'p')
+			perf_loops = strtoul(optarg, NULL, 0);
+		else if (opt == 'q')
+			nsearches = strtoul(optarg, NULL, 0);
+		else if (opt == 's')
+			search_loops = strtoul(optarg, NULL, 0);
+		else if (opt == 'a')
+			search_all = true;
+		else if (opt == 'm')
+			max_endpoint = strtoul(optarg, NULL, 0);
+		else if (opt == 'r')
+			seed = strtoul(optarg, NULL, 0);
+		else
+			usage();
+	}
+
+	maple_tree_init();
+	interval_tree_tests();
+	return 0;
+}
diff --git a/tools/testing/rbtree/rbtree_test.c b/tools/testing/rbtree/rbtree_test.c
new file mode 100644
index 000000000..585c970f6
--- /dev/null
+++ b/tools/testing/rbtree/rbtree_test.c
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * rbtree_test.c: Userspace Red Black Tree test-suite
+ * Copyright (c) 2025 Wei Yang <richard.weiyang@gmail.com>
+ */
+#include <linux/init.h>
+#include <linux/math64.h>
+#include <linux/kern_levels.h>
+#include "shared.h"
+
+#include "../../../lib/rbtree_test.c"
+
+int usage(void)
+{
+	fprintf(stderr, "Userland rbtree test cases\n");
+	fprintf(stderr, "  -n: Number of nodes in the rb-tree\n");
+	fprintf(stderr, "  -p: Number of iterations modifying the rb-tree\n");
+	fprintf(stderr, "  -c: Number of iterations modifying and verifying the rb-tree\n");
+	fprintf(stderr, "  -r: Random seed\n");
+	exit(-1);
+}
+
+void rbtree_tests(void)
+{
+	rbtree_test_init();
+	rbtree_test_exit();
+}
+
+int main(int argc, char **argv)
+{
+	int opt;
+
+	while ((opt = getopt(argc, argv, "n:p:c:r:")) != -1) {
+		if (opt == 'n')
+			nnodes = strtoul(optarg, NULL, 0);
+		else if (opt == 'p')
+			perf_loops = strtoul(optarg, NULL, 0);
+		else if (opt == 'c')
+			check_loops = strtoul(optarg, NULL, 0);
+		else if (opt == 'r')
+			seed = strtoul(optarg, NULL, 0);
+		else
+			usage();
+	}
+
+	rbtree_tests();
+	return 0;
+}
diff --git a/tools/testing/rbtree/test.h b/tools/testing/rbtree/test.h
new file mode 100644
index 000000000..f1f1b545b
--- /dev/null
+++ b/tools/testing/rbtree/test.h
@@ -0,0 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+void rbtree_tests(void);
+void interval_tree_tests(void);