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, 60 insertions, 0 deletions

diff --git a/tools/docs/parse-headers.py b/tools/docs/parse-headers.py
new file mode 100755
index 000000000..436acea4c
--- /dev/null
+++ b/tools/docs/parse-headers.py
@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2016, 2025 by Mauro Carvalho Chehab <mchehab@kernel.org>.
+# pylint: disable=C0103
+
+"""
+Convert a C header or source file ``FILE_IN``, into a ReStructured Text
+included via ..parsed-literal block with cross-references for the
+documentation files that describe the API. It accepts an optional
+``FILE_RULES`` file to describes what elements will be either ignored or
+be pointed to a non-default reference type/name.
+
+The output is written at ``FILE_OUT``.
+
+It is capable of identifying defines, functions, structs, typedefs,
+enums and enum symbols and create cross-references for all of them.
+It is also capable of distinguish #define used for specifying a Linux
+ioctl.
+
+The optional ``FILE_RULES`` contains a set of rules like:
+
+    ignore ioctl VIDIOC_ENUM_FMT
+    replace ioctl VIDIOC_DQBUF vidioc_qbuf
+    replace define V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ :c:type:`v4l2_event_motion_det`
+"""
+
+import argparse, sys
+import os.path
+
+src_dir = os.path.dirname(os.path.realpath(__file__))
+sys.path.insert(0, os.path.join(src_dir, '../lib/python'))
+from kdoc.parse_data_structs import ParseDataStructs
+from kdoc.enrich_formatter import EnrichFormatter
+
+def main():
+    """Main function"""
+    parser = argparse.ArgumentParser(description=__doc__,
+                                     formatter_class=EnrichFormatter)
+
+    parser.add_argument("-d", "--debug", action="count", default=0,
+                        help="Increase debug level. Can be used multiple times")
+    parser.add_argument("-t", "--toc", action="store_true",
+                        help="instead of a literal block, outputs a TOC table at the RST file")
+
+    parser.add_argument("file_in", help="Input C file")
+    parser.add_argument("file_out", help="Output RST file")
+    parser.add_argument("file_rules", nargs="?",
+                        help="Exceptions file (optional)")
+
+    args = parser.parse_args()
+
+    parser = ParseDataStructs(debug=args.debug)
+    parser.parse_file(args.file_in, args.file_rules)
+
+    parser.debug_print()
+    parser.write_output(args.file_in, args.file_out, args.toc)
+
+
+if __name__ == "__main__":
+    main()