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, 23 insertions, 0 deletions

diff --git a/Documentation/driver-api/wmi.rst b/Documentation/driver-api/wmi.rst
new file mode 100644
index 000000000..b847bcdcb
--- /dev/null
+++ b/Documentation/driver-api/wmi.rst
@@ -0,0 +1,23 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+==============
+WMI Driver API
+==============
+
+The WMI driver core supports a more modern bus-based interface for interacting
+with WMI devices, and an older GUID-based interface. The latter interface is
+considered to be deprecated, so new WMI drivers should generally avoid it since
+it has some issues with multiple WMI devices sharing the same GUID.
+The modern bus-based interface instead maps each WMI device to a
+:c:type:`struct wmi_device <wmi_device>`, so it supports WMI devices sharing the
+same GUID. Drivers can then register a :c:type:`struct wmi_driver <wmi_driver>`
+which will be bound to compatible WMI devices by the driver core.
+
+.. kernel-doc:: include/linux/wmi.h
+   :internal:
+
+.. kernel-doc:: drivers/platform/wmi/string.c
+   :export:
+
+.. kernel-doc:: drivers/platform/wmi/core.c
+   :export: