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, 84 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-module b/Documentation/ABI/testing/sysfs-module
new file mode 100644
index 000000000..d5b7d19bd
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-module
@@ -0,0 +1,84 @@
+What:		/sys/module/pch_phub/drivers/.../pch_mac
+Date:		August 2010
+KernelVersion:	2.6.35
+Contact:	masa-korg@dsn.okisemi.com
+Description:	Write/read GbE MAC address.
+
+What:		/sys/module/pch_phub/drivers/.../pch_firmware
+Date:		August 2010
+KernelVersion:	2.6.35
+Contact:	masa-korg@dsn.okisemi.com
+Description:	Write/read Option ROM data.
+
+
+What:		/sys/module/ehci_hcd/drivers/.../uframe_periodic_max
+Date:		July 2011
+KernelVersion:	3.1
+Contact:	Kirill Smelkov <kirr@mns.spb.ru>
+Description:	Maximum time allowed for periodic transfers per microframe (μs)
+
+		Note:
+		  USB 2.0 sets maximum allowed time for periodic transfers per
+		  microframe to be 80%, that is 100 microseconds out of 125
+		  microseconds (full microframe).
+
+		  However there are cases, when 80% max isochronous bandwidth is
+		  too limiting. For example two video streams could require 110
+		  microseconds of isochronous bandwidth per microframe to work
+		  together.
+
+		Through this setting it is possible to raise the limit so that
+		the host controller would allow allocating more than 100
+		microseconds of periodic bandwidth per microframe.
+
+		Beware, non-standard modes are usually not thoroughly tested by
+		hardware designers, and the hardware can malfunction when this
+		setting differ from default 100.
+
+What:		/sys/module/*/{coresize,initsize}
+Date:		Jan 2012
+KernelVersion:	3.3
+Contact:	Kay Sievers <kay.sievers@vrfy.org>
+Description:	Module size in bytes.
+
+What:		/sys/module/*/initstate
+Date:		Nov 2006
+KernelVersion:	2.6.19
+Contact:	Kay Sievers <kay.sievers@vrfy.org>
+Description:	Show the initialization state(live, coming, going) of
+		the module.
+
+What:		/sys/module/*/import_ns
+Date:		January 2026
+KernelVersion:	7.1
+Contact:	linux-modules@vger.kernel.org
+Description:	List of symbol namespaces imported by this module via
+		MODULE_IMPORT_NS(). Each namespace appears on a separate line.
+		This file only exists for modules that import at least one
+		namespace.
+
+What:		/sys/module/*/taint
+Date:		Jan 2012
+KernelVersion:	3.3
+Contact:	Kay Sievers <kay.sievers@vrfy.org>
+Description:	Module taint flags:
+			==  =====================
+			P   proprietary module
+			O   out-of-tree module
+			F   force-loaded module
+			C   staging driver module
+			E   unsigned module
+			K   livepatch module
+			N   in-kernel test module
+			==  =====================
+
+What:		/sys/module/grant_table/parameters/free_per_iteration
+Date:		July 2023
+KernelVersion:	6.5 but backported to all supported stable branches
+Contact:	Xen developer discussion <xen-devel@lists.xenproject.org>
+Description:	Read and write number of grant entries to attempt to free per iteration.
+
+		Note: Future versions of Xen and Linux may provide a better
+		interface for controlling the rate of deferred grant reclaim
+		or may not need it at all.
+Users:		Qubes OS (https://www.qubes-os.org)