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, 46 insertions, 0 deletions

diff --git a/include/dt-bindings/clock/berlin2.h b/include/dt-bindings/clock/berlin2.h
new file mode 100644
index 000000000..b07b8efab
--- /dev/null
+++ b/include/dt-bindings/clock/berlin2.h
@@ -0,0 +1,46 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Berlin2 BG2/BG2CD clock tree IDs
+ */
+
+#define CLKID_SYS		0
+#define CLKID_CPU		1
+#define CLKID_DRMFIGO		2
+#define CLKID_CFG		3
+#define CLKID_GFX		4
+#define CLKID_ZSP		5
+#define CLKID_PERIF		6
+#define CLKID_PCUBE		7
+#define CLKID_VSCOPE		8
+#define CLKID_NFC_ECC		9
+#define CLKID_VPP		10
+#define CLKID_APP		11
+#define CLKID_AUDIO0		12
+#define CLKID_AUDIO2		13
+#define CLKID_AUDIO3		14
+#define CLKID_AUDIO1		15
+#define CLKID_GFX3D_CORE	16
+#define CLKID_GFX3D_SYS		17
+#define CLKID_ARC		18
+#define CLKID_VIP		19
+#define CLKID_SDIO0XIN		20
+#define CLKID_SDIO1XIN		21
+#define CLKID_GFX3D_EXTRA	22
+#define CLKID_GC360		23
+#define CLKID_SDIO_DLLMST	24
+#define CLKID_GETH0		25
+#define CLKID_GETH1		26
+#define CLKID_SATA		27
+#define CLKID_AHBAPB		28
+#define CLKID_USB0		29
+#define CLKID_USB1		30
+#define CLKID_PBRIDGE		31
+#define CLKID_SDIO0		32
+#define CLKID_SDIO1		33
+#define CLKID_NFC		34
+#define CLKID_SMEMC		35
+#define CLKID_AUDIOHD		36
+#define CLKID_VIDEO0		37
+#define CLKID_VIDEO1		38
+#define CLKID_VIDEO2		39
+#define CLKID_TWD		40