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, 77 insertions, 0 deletions

diff --git a/include/dt-bindings/clock/mpc512x-clock.h b/include/dt-bindings/clock/mpc512x-clock.h
new file mode 100644
index 000000000..13c316bf2
--- /dev/null
+++ b/include/dt-bindings/clock/mpc512x-clock.h
@@ -0,0 +1,77 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * This header provides constants for MPC512x clock specs in DT bindings.
+ */
+
+#ifndef _DT_BINDINGS_CLOCK_MPC512x_CLOCK_H
+#define _DT_BINDINGS_CLOCK_MPC512x_CLOCK_H
+
+#define MPC512x_CLK_DUMMY		0
+#define MPC512x_CLK_REF			1
+#define MPC512x_CLK_SYS			2
+#define MPC512x_CLK_DIU			3
+#define MPC512x_CLK_VIU			4
+#define MPC512x_CLK_CSB			5
+#define MPC512x_CLK_E300		6
+#define MPC512x_CLK_IPS			7
+#define MPC512x_CLK_FEC			8
+#define MPC512x_CLK_SATA		9
+#define MPC512x_CLK_PATA		10
+#define MPC512x_CLK_NFC			11
+#define MPC512x_CLK_LPC			12
+#define MPC512x_CLK_MBX_BUS		13
+#define MPC512x_CLK_MBX			14
+#define MPC512x_CLK_MBX_3D		15
+#define MPC512x_CLK_AXE			16
+#define MPC512x_CLK_USB1		17
+#define MPC512x_CLK_USB2		18
+#define MPC512x_CLK_I2C			19
+#define MPC512x_CLK_MSCAN0_MCLK		20
+#define MPC512x_CLK_MSCAN1_MCLK		21
+#define MPC512x_CLK_MSCAN2_MCLK		22
+#define MPC512x_CLK_MSCAN3_MCLK		23
+#define MPC512x_CLK_BDLC		24
+#define MPC512x_CLK_SDHC		25
+#define MPC512x_CLK_PCI			26
+#define MPC512x_CLK_PSC_MCLK_IN		27
+#define MPC512x_CLK_SPDIF_TX		28
+#define MPC512x_CLK_SPDIF_RX		29
+#define MPC512x_CLK_SPDIF_MCLK		30
+#define MPC512x_CLK_SPDIF		31
+#define MPC512x_CLK_AC97		32
+#define MPC512x_CLK_PSC0_MCLK		33
+#define MPC512x_CLK_PSC1_MCLK		34
+#define MPC512x_CLK_PSC2_MCLK		35
+#define MPC512x_CLK_PSC3_MCLK		36
+#define MPC512x_CLK_PSC4_MCLK		37
+#define MPC512x_CLK_PSC5_MCLK		38
+#define MPC512x_CLK_PSC6_MCLK		39
+#define MPC512x_CLK_PSC7_MCLK		40
+#define MPC512x_CLK_PSC8_MCLK		41
+#define MPC512x_CLK_PSC9_MCLK		42
+#define MPC512x_CLK_PSC10_MCLK		43
+#define MPC512x_CLK_PSC11_MCLK		44
+#define MPC512x_CLK_PSC_FIFO		45
+#define MPC512x_CLK_PSC0		46
+#define MPC512x_CLK_PSC1		47
+#define MPC512x_CLK_PSC2		48
+#define MPC512x_CLK_PSC3		49
+#define MPC512x_CLK_PSC4		50
+#define MPC512x_CLK_PSC5		51
+#define MPC512x_CLK_PSC6		52
+#define MPC512x_CLK_PSC7		53
+#define MPC512x_CLK_PSC8		54
+#define MPC512x_CLK_PSC9		55
+#define MPC512x_CLK_PSC10		56
+#define MPC512x_CLK_PSC11		57
+#define MPC512x_CLK_SDHC2		58
+#define MPC512x_CLK_FEC2		59
+#define MPC512x_CLK_OUT0_CLK		60
+#define MPC512x_CLK_OUT1_CLK		61
+#define MPC512x_CLK_OUT2_CLK		62
+#define MPC512x_CLK_OUT3_CLK		63
+#define MPC512x_CLK_CAN_CLK_IN		64
+
+#define MPC512x_CLK_LAST_PUBLIC		64
+
+#endif