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, 63 insertions, 0 deletions

diff --git a/lib/raid/Kconfig b/lib/raid/Kconfig
new file mode 100644
index 000000000..978cd6ba0
--- /dev/null
+++ b/lib/raid/Kconfig
@@ -0,0 +1,63 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config XOR_BLOCKS
+	tristate
+
+# selected by architectures that provide an optimized XOR implementation
+config XOR_BLOCKS_ARCH
+	depends on XOR_BLOCKS
+	default y if ALPHA
+	default y if ARM
+	default y if ARM64
+	default y if CPU_HAS_LSX		# loongarch
+	default y if ALTIVEC			# powerpc
+	default y if RISCV_ISA_V
+	default y if SPARC
+	default y if S390
+	default y if X86_32
+	default y if X86_64
+	bool
+
+config XOR_KUNIT_TEST
+	tristate "KUnit tests for xor_gen" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	depends on XOR_BLOCKS
+	default KUNIT_ALL_TESTS
+	help
+	  Unit tests for the XOR library functions.
+
+	  This is intended to help people writing architecture-specific
+	  optimized versions.  If unsure, say N.
+
+config RAID6_PQ
+	tristate
+
+# selected by architectures that provide an optimized PQ implementation
+config RAID6_PQ_ARCH
+	depends on RAID6_PQ
+	default y if KERNEL_MODE_NEON		# arm32/arm64
+	default y if LOONGARCH
+	default y if ALTIVEC			# powerpc
+	default y if RISCV_ISA_V
+	default y if S390
+	default y if X86
+	bool
+
+config RAID6_PQ_KUNIT_TEST
+	tristate "KUnit tests for RAID6 PQ functions" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	depends on RAID6_PQ
+	default KUNIT_ALL_TESTS
+	help
+	  Unit tests for the RAID6 PQ library functions.
+
+	  This is intended to help people writing architecture-specific
+	  optimized versions.  If unsure, say N.
+
+config RAID6_PQ_BENCHMARK
+	bool "Automatically choose fastest RAID6 PQ functions"
+	depends on RAID6_PQ
+	default y
+	help
+	  Benchmark all available RAID6 PQ functions on init and choose the
+	  fastest one.