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, 38 insertions, 0 deletions

diff --git a/arch/alpha/Kconfig.debug b/arch/alpha/Kconfig.debug
new file mode 100644
index 000000000..f85f4281c
--- /dev/null
+++ b/arch/alpha/Kconfig.debug
@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config EARLY_PRINTK
+	bool
+	depends on ALPHA_GENERIC || ALPHA_SRM
+	default y
+
+config ALPHA_LEGACY_START_ADDRESS
+	bool "Legacy kernel start address"
+	depends on ALPHA_GENERIC
+	default n
+	help
+	  The 2.4 kernel changed the kernel start address from 0x310000
+	  to 0x810000 to make room for the Wildfire's larger SRM console.
+	  Recent consoles on Titan and Marvel machines also require the
+	  extra room.
+
+	  If you're using aboot 0.7 or later, the bootloader will examine the
+	  ELF headers to determine where to transfer control. Unfortunately,
+	  most older bootloaders -- APB or MILO -- hardcoded the kernel start
+	  address rather than examining the ELF headers, and the result is a
+	  hard lockup.
+
+	  Say Y if you have a broken bootloader.  Say N if you do not, or if
+	  you wish to run on Wildfire, Titan, or Marvel.
+
+config ALPHA_LEGACY_START_ADDRESS
+	bool
+	depends on !ALPHA_GENERIC && !ALPHA_TITAN && !ALPHA_MARVEL && !ALPHA_WILDFIRE
+	default y
+
+config MATHEMU
+	tristate "Kernel FP software completion" if DEBUG_KERNEL && !SMP
+	default y if !DEBUG_KERNEL || SMP
+	help
+	  This option is required for IEEE compliant floating point arithmetic
+	  on the Alpha. The only time you would ever not say Y is to say M in
+	  order to debug the code. Say Y unless you know what you are doing.