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, 94 insertions, 0 deletions

diff --git a/scripts/mksysmap b/scripts/mksysmap
new file mode 100755
index 000000000..c4531eacd
--- /dev/null
+++ b/scripts/mksysmap
@@ -0,0 +1,94 @@
+#!/bin/sed -f
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# sed script to filter out symbols that are not needed for System.map,
+# or not suitable for kallsyms. The input should be 'nm -n <file>'.
+#
+# System.map is used by module-init tools and some debugging
+# tools to retrieve the actual addresses of symbols in the kernel.
+#
+# readprofile starts reading symbols when _stext is found, and
+# continue until it finds a symbol which is not either of 'T', 't',
+# 'W' or 'w'.
+#
+# ---------------------------------------------------------------------------
+# Ignored symbol types
+#
+
+# a: local absolute symbols
+# N: debugging symbols
+# U: undefined global symbols
+# w: local weak symbols
+/ [aNUw] /d
+
+# ---------------------------------------------------------------------------
+# Ignored prefixes
+#  (do not forget a space before each pattern)
+
+# local symbols for ARM, MIPS, etc.
+/ \$/d
+
+# local labels, .LBB, .Ltmpxxx, .L__unnamed_xx, .LASANPC, etc.
+/ \.L/d
+
+# arm64 EFI stub namespace
+/ __efistub_/d
+
+# arm64 local symbols in PIE namespace
+/ __pi_\\$/d
+/ __pi_\.L/d
+
+# arm64 local symbols in non-VHE KVM namespace
+/ __kvm_nvhe_\$/d
+/ __kvm_nvhe_\.L/d
+
+# lld arm/aarch64/mips thunks
+/ __[[:alnum:]]*Thunk_/d
+
+# CFI type identifiers
+/ __kcfi_typeid_/d
+/ __kvm_nvhe___kcfi_typeid_/d
+/ __pi___kcfi_typeid_/d
+
+# CRC from modversions
+/ __crc_/d
+
+# EXPORT_SYMBOL (symbol name)
+/ __kstrtab_/d
+
+# EXPORT_SYMBOL (namespace)
+/ __kstrtabns_/d
+
+# MODULE_DEVICE_TABLE (symbol name)
+/ __mod_device_table__/d
+
+# ---------------------------------------------------------------------------
+# Ignored suffixes
+#  (do not forget '$' after each pattern)
+
+# arm
+/_from_arm$/d
+/_from_thumb$/d
+/_veneer$/d
+
+# ---------------------------------------------------------------------------
+# Ignored symbols (exact match)
+#  (do not forget a space before and '$' after each pattern)
+
+# for LoongArch?
+/ L0$/d
+
+# ppc
+/ _SDA_BASE_$/d
+/ _SDA2_BASE_$/d
+
+# MODULE_INFO()
+/ __UNIQUE_ID_modinfo[0-9]*$/d
+
+# ---------------------------------------------------------------------------
+# Ignored patterns
+#  (symbols that contain the pattern are ignored)
+
+# ppc stub
+/\.long_branch\./d
+/\.plt_branch\./d