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, 73 insertions, 0 deletions

diff --git a/rust/bindgen_parameters b/rust/bindgen_parameters
new file mode 100644
index 000000000..8402b0c93
--- /dev/null
+++ b/rust/bindgen_parameters
@@ -0,0 +1,73 @@
+# SPDX-License-Identifier: GPL-2.0
+
+# We want to map these types to `isize`/`usize` manually, instead of
+# define them as `int`/`long` depending on platform bitwidth.
+--blocklist-type __kernel_s?size_t
+--blocklist-type __kernel_ptrdiff_t
+
+--opaque-type xregs_state
+--opaque-type desc_struct
+--opaque-type arch_lbr_state
+--opaque-type local_apic
+
+# Packed type cannot transitively contain a `#[repr(align)]` type.
+--opaque-type alt_instr
+--opaque-type x86_msi_data
+--opaque-type x86_msi_addr_lo
+# s390-only: same packed/align issue as above (E0588).
+--opaque-type lowcore
+--opaque-type tod_clock
+--opaque-type tpi_info
+--opaque-type uv_cb.*
+--opaque-type uv_secret.*
+--opaque-type zpci_fib
+
+# If SMP is disabled, `arch_spinlock_t` is defined as a ZST which triggers a Rust
+# warning. We don't need to peek into it anyway.
+--opaque-type spinlock
+
+# enums that appear in indirect function calls should specify a cfi type
+--newtype-enum lru_status
+--with-attribute-custom-enum=lru_status='#[cfi_encoding="10lru_status"]'
+
+# `seccomp`'s comment gets understood as a doctest
+--no-doc-comments
+
+# These functions use the `__preserve_most` calling convention, which neither bindgen
+# nor Rust currently understand, and which Clang currently declares to be unstable.
+--blocklist-function __list_.*_report
+
+# These constants are sometimes not recognized by bindgen depending on config.
+# We use const helpers to aid bindgen, to avoid conflicts when constants are
+# recognized, block generation of the non-helper constants.
+--blocklist-item ARCH_SLAB_MINALIGN
+--blocklist-item ARCH_KMALLOC_MINALIGN
+--blocklist-item VM_MERGEABLE
+--blocklist-item VM_READ
+--blocklist-item VM_WRITE
+--blocklist-item VM_EXEC
+--blocklist-item VM_SHARED
+--blocklist-item VM_MAYREAD
+--blocklist-item VM_MAYWRITE
+--blocklist-item VM_MAYEXEC
+--blocklist-item VM_MAYEXEC
+--blocklist-item VM_PFNMAP
+--blocklist-item VM_IO
+--blocklist-item VM_DONTCOPY
+--blocklist-item VM_DONTEXPAND
+--blocklist-item VM_LOCKONFAULT
+--blocklist-item VM_ACCOUNT
+--blocklist-item VM_NORESERVE
+--blocklist-item VM_HUGETLB
+--blocklist-item VM_SYNC
+--blocklist-item VM_ARCH_1
+--blocklist-item VM_WIPEONFORK
+--blocklist-item VM_DONTDUMP
+--blocklist-item VM_SOFTDIRTY
+--blocklist-item VM_MIXEDMAP
+--blocklist-item VM_HUGEPAGE
+--blocklist-item VM_NOHUGEPAGE
+
+# Structs should implement `Zeroable` when all of their fields do.
+--with-derive-custom-struct .*=MaybeZeroable
+--with-derive-custom-union .*=MaybeZeroable