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, 60 insertions, 0 deletions

diff --git a/arch/powerpc/sysdev/mpic.h b/arch/powerpc/sysdev/mpic.h
new file mode 100644
index 000000000..bb460ff57
--- /dev/null
+++ b/arch/powerpc/sysdev/mpic.h
@@ -0,0 +1,60 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _POWERPC_SYSDEV_MPIC_H
+#define _POWERPC_SYSDEV_MPIC_H
+
+/*
+ * Copyright 2006-2007, Michael Ellerman, IBM Corporation.
+ */
+
+#ifdef CONFIG_PCI_MSI
+extern void mpic_msi_reserve_hwirq(struct mpic *mpic, irq_hw_number_t hwirq);
+int __init mpic_msi_init_allocator(struct mpic *mpic);
+int __init mpic_u3msi_init(struct mpic *mpic);
+#else
+static inline void mpic_msi_reserve_hwirq(struct mpic *mpic,
+					  irq_hw_number_t hwirq)
+{
+	return;
+}
+
+static inline int mpic_u3msi_init(struct mpic *mpic)
+{
+	return -1;
+}
+#endif
+
+#if defined(CONFIG_PCI_MSI) && defined(CONFIG_PPC_PASEMI)
+int __init mpic_pasemi_msi_init(struct mpic *mpic);
+#else
+static inline int mpic_pasemi_msi_init(struct mpic *mpic) { return -1; }
+#endif
+
+extern int mpic_set_irq_type(struct irq_data *d, unsigned int flow_type);
+extern void mpic_set_vector(unsigned int virq, unsigned int vector);
+extern int mpic_set_affinity(struct irq_data *d,
+			     const struct cpumask *cpumask, bool force);
+extern void mpic_reset_core(int cpu);
+
+#ifdef CONFIG_FSL_SOC
+extern int mpic_map_error_int(struct mpic *mpic, unsigned int virq, irq_hw_number_t  hw);
+void __init mpic_err_int_init(struct mpic *mpic, irq_hw_number_t irqnum);
+int __init mpic_setup_error_int(struct mpic *mpic, int intvec);
+#else
+static inline int mpic_map_error_int(struct mpic *mpic, unsigned int virq, irq_hw_number_t  hw)
+{
+	return 0;
+}
+
+
+static inline void mpic_err_int_init(struct mpic *mpic, irq_hw_number_t irqnum)
+{
+	return;
+}
+
+static inline int mpic_setup_error_int(struct mpic *mpic, int intvec)
+{
+	return -1;
+}
+#endif
+
+#endif /* _POWERPC_SYSDEV_MPIC_H */