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, 41 insertions, 0 deletions

diff --git a/lib/raid/raid6/algos.h b/lib/raid/raid6/algos.h
new file mode 100644
index 000000000..43f636be1
--- /dev/null
+++ b/lib/raid/raid6/algos.h
@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright 2003 H. Peter Anvin - All Rights Reserved
+ */
+#ifndef _PQ_IMPL_H
+#define _PQ_IMPL_H
+
+#include <linux/init.h>
+#include <linux/raid/pq_tables.h>
+
+/* Routine choices */
+struct raid6_calls {
+	const char *name;
+	void (*gen_syndrome)(int disks, size_t bytes, void **ptrs);
+	void (*xor_syndrome)(int disks, int start, int stop, size_t bytes,
+			void **ptrs);
+};
+
+struct raid6_recov_calls {
+	const char *name;
+	void (*data2)(int disks, size_t bytes, int faila, int failb,
+			void **ptrs);
+	void (*datap)(int disks, size_t bytes, int faila, void **ptrs);
+};
+
+void __init raid6_algo_add(const struct raid6_calls *algo);
+void __init raid6_algo_add_default(void);
+void __init raid6_recov_algo_add(const struct raid6_recov_calls *algo);
+
+/* for the kunit test */
+const struct raid6_calls *raid6_algo_find(unsigned int idx);
+const struct raid6_recov_calls *raid6_recov_algo_find(unsigned int idx);
+
+/* generic implementations */
+extern const struct raid6_calls raid6_intx1;
+extern const struct raid6_calls raid6_intx2;
+extern const struct raid6_calls raid6_intx4;
+extern const struct raid6_calls raid6_intx8;
+extern const struct raid6_recov_calls raid6_recov_intx1;
+
+#endif /* _PQ_IMPL_H */