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, 98 insertions, 0 deletions

diff --git a/scripts/include/hashtable.h b/scripts/include/hashtable.h
new file mode 100644
index 000000000..45abcb12b
--- /dev/null
+++ b/scripts/include/hashtable.h
@@ -0,0 +1,98 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef HASHTABLE_H
+#define HASHTABLE_H
+
+#include "array_size.h"
+#include "list.h"
+
+#define HASH_SIZE(name) (ARRAY_SIZE(name))
+
+#define HASHTABLE_DECLARE(name, size)		struct hlist_head name[size]
+
+#define HASHTABLE_DEFINE(name, size)						\
+	HASHTABLE_DECLARE(name, size) =						\
+			{ [0 ... ((size) - 1)] = HLIST_HEAD_INIT }
+
+#define hash_head(table, key)		(&(table)[(key) % HASH_SIZE(table)])
+
+static inline void __hash_init(struct hlist_head *ht, unsigned int sz)
+{
+	unsigned int i;
+
+	for (i = 0; i < sz; i++)
+		INIT_HLIST_HEAD(&ht[i]);
+}
+
+/**
+ * hash_init - initialize a hash table
+ * @table: hashtable to be initialized
+ *
+ * This has to be a macro since HASH_SIZE() will not work on pointers since
+ * it calculates the size during preprocessing.
+ */
+#define hash_init(table) __hash_init(table, HASH_SIZE(table))
+
+/**
+ * hash_add - add an object to a hashtable
+ * @table: hashtable to add to
+ * @node: the &struct hlist_node of the object to be added
+ * @key: the key of the object to be added
+ */
+#define hash_add(table, node, key)						\
+	hlist_add_head(node, hash_head(table, key))
+
+/**
+ * hash_del - remove an object from a hashtable
+ * @node: &struct hlist_node of the object to remove
+ */
+static inline void hash_del(struct hlist_node *node)
+{
+	hlist_del_init(node);
+}
+
+/**
+ * hash_for_each - iterate over a hashtable
+ * @table: hashtable to iterate
+ * @obj: the type * to use as a loop cursor for each entry
+ * @member: the name of the hlist_node within the struct
+ */
+#define hash_for_each(table, obj, member)				\
+	for (int _bkt = 0; _bkt < HASH_SIZE(table); _bkt++)		\
+		hlist_for_each_entry(obj, &table[_bkt], member)
+
+/**
+ * hash_for_each_safe - iterate over a hashtable safe against removal of
+ * hash entry
+ * @table: hashtable to iterate
+ * @obj: the type * to use as a loop cursor for each entry
+ * @tmp: a &struct hlist_node used for temporary storage
+ * @member: the name of the hlist_node within the struct
+ */
+#define hash_for_each_safe(table, obj, tmp, member)			\
+	for (int _bkt = 0; _bkt < HASH_SIZE(table); _bkt++)		\
+		hlist_for_each_entry_safe(obj, tmp, &table[_bkt], member)
+
+/**
+ * hash_for_each_possible - iterate over all possible objects hashing to the
+ * same bucket
+ * @table: hashtable to iterate
+ * @obj: the type * to use as a loop cursor for each entry
+ * @member: the name of the hlist_node within the struct
+ * @key: the key of the objects to iterate over
+ */
+#define hash_for_each_possible(table, obj, member, key)			\
+	hlist_for_each_entry(obj, hash_head(table, key), member)
+
+/**
+ * hash_for_each_possible_safe - iterate over all possible objects hashing to the
+ * same bucket safe against removals
+ * @table: hashtable to iterate
+ * @obj: the type * to use as a loop cursor for each entry
+ * @tmp: a &struct hlist_node used for temporary storage
+ * @member: the name of the hlist_node within the struct
+ * @key: the key of the objects to iterate over
+ */
+#define hash_for_each_possible_safe(table, obj, tmp, member, key)	\
+	hlist_for_each_entry_safe(obj, tmp, hash_head(table, key), member)
+
+#endif /* HASHTABLE_H */