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, 64 insertions, 0 deletions

diff --git a/tools/perf/util/expr.h b/tools/perf/util/expr.h
new file mode 100644
index 000000000..ed12e4007
--- /dev/null
+++ b/tools/perf/util/expr.h
@@ -0,0 +1,64 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef PARSE_CTX_H
+#define PARSE_CTX_H 1
+
+struct hashmap;
+struct metric_ref;
+
+struct expr_scanner_ctx {
+	char *user_requested_cpu_list;
+	int runtime;
+	bool system_wide;
+	bool is_test;
+};
+
+struct expr_parse_ctx {
+	struct hashmap	*ids;
+	struct expr_scanner_ctx sctx;
+};
+
+struct expr_id_data;
+
+struct hashmap *ids__new(void);
+void ids__free(struct hashmap *ids);
+int ids__insert(struct hashmap *ids, const char *id);
+/*
+ * Union two sets of ids (hashmaps) and construct a third, freeing ids1 and
+ * ids2.
+ */
+struct hashmap *ids__union(struct hashmap *ids1, struct hashmap *ids2);
+
+struct expr_parse_ctx *expr__ctx_new(void);
+void expr__ctx_clear(struct expr_parse_ctx *ctx);
+void expr__ctx_free(struct expr_parse_ctx *ctx);
+
+void expr__del_id(struct expr_parse_ctx *ctx, const char *id);
+int expr__add_id(struct expr_parse_ctx *ctx, const char *id);
+int expr__add_id_val(struct expr_parse_ctx *ctx, const char *id, double val);
+int expr__add_id_val_source_count(struct expr_parse_ctx *ctx, const char *id,
+				  double val, int source_count);
+int expr__add_id_val_source_count_aggr_nr(struct expr_parse_ctx *ctx, const char *id,
+					  double val, int source_count, int aggr_nr);
+int expr__add_ref(struct expr_parse_ctx *ctx, struct metric_ref *ref);
+int expr__get_id(struct expr_parse_ctx *ctx, const char *id,
+		 struct expr_id_data **data);
+bool expr__subset_of_ids(struct expr_parse_ctx *haystack,
+			 struct expr_parse_ctx *needles);
+int expr__resolve_id(struct expr_parse_ctx *ctx, const char *id,
+		     struct expr_id_data **datap);
+
+int expr__parse(double *final_val, struct expr_parse_ctx *ctx,
+		const char *expr);
+
+int expr__find_ids(const char *expr, const char *one,
+		   struct expr_parse_ctx *ids);
+
+double expr_id_data__value(const struct expr_id_data *data);
+double expr_id_data__source_count(const struct expr_id_data *data);
+double expr_id_data__aggr_nr(const struct expr_id_data *data);
+
+double expr__get_literal(const char *literal, const struct expr_scanner_ctx *ctx);
+double expr__has_event(const struct expr_parse_ctx *ctx, bool compute_ids, const char *id);
+double expr__strcmp_cpuid_str(const struct expr_parse_ctx *ctx, bool compute_ids, const char *id);
+
+#endif