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, 82 insertions, 0 deletions

diff --git a/tools/perf/util/libbfd.h b/tools/perf/util/libbfd.h
new file mode 100644
index 000000000..953886f3d
--- /dev/null
+++ b/tools/perf/util/libbfd.h
@@ -0,0 +1,82 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __PERF_LIBBFD_H
+#define __PERF_LIBBFD_H
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+struct annotate_args;
+struct build_id;
+struct dso;
+struct inline_node;
+struct symbol;
+
+#ifdef HAVE_LIBBFD_SUPPORT
+int libbfd__addr2line(const char *dso_name, u64 addr,
+		char **file, unsigned int *line, struct dso *dso,
+		bool unwind_inlines, struct inline_node *node,
+		struct symbol *sym);
+
+
+void dso__free_a2l_libbfd(struct dso *dso);
+
+int symbol__disassemble_libbfd(const char *filename, struct symbol *sym,
+			     struct annotate_args *args);
+
+int libbfd__read_build_id(const char *filename, struct build_id *bid);
+
+int libbfd_filename__read_debuglink(const char *filename, char *debuglink, size_t size);
+
+int symbol__disassemble_bpf_libbfd(struct symbol *sym, struct annotate_args *args);
+
+#else // !defined(HAVE_LIBBFD_SUPPORT)
+#include "annotate.h"
+
+static inline int libbfd__addr2line(const char *dso_name __always_unused,
+				u64 addr __always_unused,
+				char **file __always_unused,
+				unsigned int *line __always_unused,
+				struct dso *dso __always_unused,
+				bool unwind_inlines __always_unused,
+				struct inline_node *node __always_unused,
+				struct symbol *sym __always_unused)
+{
+	return -1;
+}
+
+
+static inline void dso__free_a2l_libbfd(struct dso *dso __always_unused)
+{
+}
+
+static inline int symbol__disassemble_libbfd(const char *filename __always_unused,
+					struct symbol *sym __always_unused,
+					struct annotate_args *args __always_unused)
+{
+	return -1;
+}
+
+static inline int libbfd__read_build_id(const char *filename __always_unused,
+					struct build_id *bid __always_unused)
+{
+	return -1;
+}
+
+static inline int libbfd_filename__read_debuglink(const char *filename __always_unused,
+						char *debuglink __always_unused,
+						size_t size __always_unused)
+{
+	return -1;
+}
+
+static inline int symbol__disassemble_bpf_libbfd(struct symbol *sym __always_unused,
+						 struct annotate_args *args __always_unused)
+{
+	return SYMBOL_ANNOTATE_ERRNO__NO_LIBOPCODES_FOR_BPF;
+}
+
+#endif // defined(HAVE_LIBBFD_SUPPORT)
+
+#endif /* __PERF_LIBBFD_H */