[PATCH v4 bpf-next 2/4] libbpf: auto-set PERF_EVENT_ARRAY size to number of CPUs

[Andrii Nakryiko <andriin@fb.com>]

For BPF_MAP_TYPE_PERF_EVENT_ARRAY typically correct size is number of
possible CPUs. This is impossible to specify at compilation time. This
change adds automatic setting of PERF_EVENT_ARRAY size to number of
system CPUs, unless non-zero size is specified explicitly. This allows
to adjust size for advanced specific cases, while providing convenient
and logical defaults.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
---
 tools/lib/bpf/libbpf.c | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e2bf3eedd0ae..a5ae7207d19a 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2118,6 +2118,7 @@ static int
 bpf_object__create_maps(struct bpf_object *obj)
 {
 	struct bpf_create_map_attr create_attr = {};
+	int nr_cpus = 0;
 	unsigned int i;
 	int err;
 
@@ -2140,7 +2141,22 @@ bpf_object__create_maps(struct bpf_object *obj)
 		create_attr.map_flags = def->map_flags;
 		create_attr.key_size = def->key_size;
 		create_attr.value_size = def->value_size;
-		create_attr.max_entries = def->max_entries;
+		if (def->type == BPF_MAP_TYPE_PERF_EVENT_ARRAY &&
+		    !def->max_entries) {
+			if (!nr_cpus)
+				nr_cpus = libbpf_num_possible_cpus();
+			if (nr_cpus < 0) {
+				pr_warning("failed to determine number of system CPUs: %d\n",
+					   nr_cpus);
+				err = nr_cpus;
+				goto err_out;
+			}
+			pr_debug("map '%s': setting size to %d\n",
+				 map->name, nr_cpus);
+			create_attr.max_entries = nr_cpus;
+		} else {
+			create_attr.max_entries = def->max_entries;
+		}
 		create_attr.btf_fd = 0;
 		create_attr.btf_key_type_id = 0;
 		create_attr.btf_value_type_id = 0;
@@ -2157,9 +2173,10 @@ bpf_object__create_maps(struct bpf_object *obj)
 		*pfd = bpf_create_map_xattr(&create_attr);
 		if (*pfd < 0 && (create_attr.btf_key_type_id ||
 				 create_attr.btf_value_type_id)) {
-			cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
+			err = -errno;
+			cp = libbpf_strerror_r(err, errmsg, sizeof(errmsg));
 			pr_warning("Error in bpf_create_map_xattr(%s):%s(%d). Retrying without BTF.\n",
-				   map->name, cp, errno);
+				   map->name, cp, err);
 			create_attr.btf_fd = 0;
 			create_attr.btf_key_type_id = 0;
 			create_attr.btf_value_type_id = 0;
@@ -2171,11 +2188,11 @@ bpf_object__create_maps(struct bpf_object *obj)
 		if (*pfd < 0) {
 			size_t j;
 
-			err = *pfd;
+			err = -errno;
 err_out:
-			cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
-			pr_warning("failed to create map (name: '%s'): %s\n",
-				   map->name, cp);
+			cp = libbpf_strerror_r(err, errmsg, sizeof(errmsg));
+			pr_warning("failed to create map (name: '%s'): %s(%d)\n",
+				   map->name, cp, err);
 			for (j = 0; j < i; j++)
 				zclose(obj->maps[j].fd);
 			return err;
-- 
2.17.1