[PATCH v2 bpf-next 2/3] libbpf: auto-set PERF_EVENT_ARRAY size to number of CPUs

[Andrii Nakryiko]

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 <andr...@fb.com>
---
 tools/lib/bpf/libbpf.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index c74cc535902a..8f2b8a081ba7 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2114,6 +2114,7 @@ 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;
 
@@ -2136,7 +2137,21 @@ 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);
+                               return nr_cpus;
+                       }
+                       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;
-- 
2.17.1