[bpf-next v1 1/2] libbpf: introduce new API libbpf_num_online_cpus

[tong@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]

From: Tonghao Zhang <tong@xxxxxxxxxxxxx>

Adding a new API libbpf_num_online_cpus() that helps user with
fetching online CPUs number.

It's useful in system which number of online CPUs is different with
possible CPUs.

Signed-off-by: Tonghao Zhang <tong@xxxxxxxxxxxxx>
Cc: Quentin Monnet <quentin@xxxxxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
Cc: Song Liu <song@xxxxxxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: KP Singh <kpsingh@xxxxxxxxxx>
Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
Cc: Hao Luo <haoluo@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 tools/lib/bpf/libbpf.c   | 47 ++++++++++++++++++++++++++++++----------
 tools/lib/bpf/libbpf.h   |  7 ++++++
 tools/lib/bpf/libbpf.map |  1 +
 3 files changed, 43 insertions(+), 12 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 27d9faa80471..b84904f79ffd 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -12192,30 +12192,53 @@ int parse_cpu_mask_file(const char *fcpu, bool **mask, int *mask_sz)
 	return parse_cpu_mask_str(buf, mask, mask_sz);
 }
 
-int libbpf_num_possible_cpus(void)
+static int num_cpus(const char *fcpu)
 {
-	static const char *fcpu = "/sys/devices/system/cpu/possible";
-	static int cpus;
-	int err, n, i, tmp_cpus;
+	int err, n, i, cpus;
 	bool *mask;
 
-	tmp_cpus = READ_ONCE(cpus);
-	if (tmp_cpus > 0)
-		return tmp_cpus;
-
 	err = parse_cpu_mask_file(fcpu, &mask, &n);
 	if (err)
 		return libbpf_err(err);
 
-	tmp_cpus = 0;
+	cpus = 0;
 	for (i = 0; i < n; i++) {
 		if (mask[i])
-			tmp_cpus++;
+			cpus++;
 	}
 	free(mask);
 
-	WRITE_ONCE(cpus, tmp_cpus);
-	return tmp_cpus;
+	return cpus;
+}
+
+int libbpf_num_online_cpus(void)
+{
+	static int online_cpus;
+	int cpus;
+
+	cpus = READ_ONCE(online_cpus);
+	if (cpus > 0)
+		return cpus;
+
+	cpus = num_cpus("/sys/devices/system/cpu/online");
+
+	WRITE_ONCE(online_cpus, cpus);
+	return cpus;
+}
+
+int libbpf_num_possible_cpus(void)
+{
+	static int possible_cpus;
+	int cpus;
+
+	cpus = READ_ONCE(possible_cpus);
+	if (cpus > 0)
+		return cpus;
+
+	cpus = num_cpus("/sys/devices/system/cpu/possible");
+
+	WRITE_ONCE(possible_cpus, cpus);
+	return cpus;
 }
 
 static int populate_skeleton_maps(const struct bpf_object *obj,
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 898db26e42e9..e433575ff865 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -1332,6 +1332,13 @@ LIBBPF_API int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type,
  */
 LIBBPF_API int libbpf_num_possible_cpus(void);
 
+/**
+ * @brief **libbpf_num_online_cpus()** is a helper function to get the
+ * number of online CPUs that the host kernel supports and expects.
+ * @return number of online CPUs; or error code on failure
+ */
+LIBBPF_API int libbpf_num_online_cpus(void);
+
 struct bpf_map_skeleton {
 	const char *name;
 	struct bpf_map **map;
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 11c36a3c1a9f..384fb6333f3f 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -381,6 +381,7 @@ LIBBPF_1.1.0 {
 		user_ring_buffer__reserve;
 		user_ring_buffer__reserve_blocking;
 		user_ring_buffer__submit;
+		libbpf_num_online_cpus;
 } LIBBPF_1.0.0;
 
 LIBBPF_1.2.0 {
-- 
2.27.0