[PATCH] libbpf: provide num present cpus

[Carlos Rodriguez-Fernandez <carlosrodrifernandez@xxxxxxxxx>]

It allows tools to iterate over CPUs present
in the system that are actually running processes,
which can be less than the number of possible CPUs.

Signed-off-by: Carlos Rodriguez-Fernandez <carlosrodrifernandez@xxxxxxxxx>
---
 src/libbpf.c | 32 +++++++++++++++++++++++++++-----
 src/libbpf.h |  8 +++++---
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/src/libbpf.c b/src/libbpf.c
index 214f828..e42d252 100644
--- a/src/libbpf.c
+++ b/src/libbpf.c
@@ -12615,14 +12615,26 @@ 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)
+typedef enum {POSSIBLE=0, PRESENT, NUM_TYPES } CPU_TOPOLOGY_SYSFS_TYPE;
+
+static const char *cpu_topology_sysfs_path_by_type(const CPU_TOPOLOGY_SYSFS_TYPE type) {
+	const static char *possible_sysfs_path = "/sys/devices/system/cpu/possible";
+	const static char *present_sysfs_path = "/sys/devices/system/cpu/present";
+	switch(type) {
+		case POSSIBLE: return possible_sysfs_path;
+		case PRESENT: return present_sysfs_path;
+		default: return possible_sysfs_path;
+	}
+}
+
+int libbpf_num_cpus_by_topology_sysfs_type(const CPU_TOPOLOGY_SYSFS_TYPE type)
 {
-	static const char *fcpu = "/sys/devices/system/cpu/possible";
-	static int cpus;
+	const char *fcpu = cpu_topology_sysfs_path_by_type(type);
+	static int cpus[NUM_TYPES];
 	int err, n, i, tmp_cpus;
 	bool *mask;
 
-	tmp_cpus = READ_ONCE(cpus);
+	tmp_cpus = READ_ONCE(cpus[type]);
 	if (tmp_cpus > 0)
 		return tmp_cpus;
 
@@ -12637,10 +12649,20 @@ int libbpf_num_possible_cpus(void)
 	}
 	free(mask);
 
-	WRITE_ONCE(cpus, tmp_cpus);
+	WRITE_ONCE(cpus[type], tmp_cpus);
 	return tmp_cpus;
 }
 
+int libbpf_num_possible_cpus(void)
+{
+	return libbpf_num_cpus_by_topology_sysfs_type(POSSIBLE);
+}
+
+int libbpf_num_present_cpus(void)
+{
+	return libbpf_num_cpus_by_topology_sysfs_type(PRESENT);
+}
+
 static int populate_skeleton_maps(const struct bpf_object *obj,
 				  struct bpf_map_skeleton *maps,
 				  size_t map_cnt)
diff --git a/src/libbpf.h b/src/libbpf.h
index 754da73..a34152c 100644
--- a/src/libbpf.h
+++ b/src/libbpf.h
@@ -1433,9 +1433,10 @@ LIBBPF_API int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type,
 				       enum bpf_func_id helper_id, const void *opts);
 
 /**
- * @brief **libbpf_num_possible_cpus()** is a helper function to get the
- * number of possible CPUs that the host kernel supports and expects.
- * @return number of possible CPUs; or error code on failure
+ * @brief **libbpf_num_possible_cpus()**, and **libbpf_num_present_cpus()**
+ * are helper functions to get the number of possible, and present CPUs respectivelly.
+ * See for more information: https://www.kernel.org/doc/html/latest/admin-guide/cputopology.html
+ * @return number of CPUs; or error code on failure
  *
  * Example usage:
  *
@@ -1447,6 +1448,7 @@ LIBBPF_API int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type,
  *     bpf_map_lookup_elem(per_cpu_map_fd, key, values);
  */
 LIBBPF_API int libbpf_num_possible_cpus(void);
+LIBBPF_API int libbpf_num_present_cpus(void);
 
 struct bpf_map_skeleton {
 	const char *name;
-- 
2.41.0