[PATCH v5 bpf-next 1/2] bpf: add a new API libbpf_num_possible_cpus()

[Hechao Li]

Adding a new API libbpf_num_possible_cpus() that helps user with
per-CPU map operations.

Signed-off-by: Hechao Li <hech...@fb.com>
---
 tools/lib/bpf/libbpf.c   | 57 ++++++++++++++++++++++++++++++++++++++++
 tools/lib/bpf/libbpf.h   | 16 +++++++++++
 tools/lib/bpf/libbpf.map |  1 +
 3 files changed, 74 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ba89d9727137..dd8b2cd5d3a7 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -3827,3 +3827,60 @@ void bpf_program__bpil_offs_to_addr(struct 
bpf_prog_info_linear *info_linear)
                                             desc->array_offset, addr);
        }
 }
+
+int libbpf_num_possible_cpus(void)
+{
+       static const char *fcpu = "/sys/devices/system/cpu/possible";
+       int len = 0, n = 0, il = 0, ir = 0;
+       unsigned int start = 0, end = 0;
+       static int cpus;
+       char buf[128];
+       int error = 0;
+       int fd = -1;
+
+       if (cpus > 0)
+               return cpus;
+
+       fd = open(fcpu, O_RDONLY);
+       if (fd < 0) {
+               error = errno;
+               pr_warning("Failed to open file %s: %s\n",
+                          fcpu, strerror(error));
+               return -error;
+       }
+       len = read(fd, buf, sizeof(buf));
+       close(fd);
+       if (len <= 0) {
+               error = len ? errno : EINVAL;
+               pr_warning("Failed to read # of possible cpus from %s: %s\n",
+                          fcpu, strerror(error));
+               return -error;
+       }
+       if (len == sizeof(buf)) {
+               pr_warning("File %s size overflow\n", fcpu);
+               return -EOVERFLOW;
+       }
+       buf[len] = '\0';
+
+       for (ir = 0, cpus = 0; ir <= len; ir++) {
+               /* Each sub string separated by ',' has format \d+-\d+ or \d+ */
+               if (buf[ir] == ',' || buf[ir] == '\0') {
+                       buf[ir] = '\0';
+                       n = sscanf(&buf[il], "%u-%u", &start, &end);
+                       if (n <= 0) {
+                               pr_warning("Failed to get # CPUs from %s\n",
+                                          &buf[il]);
+                               return -EINVAL;
+                       } else if (n == 1) {
+                               end = start;
+                       }
+                       cpus += end - start + 1;
+                       il = ir + 1;
+               }
+       }
+       if (cpus <= 0) {
+               pr_warning("Invalid #CPUs %d from %s\n", cpus, fcpu);
+               return -EINVAL;
+       }
+       return cpus;
+}
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 1af0d48178c8..2e594a0fa961 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -454,6 +454,22 @@ bpf_program__bpil_addr_to_offs(struct bpf_prog_info_linear 
*info_linear);
 LIBBPF_API void
 bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear);
 
+/*
+ * A helper function to get the number of possible CPUs before looking up
+ * per-CPU maps. Negative errno is returned on failure.
+ *
+ * Example usage:
+ *
+ *     int ncpus = libbpf_num_possible_cpus();
+ *     if (ncpus < 0) {
+ *          // error handling
+ *     }
+ *     long values[ncpus];
+ *     bpf_map_lookup_elem(per_cpu_map_fd, key, values);
+ *
+ */
+LIBBPF_API int libbpf_num_possible_cpus(void);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 46dcda89df21..2c6d835620d2 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -172,4 +172,5 @@ LIBBPF_0.0.4 {
                btf_dump__new;
                btf__parse_elf;
                bpf_object__load_xattr;
+               libbpf_num_possible_cpus;
 } LIBBPF_0.0.3;
-- 
2.17.1