Re: [PATCH v2 bpf-next] libbpf: Use sysconf to simplify libbpf_num_possible_cpus

On Wed, Sep 22, 2021 at 2:22 PM Daniel Borkmann [off-list ref] wrote:

On 9/22/21 9:07 AM, Muhammad Falak R Wani wrote:

quoted

Simplify libbpf_num_possible_cpus by using sysconf(_SC_NPROCESSORS_CONF)
instead of parsing a file.
This patch is a part ([0]) of libbpf-1.0 milestone.

[0] Closes: https://github.com/libbpf/libbpf/issues/383

Signed-off-by: Muhammad Falak R Wani <redacted>
---
  tools/lib/bpf/libbpf.c | 17 ++++-------------
  1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ef5db34bf913..f1c0abe5b58d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -10898,25 +10898,16 @@ int parse_cpu_mask_file(const char *fcpu, bool **mask, int *mask_sz)

  int libbpf_num_possible_cpus(void)
  {
-     static const char *fcpu = "/sys/devices/system/cpu/possible";
      static int cpus;
-     int err, n, i, tmp_cpus;
-     bool *mask;
+     int tmp_cpus;

      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;
-     for (i = 0; i < n; i++) {
-             if (mask[i])
-                     tmp_cpus++;
-     }
-     free(mask);
+     tmp_cpus = sysconf(_SC_NPROCESSORS_CONF);
+     if (tmp_cpus < 1)
+             return libbpf_err(-EINVAL);

This approach is unfortunately broken, see also commit e00c7b216f34 ("bpf: fix
multiple issues in selftest suite and samples") for more details:

Oh, that predates me. Thanks, Daniel!

Sorry, Muhammad, seems like current implementation is there for a
reason and will have to stay. Thanks a lot for working on this,
though. Hopefully you can help with other issues, though.

[...]

Thanks,
Daniel

quoted

      WRITE_ONCE(cpus, tmp_cpus);
      return tmp_cpus;