Re: [PATCH v2 bpf-next 1/2] bpf: add a new API libbpf_num_possible_cpus()

[Martin Lau <kafai@fb.com>]

On Thu, Jun 06, 2019 at 09:58:36AM -0700, Hechao Li wrote:
> Adding a new API libbpf_num_possible_cpus() that helps user with
> per-CPU map operations.
> 
> Signed-off-by: Hechao Li <hechaol@fb.com>
> ---
>  tools/lib/bpf/libbpf.c   | 53 ++++++++++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf.h   | 16 ++++++++++++
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 70 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index ba89d9727137..ea80ba6f7dfd 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3827,3 +3827,56 @@ 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 = errno;
What is the errno when len == 0?

> +		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;
> +		}
> +	}
> +	return cpus;
> +}
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 1af0d48178c8..f5e82eb2e5d4 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
>