netdev - Re: [PATCH] bpf: selftests: handle sparse CPU allocations

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Message-ID: <CAH3MdRUKvPpxmOL_BGjsKeiWTEWCgKNiDKGsXsVzDLbY5F+Dzw@mail.gmail.com>
Date:   Wed, 30 Jan 2019 21:25:17 -0800
From:   Y Song <ys114321@...il.com>
To:     Martynas Pumputis <m@...bda.lt>
Cc:     netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH] bpf: selftests: handle sparse CPU allocations

[ My reply somehow rejected by netdev, this is to send it again. ]

On Wed, Jan 30, 2019 at 1:19 AM Martynas Pumputis <m@...bda.lt> wrote:
>
> Previously, bpf_num_possible_cpus() had a bug when calculating a
> number of possible CPUs in the case of sparse CPU allocations, as
> it was considering only the first range or element of
> /sys/devices/system/cpu/possible.
>
> E.g. in the case of "0,2-3" (CPU 1 is not available), the function
> returned 1 instead of 3.
>
> This patch fixes the function by making it parse all CPU ranges and
> elements.
>
> Signed-off-by: Martynas Pumputis <m@...bda.lt>
> ---
>  tools/testing/selftests/bpf/bpf_util.h | 29 +++++++++++++++++---------
>  1 file changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/bpf_util.h b/tools/testing/selftests/bpf/bpf_util.h
> index 315a44fa32af..8cab50408204 100644
> --- a/tools/testing/selftests/bpf/bpf_util.h
> +++ b/tools/testing/selftests/bpf/bpf_util.h
> @@ -13,7 +13,7 @@ static inline unsigned int bpf_num_possible_cpus(void)
>         unsigned int start, end, possible_cpus = 0;
>         char buff[128];
>         FILE *fp;
> -       int n;
> +       int n, i, j = 0;
>
>         fp = fopen(fcpu, "r");
>         if (!fp) {
> @@ -21,17 +21,26 @@ static inline unsigned int bpf_num_possible_cpus(void)
>                 exit(1);
>         }
>
> -       while (fgets(buff, sizeof(buff), fp)) {
> -               n = sscanf(buff, "%u-%u", &start, &end);
> -               if (n == 0) {
> -                       printf("Failed to retrieve # possible CPUs!\n");
> -                       exit(1);
> -               } else if (n == 1) {
> -                       end = start;
> +       if (!fgets(buff, sizeof(buff), fp)) {
> +               printf("Failed to read %s!\n", fcpu);
> +               exit(1);
> +       }
> +
> +       for (i = 0; i <= strlen(buff); i++) {
> +               if (buff[i] == ',' || buff[i] == '\0') {
> +                       buff[i] = '\0';

This does not sound right. For example, the cpu list "0,2-3",
you will change "," to '\0" so buffer becomes "0\02-3".
The next iteration you will get strlen(buff) = 1.
The "2-3" will be skipped.

> +                       n = sscanf(&buff[j], "%u-%u", &start, &end);
> +                       if (n <= 0) {
> +                               printf("Failed to retrieve # possible CPUs!\n");
> +                               exit(1);
> +                       } else if (n == 1) {
> +                               end = start;
> +                       }
> +                       possible_cpus += end - start + 1;
> +                       j = i + 1;
>                 }
> -               possible_cpus = start == 0 ? end + 1 : 0;
> -               break;
>         }
> +
>         fclose(fp);
>
>         return possible_cpus;
> --
> 2.20.1
>