Re: [PATCH] fs/proc/array.c: slightly improve render_sigset_t

[Kees Cook]

On Tue, Sep 20, 2016 at 3:28 PM, Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> wrote:
> format_decode and vsnprintf occasionally show up in perf top, so I
> went looking for places that might not need the full printf
> power. With the help of kprobes, I gathered some statistics on which
> format strings we mostly pass to vsnprintf. On a trivial desktop
> workload, I hit "%x" 25% of the time, so something apparently reads
> /proc/pid/status (which does 5*16 printf("%x") calls) a lot.
>
> With this patch, reading /proc/pid/status is 30% faster according to
> this microbenchmark:
>
>         char buf[4096];
>         int i, fd;
>         for (i = 0; i < 10000; ++i) {
>                 fd = open("/proc/self/status", O_RDONLY);
>                 read(fd, buf, sizeof(buf));
>                 close(fd);
>         }
>
> Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>

Heheh. Nice. :)

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

Out of curiosity, what other stuff ended up near the top? I'd be
curious to see your kprobes too.

-Kees

> ---
>  fs/proc/array.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 88c7de12197b..7f73b689a15c 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -251,7 +251,7 @@ void render_sigset_t(struct seq_file *m, const char *header,
>                 if (sigismember(set, i+2)) x |= 2;
>                 if (sigismember(set, i+3)) x |= 4;
>                 if (sigismember(set, i+4)) x |= 8;
> -               seq_printf(m, "%x", x);
> +               seq_putc(m, hex_asc[x]);
>         } while (i >= 4);
>
>         seq_putc(m, '\n');
> --
> 2.1.4
>



-- 
Kees Cook
Nexus Security