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

[Rasmus Villemoes]

On Wed, Sep 21 2016, Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> 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.

I don't have the results from the other machine handy, but it very much
depends on what one is doing. I ran a 'find $HOME ...', and "%.2x" ended
up accounting for 99%. Turns out ecryptfs does a lot of bin-to-hex
conversions, but very inefficiently (calling sprintf("%.2x") in a
loop...). Patch sent.

Other than that, the top consists of stuff like

  "%c%c " (/proc/pid/smaps, VmFlags:)
  " %s" (/proc/cpuinfo, the flags: line)
  "%*s" (maybe from seq_pad, but not sure who the user of that is)

and a lot of other short strings which are rather hard to trace to their
source, but presumably most are related to some /proc file.

The kprobe is just

  echo 'p:vsnprintf vsnprintf fmt=+0(%dx):string' > /sys/kernel/debug/tracing/kprobe_events

This doesn't escape the strings, so embedded newlines mess up the trace
buffer slightly. I used this little piece of line noise to extract the
format strings.

  use File::Slurp;
  my $txt = read_file($ARGV[0]);
  while ($txt =~ m/fmt="(.*?)"\n/mg) {
      $_ = $1;
      s/\\/\\\\/g; s/\n/\\n/g;
      s/\t/\\t/g;  s/"/\\"/g;
      print "\"$_\"\n";
  }

Rasmus