On Wed, Sep 21 2016, Kees Cook <keesc...@chromium.org> wrote:
> On Tue, Sep 20, 2016 at 3:28 PM, Rasmus Villemoes
> <li...@rasmusvillemoes.dk> 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 <li...@rasmusvillemoes.dk>
>
> Heheh. Nice. :)
>
> Acked-by: Kees Cook <keesc...@chromium.org>
>
> 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
