Subject: Re: [PATCH 0/9] mm: Hardened usercopy

From: Ingo Molnar <[email-masked]>
Date: Fri, 8 Jul 2016 20:23:44 +0200

* Linus Torvalds <[email-masked]> wrote:

> On Fri, Jul 8, 2016 at 1:46 AM, Ingo Molnar <[email-masked]> wrote:
> >
> > Could you please try to find some syscall workload that does many small user
> > copies and thus excercises this code path aggressively?
> 
> Any stat()-heavy path will hit cp_new_stat() very heavily. Think the
> usual kind of "traverse the whole tree looking for something". "git
> diff" will do it, just checking that everything is up-to-date.
> 
> That said, other things tend to dominate.

So I think a cached 'find /usr >/dev/null' might be a good one as well:

 triton:~/tip> strace -c find /usr >/dev/null
 % time     seconds  usecs/call     calls    errors syscall
 ------ ----------- ----------- --------- --------- ----------------
  47.09    0.006518           0    254697           newfstatat
  26.20    0.003627           0    254795           getdents
  14.45    0.002000           0   1147411           fcntl
   7.33    0.001014           0    509811           close
   3.28    0.000454           0    128220         1 openat
   1.52    0.000210           0    128230           fstat
   0.27    0.000016           0     12810           write
   0.00    0.000000           0        10           read

 triton:~/tip> perf stat --repeat 3 -e cycles:u,cycles:k,cycles find /usr >/dev/null

 Performance counter stats for 'find /usr' (3 runs):

     1,594,437,143      cycles:u                                                      ( +-  2.76% )
     2,570,544,009      cycles:k                                                      ( +-  2.50% )
     4,164,981,152      cycles                                                        ( +-  2.59% )

       0.929883686 seconds time elapsed                                          ( +-  2.57% )

... and it's dominated by kernel overhead, with a fair amount of memcpy overhead 
as well:

   1.22%  find     [kernel.kallsyms]   [k] copy_user_enhanced_fast_string                                                                                                            

But maybe there are simple shell commands that are even more user-memcpy intense? 

Thanks,

	Ingo