Re: acl_permission_check: disgusting performance

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Linus Torvalds (torvalds@linux-foundation.org):
> On Fri, May 13, 2011 at 9:16 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Looks ok to me. And generates good code for acl_permission_check
> > without CONFIG_USER_NS.
> >
> > I'll see how much that function drops on the kernel profiles..
> 
> Yup, looking good.
> 
> For my "kernel make with no changes" workload, it dropped from
> 
>     1.28%           make  [kernel.kallsyms]             [k] acl_permission_check
> 
> to
> 
>      0.88%           make  [kernel.kallsyms]             [k]
> acl_permission_check
> 
> which is pretty much exactly the expected 30% drop from no longer
> having that expensive load of user_ns.
> 
> Of course, that 30% improvement is just a 0.4% performance improvement
> in the big picture, but hey, almost half a percentage point on a real
> load from just one single function in the kernel is definitely worth
> doing.

That's great, thanks for the help.

> Do you want to carry this for 2.6.40, or should I just apply it?

It makes no user-visible difference so I'd say just apply it.

thanks,
-serge