Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal

[Dave Hansen <dave.hansen@linux.intel.com>]

On 10/05/2015 01:22 PM, Linus Torvalds wrote:
> On Mon, Oct 5, 2015 at 5:23 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote:
>> One thing I've been noticing on Skylake is that barriers (implicit and
>> explicit) are showing up more in profiles.
> 
> Ahh, you're on skylake?

Yup.

> It's entirely possible that the issue is that the whole
> "stac/mov/clac" is much more expensive because skylake actually ends
> up supporting those AC instructions. That would make sense.
> 
> We could probably do them outside the loop, rather than tightly around
> the actual move instructions. Peter (hpa), is there some sane
> interface to try to do that?

iov_iter_fault_in_readable() is just going and touching a single word in
the page so that it is faulted in, or a pair of words if it manages to
cross a page boundary (which isn't happening here).  I'm not sure
there's a loop to move them out of here (for the prefaulting part).

We could theoretically expand the stac/clac to be around the pair of
__get_user()s in fault_in_pages_readable() but that would only help the
case where we are crossing a page boundary.

Although I was probably wrong about the source of the overhead, the
point still remains that the prefaulting is eating cycles for no
practical benefit.

>>  What we're seeing here
>> probably isn't actually stac/clac overhead, but the cost of finishing
>> some other operations that are outstanding before we can proceed through
>> here.
> 
> I suspect it actually _is_ stac/clac overhead. It might well be that
> clac/stac ends up serializing loads some way. Last I heard, they were
> reasonably cheap but certainly not free - and when we're talking about
> something that just loops over bringing the line into cache, it might
> be relatively expensive.
> 
> How did you do the profile? Use "-e cycles:pp" to get the precise
> profile information, which should actually attribute the cost to the
> instruction that really causes it.

It reduced the skid a bit.

Plain (no -e"):
>        │      stac
>  24.57 │      mov    (%rcx),%sil
>  15.70 │      clac
>  28.77 │      test   %eax,%eax
>   2.15 │      mov    %sil,-0x1(%rbp)
>   8.93 │    ↓ jne    66
>   2.31 │      movslq %edx,%rdx

With "-e cycles:pp":
>        │      sub    $0x8,%rsp
>  24.57 │      stac
>  15.49 │      mov    (%rcx),%sil
>  29.06 │      clac
>   2.24 │      test   %eax,%eax
>   8.77 │      mov    %sil,-0x1(%rbp)
>   2.22 │    ↓ jne    66
>        │      movslq %edx,%rdx