Re: [x86/copy_mc] a0ac629ebe: fio.read_iops -43.3% regression

[Dan Williams]

On Thu, Aug 6, 2020 at 6:35 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * kernel test robot <rong.a.chen@xxxxxxxxx> wrote:
>
> > Greeting,
> >
> > FYI, we noticed a -43.3% regression of fio.read_iops due to commit:
> >
> >
> > commit: a0ac629ebe7b3d248cb93807782a00d9142fdb98 ("x86/copy_mc: Introduce copy_mc_generic()")
> > url: https://github.com/0day-ci/linux/commits/Dan-Williams/Renovate-memcpy_mcsafe-with-copy_mc_to_-user-kernel/20200802-014046
> >
> >
> > in testcase: fio-basic
> > on test machine: 96 threads Intel(R) Xeon(R) Gold 6252 CPU @ 2.10GHz with 256G memory
> > with following parameters:
>
> So this performance regression, if it isn't a spurious result, looks
> concerning. Is this expected?

This is not expected and I think delays these patches until I'm back
from leave in a few weeks. I know that we might lose some inlining
effect due to replacing native memcpy, but I did not expect it would
have an impact like this. In my testing I was seeing a performance
improvement from replacing the careful / open-coded copy with rep;
mov;, which increases the surprise of this result.