linux-kernel - Re: [x86/copy_mc] a0ac629ebe: fio.read

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Message-ID: <CAPcyv4hS7K0Arrd+C0LhjrFH=yGJf3g55_WkHOET4z58AcWrJw@mail.gmail.com>
Date:   Thu, 6 Aug 2020 08:19:52 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     kernel test robot <rong.a.chen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        X86 ML <x86@...nel.org>, stable <stable@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Vivek Goyal <vgoyal@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tony Luck <tony.luck@...el.com>,
        Erwin Tsaur <erwin.tsaur@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        0day robot <lkp@...el.com>, lkp@...ts.01.org
Subject: Re: [x86/copy_mc] a0ac629ebe: fio.read_iops -43.3% regression

On Thu, Aug 6, 2020 at 6:35 AM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * kernel test robot <rong.a.chen@...el.com> 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.