Re: [LKP] Re: [mm/gup] a308c71bf1: stress-ng.vm-splice.ops_per_sec -95.6% regression

[Xing Zhengjun]

On 11/5/2020 2:29 AM, Linus Torvalds wrote:
> On Mon, Nov 2, 2020 at 1:15 AM kernel test robot <rong.a.chen@xxxxxxxxx> wrote:
> > Greeting,
> >
> > FYI, we noticed a -95.6% regression of stress-ng.vm-splice.ops_per_sec due to commit:
> >
> > commit: a308c71bf1e6e19cc2e4ced31853ee0fc7cb439a ("mm/gup: Remove enfornced COW mechanism")
> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
>
> Note that this is just the reverse of the previous 2000% improvement
> reported by the test robot here:
>
>      https://lore.kernel.org/lkml/20200611040453.GK12456@shao2-debian/
>
> and the explanation seems to remain the same:
>
>      https://lore.kernel.org/lkml/CAG48ez1v1b4X5LgFya6nvi33-TWwqna_dc5jGFVosqQhdn_Nkg@xxxxxxxxxxxxxx/
>
> IOW, this is testing a special case (zero page lookup) that the "force
> COW" patches happened to turn into a regular case (COW creating a
> regular page from the zero page).
>
> The question is whether we should care about the zero page for gup_fast lookup.
>
> If we do care, then the proper fix is likely simply to allow the zero
> page in fast-gup, the same way we already do in slow-gup.
>
> ENTIRELY UNTESTED PATCH ATTACHED.
>
> Rong - mind testing this? I don't think the zero-page _should_ be
> something that real loads care about, but hey, maybe people do want to
> do things like splice zeroes very efficiently..

I test the patch, the regression still existed.

=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/nr_threads/disk/testtime/class/cpufreq_governor/ucode:

lkp-csl-2sp5/stress-ng/debian-10.4-x86_64-20200603.cgz/x86_64-rhel-8.3/gcc-9/100%/1HDD/30s/pipe/performance/0x5002f01

commit:
  1a0cf26323c80e2f1c58fc04f15686de61bfab0c
  a308c71bf1e6e19cc2e4ced31853ee0fc7cb439a
  da5ba9980aa2211c1e2a89fc814abab2fea6f69d (debug patch)

1a0cf26323c80e2f a308c71bf1e6e19cc2e4ced3185 da5ba9980aa2211c1e2a89fc814
---------------- --------------------------- ---------------------------
         %stddev     %change         %stddev     %change         %stddev
             \          |                \          |                \
 3.406e+09           -95.6%   1.49e+08           -96.4%  1.213e+08 
   stress-ng.vm-splice.ops
 1.135e+08           -95.6%    4965911           -96.4%    4041777 
   stress-ng.vm-splice.ops_per_sec

> And note the "untested" part of the patch. It _looks_ fairly obvious,
> but maybe I'm missing something.
>
>              Linus
>
>
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--
Zhengjun Xing