linux-kernel - Re: [mm/gup] 47e29d32af: phoronix-test-suite.npb.FT.A.total_mop

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Date:   Wed, 18 Nov 2020 10:17:27 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     John Hubbard <jhubbard@...dia.com>,
        kernel test robot <oliver.sang@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kbuild test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        feng tang <feng.tang@...el.com>,
        Zhengjun Xing <zhengjun.xing@...el.com>,
        guobing.chen@...el.com, ming.a.chen@...el.com, frank.du@...el.com,
        Shuhua.Fan@...el.com, wangyang.guo@...el.com,
        Wenhuan.Huang@...el.com, jessica.ji@...el.com, shan.kang@...el.com,
        guangli.li@...el.com, tiejun.li@...el.com, yu.ma@...el.com,
        dapeng1.mi@...el.com, jiebin.sun@...el.com, gengxin.xie@...el.com,
        fan.zhao@...el.com
Subject: Re: [mm/gup] 47e29d32af: phoronix-test-suite.npb.FT.A.total_mop_s
 -45.0% regression

On Wed, Nov 18, 2020 at 5:51 AM Jan Kara <jack@...e.cz> wrote:
>
> On Mon 16-11-20 19:35:31, John Hubbard wrote:
> >
> > On 11/16/20 6:48 PM, kernel test robot wrote:
> > >
> > > Greeting,
> > >
> > > FYI, we noticed a -45.0% regression of phoronix-test-suite.npb.FT.A.total_mop_s due to commit:
> > >
> >
> > That's a huge slowdown...
> >
> > >
> > > commit: 47e29d32afba11b13efb51f03154a8cf22fb4360 ("mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages")
> > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> >
> > ...but that commit happened in April, 2020. Surely if this were a serious
> > issue we would have some other indication...is this worth following up
> > on?? I'm inclined to ignore it, honestly.
>
> Why this was detected so late is a fair question although it doesn't quite
> invalidate the report...

I don't know what specifically happened in this case, perhaps someone
from the lkp team can comment? However, the myth / contention that
"surely someone else would have noticed by now" is why the lkp project
was launched. Kernels regressed without much complaint and it wasn't
until much later in the process, around the time enterprise distros
rebased to new kernels, did end users start filing performance loss
regression reports. Given -stable kernel releases, 6-7 months is still
faster than many end user upgrade cycles to new kernel baselines.