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

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Date:   Fri, 20 Nov 2020 10:42:35 +0800
From:   Oliver Sang <oliver.sang@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jan Kara <jack@...e.cz>, John Hubbard <jhubbard@...dia.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, philip.li@...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 10:17:27AM -0800, Dan Williams wrote:
> 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? 

- some extra phoronix test suites are enabled/fixed gradually so we will have
better coverage
- we scan kernel releases within the year to baseline the performance, it may
trigger bisection if one release has regressed and not recovered.

With this continuous effort, 0-day ci can detect the changes on mainline.

> 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.