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

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Date:   Wed, 18 Nov 2020 11:32:58 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>, Jan Kara <jack@...e.cz>
CC:     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 11/18/20 10:17 AM, 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? 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.
> 

I see, thanks for explaining. I'll take a peek, then.

thanks,
-- 
John Hubbard
NVIDIA