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

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Date:   Wed, 18 Nov 2020 14:49:52 +0100
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        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>,
        Dan Williams <dan.j.williams@...el.com>,
        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,
        lkp@...el.com, ying.huang@...el.com, feng.tang@...el.com,
        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 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... The NPB benchmark appears to be a supercomputing
benchmark so concievably it could be heavily using THPs. The question is
why it would be a heavy user of pinning as well but even that is imaginable
considering that MPI is in use etc.

So maybe it is worth trying to reproduce this because heavy THP + pinning
users might be indeed rare and only those would show regressions in THP
pinning performance...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR