linux-kernel - Re: [mm/mremap] ecf8443e51: vm-scalability.throughput -29.4% regression

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Date:   Mon, 14 Jun 2021 07:58:07 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     kernel test robot <oliver.sang@...el.com>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        0day robot <lkp@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        "Huang, Ying" <ying.huang@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Nick Piggin <npiggin@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [mm/mremap] ecf8443e51: vm-scalability.throughput -29.4% regression

On Mon, Jun 14, 2021 at 7:39 AM kernel test robot <oliver.sang@...el.com> wrote:
>

> FYI, we noticed a -29.4% regression of vm-scalability.throughput due to commit:
> ecf8443e51a8 ("[PATCH v7 02/11] mm/mremap: Fix race between MOVE_PUD mremap and pageout")

Ouch.

I guess it's not a huge surprise, but that's a fairly large regression.

Probably because the pud lock is just one single lock ("No scalability
reason to split PUD locks yet").

What happens if pud_lockptr() were to do the same thing that pmd_lockptr() does?

                Linus