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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 5 Nov 2018 09:50:09 -0800
Message-ID: <CAHk-=wi8BNNTp+Xt4ROz1TqGgFT-c6-Hmmc9yW0B6p2FTFwJTg@mail.gmail.com>
Subject: Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression
To:     rong.a.chen@intel.com
Cc:     yang.shi@linux.alibaba.com, vbabka@suse.cz,
        kirill.shutemov@linux.intel.com, mhocko@kernel.org,
        willy@infradead.org, ldufour@linux.vnet.ibm.com,
        Colin King <colin.king@canonical.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        lkp@01.org
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On Sun, Nov 4, 2018 at 9:08 PM kernel test robot <rong.a.chen@intel.com> wr=
ote:
>
> FYI, we noticed a -64.1% regression of will-it-scale.per_thread_ops
> due to commit 9bc8039e715d ("mm: brk: downgrade mmap_sem to read when
> shrinking")

Ugh. That looks pretty bad.

> in testcase: will-it-scale
> on test machine: 8 threads Ivy Bridge with 16G memory
> with following parameters:
>
>         nr_task: 100%
>         mode: thread
>         test: brk1
>         ucode: 0x20
>         cpufreq_governor: performance

The reason seems to be way more scheduler time due to lots more
context switches:

>   34925294 =C2=B1 18%    +270.3%  1.293e+08 =C2=B1  4%  will-it-scale.tim=
e.voluntary_context_switches

Yang Shi, would you mind taking a look at what's going on?

              Linus