linux-kernel - Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread

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Message-ID: <CAHk-=wi8BNNTp+Xt4ROz1TqGgFT-c6-Hmmc9yW0B6p2FTFwJTg@mail.gmail.com>
Date:   Mon, 5 Nov 2018 09:50:09 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     rong.a.chen@...el.com
Cc:     yang.shi@...ux.alibaba.com, vbabka@...e.cz,
        kirill.shutemov@...ux.intel.com, mhocko@...nel.org,
        willy@...radead.org, ldufour@...ux.vnet.ibm.com,
        Colin King <colin.king@...onical.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        lkp@...org
Subject: Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression

On Sun, Nov 4, 2018 at 9:08 PM kernel test robot <rong.a.chen@...el.com> wrote:
>
> 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 ± 18%    +270.3%  1.293e+08 ±  4%  will-it-scale.time.voluntary_context_switches

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

              Linus