Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression

[Linus Torvalds]

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

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

              Linus