Re: [LKP] [fsnotify] 60f7ed8c7c: will-it-scale.per_thread_ops -5.9% regression

[Amir Goldstein]

On Sun, Sep 30, 2018 at 9:50 AM kernel test robot <rong.a.chen@xxxxxxxxx> wrote:
>
> Greeting,
>
> FYI, we noticed a -5.9% regression of will-it-scale.per_thread_ops due to commit:
>
>
> commit: 60f7ed8c7c4d06aeda448c6da74621552ee739aa ("fsnotify: send path type events to group with super block marks")
> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
>
> in testcase: will-it-scale
> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory
> with following parameters:
>
>         nr_task: 16
>         mode: thread
>         test: unlink2
>         cpufreq_governor: performance
>
> test-description: Will It Scale takes a testcase and runs it from 1 through to n parallel copies to see if the testcase will scale. It builds both a process and threads based test in order to see any differences between the two.
> test-url: https://github.com/antonblanchard/will-it-scale
>
>
>
> Details are as below:
> -------------------------------------------------------------------------------------------------->
>
>
> To reproduce:
>
>         git clone https://github.com/intel/lkp-tests.git
>         cd lkp-tests
>         bin/lkp install job.yaml  # job file is attached in this email
>         bin/lkp run     job.yaml
>
> =========================================================================================
> compiler/cpufreq_governor/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
>   gcc-7/performance/x86_64-rhel-7.2/thread/16/debian-x86_64-2018-04-03.cgz/lkp-bdw-ep3d/unlink2/will-it-scale
>
> commit:
>   1e6cb72399 ("fsnotify: add super block object type")
>   60f7ed8c7c ("fsnotify: send path type events to group with super block marks")
>

I have to admit this looks strange.
All this commit does is dereference mnt->mnt.mnt_sb and then
sb->s_fsnotify_mask/sb->s_fsnotify_marks to find that they are zero.
AFAICT there should be no extra contention added by this commit and it's
hard to believe that parallel unlink workload would suffer from this change.

I will try to install lkp-tests to verify this on my own system, but
until proven
otherwise I will regard this as false positive.

Thanks,
Amir.