linux-ext4 - Re: [fsnotify] 4c40d6efc8: unixbench.score -3.3% regression

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Date:   Sat, 31 Jul 2021 12:27:43 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     0day robot <lkp@...el.com>, LKML <linux-kernel@...r.kernel.org>,
        lkp@...ts.01.org, ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...ux.intel.com, Jan Kara <jack@...e.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Theodore Tso <tytso@....edu>,
        Dave Chinner <david@...morbit.com>,
        David Howells <dhowells@...hat.com>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>, kernel@...labora.com,
        Mel Gorman <mgorman@...hsingularity.net>,
        kernel test robot <oliver.sang@...el.com>
Subject: Re: [fsnotify] 4c40d6efc8: unixbench.score -3.3% regression

On Sat, Jul 31, 2021 at 9:20 AM kernel test robot <oliver.sang@...el.com> wrote:
>
>
>
> Greeting,
>
> FYI, we noticed a -3.3% regression of unixbench.score due to commit:
>
>
> commit: 4c40d6efc8b22b88a45c335ffd6d25b55d769f5b ("[PATCH v4 08/16] fsnotify: pass arguments of fsnotify() in struct fsnotify_event_info")
> url: https://github.com/0day-ci/linux/commits/Gabriel-Krisman-Bertazi/File-system-wide-monitoring/20210721-001444
> base: https://git.kernel.org/cgit/linux/kernel/git/jack/linux-fs.git fsnotify
>
> in testcase: unixbench
> on test machine: 96 threads 2 sockets Intel(R) Xeon(R) CPU @ 2.30GHz with 128G memory
> with following parameters:
>
>         runtime: 300s
>         nr_task: 1
>         test: pipe
>         cpufreq_governor: performance
>         ucode: 0x4003006
>
> test-description: UnixBench is the original BYTE UNIX benchmark suite aims to test performance of Unix-like system.
> test-url: https://github.com/kdlucas/byte-unixbench
>
> In addition to that, the commit also has significant impact on the following tests:
>
> +------------------+-------------------------------------------------------------------------------------+
> | testcase: change | will-it-scale: will-it-scale.per_thread_ops -1.3% regression                        |
> | test machine     | 192 threads 4 sockets Intel(R) Xeon(R) Platinum 9242 CPU @ 2.30GHz with 192G memory |
> | test parameters  | cpufreq_governor=performance                                                        |
> |                  | mode=thread                                                                         |
> |                  | nr_task=100%                                                                        |
> |                  | test=eventfd1                                                                       |
> |                  | ucode=0x5003006                                                                     |
> +------------------+-------------------------------------------------------------------------------------+
>
>
> If you fix the issue, kindly add following tag
> Reported-by: kernel test robot <oliver.sang@...el.com>
>

Gabriel,

It looks like my change throws away much of the performance gain for
small IO on pipes without any watches that was achieved by commit
71d734103edf ("fsnotify: Rearrange fast path to minimise overhead
when there is no watcher").

I think the way to fix it is to lift the optimization in __fsnotify()
to the fsnotify_parent() inline wrapper as Mel considered doing
but was not sure it was worth the effort at the time.

It's not completely trivial. I think it requires setting a flag
MNT_FSNOTIFY_WATCHED when there are watches on the
vfsmount. I will look into it.

Thanks,
Amir.