On Sat, Jul 31, 2021 at 9:20 AM kernel test robot <oliver.sang@xxxxxxxxx> 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@xxxxxxxxx>
>
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.
