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

[Amir Goldstein <amir73il@gmail.com>]

On Sat, Jul 31, 2021 at 10:51 PM Gabriel Krisman Bertazi
<krisman@collabora.com> wrote:
>
> Amir Goldstein <amir73il@gmail.com> writes:
>
> > On Sat, Jul 31, 2021 at 9:20 AM kernel test robot <oliver.sang@intel.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@intel.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.
>
> Amir,
>
> Since this patch is a clean up, would you mind if I drop it from my
> series and base my work on top of mainline? Eventually, we can rebase
> this patch, when the performance issue is addressed.
>
> I ask because I'm about to send a v5 and I'm not sure if I should wait
> to have this fixed.

I guess you mean that you want to add the sb to fsnotify() args list.
I don't mind, it's up to Jan.

Thanks,
Amir.