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

[Amir Goldstein]

On Mon, Oct 1, 2018 at 12:32 PM Jan Kara <jack@xxxxxxx> wrote:
>
> On Sun 30-09-18 12:00:46, Amir Goldstein wrote:
[...]
> > > 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.
>
> Well, it could be those additional fetches of
> sb->s_fsnotify_mask/sb->s_fsnotify_marks if they happen to be cache cold.
> Or it could be just code layout differences (i.e., compiler is not able to
> optimize resulting code as good or the code layout just happens to align
> with cache lines in a wrong way or something like that). Anyway, without
> being able to reproduce this and do detailed comparison of perf profiles I
> don't think we'll be able to tell.
>

Indeed, I am still trying to figure out how to run lkp in my test env.

Thanks,
Amir.