Re: [PATCH] fsnotify: Rearrange fast path to minimise overhead when there is no watcher

[Amir Goldstein]

On Mon, Jun 8, 2020 at 7:50 PM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Jun 08, 2020 at 05:19:43PM +0200, Jan Kara wrote:
> > > This is showing that the latencies are improved by roughly 2-9%. The
> > > variability is not shown but some of these results are within the noise
> > > as this workload heavily overloads the machine. That said, the system CPU
> > > usage is reduced by quite a bit so it makes sense to avoid the overhead
> > > even if it is a bit tricky to detect at times. A perf profile of just 1
> > > group of tasks showed that 5.14% of samples taken were in either fsnotify()
> > > or fsnotify_parent(). With the patch, 2.8% of samples were in fsnotify,
> > > mostly function entry and the initial check for watchers.  The check for
> > > watchers is complicated enough that inlining it may be controversial.
> > >
> > > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> >
> > Thanks for the patch! I have to tell I'm surprised this small reordering
> > helps so much. For pipe inode we will bail on:
> >
> >        if (!to_tell->i_fsnotify_marks && !sb->s_fsnotify_marks &&
> >            (!mnt || !mnt->mnt_fsnotify_marks))
> >                return 0;
> >
> > So what we save with the reordering is sb->s_fsnotify_mask and
> > mnt->mnt_fsnotify_mask fetch but that should be the same cacheline as
> > sb->s_fsnotify_marks and mnt->mnt_fsnotify_marks, respectively.
>
> It is likely that the contribution of that change is marginal relative
> to the fsnotify_parent() call. I'll know by tomorrow morning at the latest.
>
> > We also
> > save a function call of fsnotify_parent() but I would think that is very
> > cheap (compared to the whole write path) as well.
> >
>
> To be fair, it is cheap but with this particular workload, we call
> vfs_write() a *lot* and the path is not that long so it builds up to 5%
> of samples overall. Given that these were anonymous pipes, it surprised
> me to see fsnotify at all which is why I took a closer look.
>

I should note that after:
7c49b8616460 fs/notify: optimize inotify/fsnotify code for unwatched files
Which speaks of a similar workload,
the code looked quite similar to your optimization.

It was:
60f7ed8c7c4d fsnotify: send path type events to group with super block marks

That started accessing ->x_fsnotify_mask before ->x_fsnotify_marks,
although I still find it hard to believe that this makes a real difference.

Thanks,
Amir.