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

[Amir Goldstein]

On Mon, Jun 8, 2020 at 9:12 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> > > > didn't look too closely at your series as I'm not familiar with fsnotify
> > > > in general. However, at a glance it looks like fsnotify_parent() executes
> > > > a substantial amount of code even if there are no watchers but I could
> > > > be wrong.
> > > >
> > >
> > > I don't about substantial, I would say it is on par with the amount of
> > > code that you tries to optimize out of fsnotify().
> > >
> > > Before bailing out with DCACHE_FSNOTIFY_PARENT_WATCHED
> > > test, it also references d_inode->i_sb,  real_mount(path->mnt)
> > > and fetches all their ->x_fsnotify_mask fields.
> > >
> > > I changed the call pattern from open/modify/... hooks from:
> > > fsnotify_parent(...);
> > > fsnotify(...);
> > >
> > > to:
> > > fsnotify_parent(...); /* which calls fsnotify() */
> > >
> > > So the NULL marks optimization could be done in beginning of
> > > fsnotify_parent() and it will be just as effective as it is in fsnotify().
> > >
> >
> > Something like that may be required because
> >
> >                               5.7.0                  5.7.0                  5.7.0                  5.7.0
> >                             vanilla      fastfsnotify-v1r1      fastfsnotify-v2r1          amir-20200608
> > Amean     1       0.4837 (   0.00%)      0.4630 *   4.27%*      0.4597 *   4.96%*      0.4967 *  -2.69%*
> > Amean     3       1.5447 (   0.00%)      1.4557 (   5.76%)      1.5310 (   0.88%)      1.6587 *  -7.38%*
> > Amean     5       2.6037 (   0.00%)      2.4363 (   6.43%)      2.4237 (   6.91%)      2.6400 (  -1.40%)
> > Amean     7       3.5987 (   0.00%)      3.4757 (   3.42%)      3.6543 (  -1.55%)      3.9040 *  -8.48%*
> > Amean     12      5.8267 (   0.00%)      5.6983 (   2.20%)      5.5903 (   4.06%)      6.2593 (  -7.43%)
> > Amean     18      8.4400 (   0.00%)      8.1327 (   3.64%)      7.7150 *   8.59%*      8.9940 (  -6.56%)
> > Amean     24     11.0187 (   0.00%)     10.0290 *   8.98%*      9.8977 *  10.17%*     11.7247 *  -6.41%*
> > Amean     30     13.1013 (   0.00%)     12.8510 (   1.91%)     12.2087 *   6.81%*     14.0290 *  -7.08%*
> > Amean     32     13.9190 (   0.00%)     13.2410 (   4.87%)     13.2900 (   4.52%)     14.7140 *  -5.71%*
> >
> > vanilla and fastnotify-v1r1 are the same. fastfsnotify-v2r1 is just the
> > fsnotify_parent() change which is mostly worse and may indicate that the
> > first patch was reasonable. amir-20200608 is your branch as of today and
> > it appears to introduce a substantial regression albeit in an extreme case
> > where fsnotify overhead is visible. The regressions are mostly larger
> > than noise with the caveat it may be machine specific given that the
> > machine is overloaded. I accept that adding extra functional to fsnotify
> > may be desirable but ideally it would not hurt the case where there are
> > no watchers at all.
> >
>
> Of course.
> And thanks for catching this regression even before I posted the patches :-)
>
> > So what's the right way forward? The patch as-is even though the fsnotify()
> > change itself may be marginal, a patch that just inlines the fast path
> > of fsnotify_parent or wait for the additional functionality and try and
> > address the overhead on top?
> >
> >
>
> Let me add your optimizations on top of my branch with the needed
> adaptations and send you a branch for testing.

https://github.com/amir73il/linux/commits/fsnotify_name-for-mel

Cheers,
Amir.