Re: [RFC] fs: Use slab constructor to initialize conn objects in fsnotify

[Joel Fernandes]

On Thu, Apr 23, 2020 at 6:48 AM Jan Kara <jack@xxxxxxx> wrote:
>
> On Thu 23-04-20 08:24:23, Amir Goldstein wrote:
> > On Thu, Apr 23, 2020 at 7:45 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, Apr 23, 2020 at 12:40:50AM -0400, Joel Fernandes (Google) wrote:
> > > > While reading the famous slab paper [1], I noticed that the conn->lock
> > > > spinlock and conn->list hlist in fsnotify code is being initialized
> > > > during every object allocation. This seems a good fit for the
> > > > constructor within the slab to take advantage of the slab design. Move
> > > > the initializtion to that.
> > > >
> > > >        spin_lock_init(&conn->lock);
> > > >        INIT_HLIST_HEAD(&conn->list);
> > > >
> > > > [1] https://pdfs.semanticscholar.org/1acc/3a14da69dd240f2fbc11d00e09610263bdbd.pdf
> > > >
> > >
> > > The commit message could be better. Just to clarify, doing it this way is
> > > more efficient because the object will only have its spinlock init and hlist
> > > init happen during object construction, not object allocation.
> > >
> >
> > This change may be correct, but completely unjustified IMO.
> > conn objects are very rarely allocated, from user syscall path only.
> > I see no reason to micro optimize this.
> >
> > Perhaps there is another justification to do this, but not efficiency.
>
> Thanks for the suggestion Joel but I agree with Amir here. In principle
> using constructor is correct however it puts initialization of object in
> two places which makes the code harder to follow and the allocation of
> connector does not happen frequently enough for optimizing out these two
> stores to matter in any tangible way.

Thanks a lot Jan and Amir for your comments on the RFC patch. I am
glad I got learn about this concept and appreciate the discussion very
much.

I agree with your analysis about the lack of constructor benefit with
infrequent allocations, the other ones being: splitting object
initialization into 2 code paths and also dirtying the entire page and
the L1 cache that Matthew mentioned.

 - Joel