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

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Apr 23, 2020 at 7:40 AM Matthew Wilcox <willy@infradead.org> 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.
>
> The slab paper was written a number of years ago when CPU caches were
> not as they are today.  With this patch, every time you allocate a
> new page, we dirty the entire page, and then the dirty cachelines will
> gradually fall out of cache as the other objects on the page are not used
> immediately.  Then, when we actually use one of the objects on the page,
> we bring those cachelines back in and dirty them again by initialising
> 'type' and 'obj'.  The two stores to initialise lock and list are almost
> free when done in fsnotify_attach_connector_to_object(), but are costly
> when done in a slab constructor.

Thanks a lot for this reasoning. Basically, you're saying when a slab
allocates a page, it would construct all objects which end up dirtying
the entire page before the object is even allocated. That makes sense.

There's one improvement (although probably verys small) that the paper mentions:
Also according to the paper you referenced, the instruction cache is
what would also benefit. Those spinlock and hlist initialization
instructions wouldn't cost L1 I-cache footprint for every allocation.

> There are very few places where a slab constructor is justified with a
> modern CPU.  We've considered removing the functionality before.

I see, thanks again for the insights.

 - Joel

>
> > @@ -479,8 +479,6 @@ static int fsnotify_attach_connector_to_object(fsnotify_connp_t *connp,
> >       conn = kmem_cache_alloc(fsnotify_mark_connector_cachep, GFP_KERNEL);
> >       if (!conn)
> >               return -ENOMEM;
> > -     spin_lock_init(&conn->lock);
> > -     INIT_HLIST_HEAD(&conn->list);
> >       conn->type = type;
> >       conn->obj = connp;
> >       /* Cache fsid of filesystem containing the object */
> > --
> > 2.26.1.301.g55bc3eb7cb9-goog