Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

[Shakeel Butt]

On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jul 03, 2018 at 06:09:05PM +0300, Kirill Tkhai wrote:
> > +++ b/mm/vmscan.c
> > @@ -169,6 +169,49 @@ unsigned long vm_total_pages;
> >  static LIST_HEAD(shrinker_list);
> >  static DECLARE_RWSEM(shrinker_rwsem);
> >
> > +#ifdef CONFIG_MEMCG_KMEM
> > +static DEFINE_IDR(shrinker_idr);
> > +static int shrinker_nr_max;
>
> So ... we've now got a list_head (shrinker_list) which contains all of
> the shrinkers, plus a shrinker_idr which contains the memcg-aware shrinkers?
>
> Why not replace the shrinker_list with the shrinker_idr?  It's only used
> twice in vmscan.c:
>
> void register_shrinker_prepared(struct shrinker *shrinker)
> {
>         down_write(&shrinker_rwsem);
>         list_add_tail(&shrinker->list, &shrinker_list);
>         up_write(&shrinker_rwsem);
> }
>
>         list_for_each_entry(shrinker, &shrinker_list, list) {
> ...
>
> The first is simply idr_alloc() and the second is
>
>         idr_for_each_entry(&shrinker_idr, shrinker, id) {
>
> I understand there's a difference between allocating the shrinker's ID and
> adding it to the list.  You can do this by calling idr_alloc with NULL
> as the pointer, and then using idr_replace() when you want to add the
> shrinker to the list.  idr_for_each_entry() skips over NULL entries.
>
> This will actually reduce the size of each shrinker and be more
> cache-efficient when calling the shrinkers.  I think we can also get
> rid of the shrinker_rwsem eventually, but let's leave it for now.

Can you explain how you envision shrinker_rwsem can be removed? I am
very much interested in doing that.

thanks,
Shakeel