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

[Shakeel Butt]

On Tue, Jul 3, 2018 at 9:17 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>
> Hi, Shakeel,
>
> On 03.07.2018 18:46, Shakeel Butt wrote:
> > 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.
>
> Have you tried to do some games with SRCU? It looks like we just need to
> teach count_objects() and scan_objects() to work with semi-destructed
> shrinkers. Though, this looks this will make impossible to introduce
> shrinkers, which do synchronize_srcu() in scan_objects() for example.
> Not sure, someone will actually use this, and this is possible to consider
> as limitation.
>

Hi Kirill, I tried SRCU and the discussion is at
https://lore.kernel.org/lkml/20171117173521.GA21692@xxxxxxxxxxxxx/T/#u

Paul E. McKenney suggested to enable SRCU unconditionally. So, to use
SRCU for shrinkers, we first have to push unconditional SRCU.

Tetsuo had another lockless solution which was a bit involved but does
not depend on SRCU.

thanks,
Shakeel