Re: [v5 PATCH 08/11] mm: vmscan: use per memcg nr_deferred of shrinker

[Yang Shi]

On Fri, Jan 29, 2021 at 7:13 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 1/28/21 12:33 AM, Yang Shi wrote:
> > Use per memcg's nr_deferred for memcg aware shrinkers.  The shrinker's nr_deferred
> > will be used in the following cases:
> >     1. Non memcg aware shrinkers
> >     2. !CONFIG_MEMCG
> >     3. memcg is disabled by boot parameter
> >
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > ---
> >  mm/vmscan.c | 87 ++++++++++++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 73 insertions(+), 14 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 20be0db291fe..e1f8960f5cf6 100644
>
> ...
>
> > @@ -653,14 +717,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> >               next_deferred = 0;
> >       /*
> >        * move the unused scan count back into the shrinker in a
> > -      * manner that handles concurrent updates. If we exhausted the
> > -      * scan, there is no need to do an update.
> > +      * manner that handles concurrent updates.
> >        */
> > -     if (next_deferred > 0)
> > -             new_nr = atomic_long_add_return(next_deferred,
> > -                                             &shrinker->nr_deferred[nid]);
> > -     else
> > -             new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
>
> Why not keep this write avoidance if next_deferred == 0 in the new helper?

Because IMHO the write avoidance may not make too much difference
since nr_deferred was updated for the most cases per my tracing
result. So I thought this would make the code more clean.

>
> > +     new_nr = set_nr_deferred(next_deferred, shrinker, shrinkctl);
> >
> >       trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan);
> >       return freed;
> >
>