Re: [External] Re: [RFC PATCH v3 08/12] mm: memcontrol: introduce memcg_reparent_ops

[Muchun Song]

On Wed, May 26, 2021 at 1:46 AM Roman Gushchin <guro@xxxxxx> wrote:
>
> On Wed, Apr 21, 2021 at 03:00:55PM +0800, Muchun Song wrote:
> > In the previous patch, we know how to make the lruvec lock safe when the
> > LRU pages reparented. We should do something like following.
> >
> >     memcg_reparent_objcgs(memcg)
> >         1) lock
> >         // lruvec belongs to memcg and lruvec_parent belongs to parent memcg.
> >         spin_lock(&lruvec->lru_lock);
> >         spin_lock(&lruvec_parent->lru_lock);
> >
> >         2) do reparent
> >         // Move all the pages from the lruvec list to the parent lruvec list.
> >
> >         3) unlock
> >         spin_unlock(&lruvec_parent->lru_lock);
> >         spin_unlock(&lruvec->lru_lock);
> >
> > Apart from the page lruvec lock, the deferred split queue lock (THP only)
> > also needs to do something similar. So we extracted the necessary 3 steps
> > in the memcg_reparent_objcgs().
> >
> >     memcg_reparent_objcgs(memcg)
> >         1) lock
> >         memcg_reparent_ops->lock(memcg, parent);
> >
> >         2) reparent
> >         memcg_reparent_ops->reparent(memcg, reparent);
> >
> >         3) unlock
> >         memcg_reparent_ops->unlock(memcg, reparent);
> >
> > Now there are two different locks (e.g. lruvec lock and deferred split
> > queue lock) need to use this infrastructure. In the next patch, we will
> > use those APIs to make those locks safe when the LRU pages reparented.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  include/linux/memcontrol.h | 11 +++++++++++
> >  mm/memcontrol.c            | 49 ++++++++++++++++++++++++++++++++++++++++++++--
> >  2 files changed, 58 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 228263f2c82b..b12847b0be09 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -355,6 +355,17 @@ struct mem_cgroup {
> >       /* WARNING: nodeinfo must be the last member here */
> >  };
> >
> > +struct memcg_reparent_ops {
> > +     struct list_head list;
> > +
> > +     /* Irq is disabled before calling those functions. */
> > +     void (*lock)(struct mem_cgroup *memcg, struct mem_cgroup *parent);
> > +     void (*unlock)(struct mem_cgroup *memcg, struct mem_cgroup *parent);
> > +     void (*reparent)(struct mem_cgroup *memcg, struct mem_cgroup *parent);
> > +};
> > +
> > +void __init register_memcg_repatent(struct memcg_reparent_ops *ops);
> > +
> >  /*
> >   * size of first charge trial. "32" comes from vmscan.c's magic value.
> >   * TODO: maybe necessary to use big numbers in big irons.
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index a48403e5999c..f88fe2f06f5b 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -330,6 +330,41 @@ static struct obj_cgroup *obj_cgroup_alloc(void)
> >       return objcg;
> >  }
> >
> > +static LIST_HEAD(reparent_ops_head);
>
> Because this list is completely static, why not make a build-time initialized
> array instead?

I didn't think of using an array before. The first idea that popped out
was a list. But you remind me of the array. I'd love to replace it with
the array.

Thanks, Roman.

> I guess it's a more canonical way of solving problems like this.
> The proposed API looks good to me.