Re: [External] [PATCH v4 5/5] mm/memcg: Improve refill_obj_stock() performance

[Waiman Long]

On 4/19/21 11:00 AM, Shakeel Butt wrote:
> On Sun, Apr 18, 2021 at 11:07 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@xxxxxxxxxx> wrote:
> > > There are two issues with the current refill_obj_stock() code. First of
> > > all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
> > > atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
> > > and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
> > > be used again which leads to another call to drain_obj_stock() and
> > > obj_cgroup_get() as well as atomically retrieve the available byte from
> > > obj_cgroup. That is costly. Instead, we should just uncharge the excess
> > > pages, reduce the stock bytes and be done with it. The drain_obj_stock()
> > > function should only be called when obj_cgroup changes.
> > >
> > > Secondly, when charging an object of size not less than a page in
> > > obj_cgroup_charge(), it is possible that the remaining bytes to be
> > > refilled to the stock will overflow a page and cause refill_obj_stock()
> > > to uncharge 1 page. To avoid the additional uncharge in this case,
> > > a new overfill flag is added to refill_obj_stock() which will be set
> > > when called from obj_cgroup_charge().
> > >
> > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > > ---
> > >   mm/memcontrol.c | 23 +++++++++++++++++------
> > >   1 file changed, 17 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index a6dd18f6d8a8..d13961352eef 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -3357,23 +3357,34 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> > >          return false;
> > >   }
> > >
> > > -static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> > > +static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
> > > +                            bool overfill)
> > >   {
> > >          unsigned long flags;
> > >          struct obj_stock *stock = get_obj_stock(&flags);
> > > +       unsigned int nr_pages = 0;
> > >
> > >          if (stock->cached_objcg != objcg) { /* reset if necessary */
> > > -               drain_obj_stock(stock);
> > > +               if (stock->cached_objcg)
> > > +                       drain_obj_stock(stock);
> > >                  obj_cgroup_get(objcg);
> > >                  stock->cached_objcg = objcg;
> > >                  stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
> > >          }
> > >          stock->nr_bytes += nr_bytes;
> > >
> > > -       if (stock->nr_bytes > PAGE_SIZE)
> > > -               drain_obj_stock(stock);
> > > +       if (!overfill && (stock->nr_bytes > PAGE_SIZE)) {
> > > +               nr_pages = stock->nr_bytes >> PAGE_SHIFT;
> > > +               stock->nr_bytes &= (PAGE_SIZE - 1);
> > > +       }
> > >
> > >          put_obj_stock(flags);
> > > +
> > > +       if (nr_pages) {
> > > +               rcu_read_lock();
> > > +               __memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages);
> > > +               rcu_read_unlock();
> > > +       }
> > It is not safe to call __memcg_kmem_uncharge() under rcu lock
> > and without holding a reference to memcg. More details can refer
> > to the following link.
> >
> > https://lore.kernel.org/linux-mm/20210319163821.20704-2-songmuchun@xxxxxxxxxxxxx/
> >
> > In the above patchset, we introduce obj_cgroup_uncharge_pages to
> > uncharge some pages from object cgroup. You can use this safe
> > API.
> I would recommend just rebase the patch series over the latest mm tree.
I see, I will rebase it to the latest mm tree.

Thanks,
Longman