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

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@redhat.com> 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@redhat.com>
> ---
>  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@bytedance.com/

In the above patchset, we introduce obj_cgroup_uncharge_pages to
uncharge some pages from object cgroup. You can use this safe
API.

Thanks.

>  }
>
>  int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
> @@ -3410,7 +3421,7 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
>
>         ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
>         if (!ret && nr_bytes)
> -               refill_obj_stock(objcg, PAGE_SIZE - nr_bytes);
> +               refill_obj_stock(objcg, PAGE_SIZE - nr_bytes, true);
>
>         css_put(&memcg->css);
>         return ret;
> @@ -3418,7 +3429,7 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
>
>  void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
>  {
> -       refill_obj_stock(objcg, size);
> +       refill_obj_stock(objcg, size, false);
>  }
>
>  #endif /* CONFIG_MEMCG_KMEM */
> --
> 2.18.1
>