Re: [PATCH v8 4/8] mm: Provide a new count_objcg_events() API for batch event updates.

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Sep 27, 2024 at 7:16 PM Kanchana P Sridhar
<kanchana.p.sridhar@intel.com> wrote:
>
> With the introduction of zswap_store() swapping out large folios,
> we need to efficiently update the objcg's memcg events once per
> successfully stored folio. For instance, the 'ZSWPOUT' event needs
> to be incremented by folio_nr_pages().
>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
> ---
>  include/linux/memcontrol.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 15c2716f9aa3..f47fd00c5eea 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1778,6 +1778,21 @@ static inline void count_objcg_event(struct obj_cgroup *objcg,
>         rcu_read_unlock();
>  }
>
> +static inline void count_objcg_events(struct obj_cgroup *objcg,
> +                                     enum vm_event_item idx,
> +                                     unsigned long count)
> +{
> +       struct mem_cgroup *memcg;
> +
> +       if (!memcg_kmem_online())
> +               return;
> +
> +       rcu_read_lock();
> +       memcg = obj_cgroup_memcg(objcg);
> +       count_memcg_events(memcg, idx, count);
> +       rcu_read_unlock();
> +}

Instead of replicating the code in count_objcg_event(), we should
change count_objcg_event() to become count_objcg_events() (i.e. add a
count parameter). The existing callers can pass in 1, there's only 3
of them anyway (2 after patch 6), and they are all in zswap.

> +
>  #else
>  static inline bool mem_cgroup_kmem_disabled(void)
>  {
> @@ -1834,6 +1849,11 @@ static inline void count_objcg_event(struct obj_cgroup *objcg,
>  {
>  }
>
> +static inline void count_objcg_events(struct obj_cgroup *objcg,
> +                                     enum vm_event_item idx,
> +                                     unsigned long count)
> +{
> +}
>  #endif /* CONFIG_MEMCG */
>
>  #if defined(CONFIG_MEMCG) && defined(CONFIG_ZSWAP)
> --
> 2.27.0
>