Re: [PATCH] mm: memcontrol: fix missing wakeup polling thread

[Johannes Weiner]

On Wed, Nov 04, 2020 at 08:55:58PM +0800, Muchun Song wrote:
> When wen poll the memory.swap.events, we can miss being waked up when the
> swap event occurs. Because we didn't notify.
> 
> Fixes: f3a53a3a1e5b ("mm, memcontrol: implement memory.swap.events")
> Signed-off-by: Muchun Song <songmuc...@bytedance.com>

Good catch!

> ---
>  include/linux/memcontrol.h | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 0f4dd7829fb2..2456cb737329 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1071,15 +1071,29 @@ static inline void count_memcg_event_mm(struct 
> mm_struct *mm,
>       rcu_read_unlock();
>  }
>  
> +static inline bool is_swap_memory_event(enum memcg_memory_event event)
> +{
> +     return event == MEMCG_SWAP_HIGH || event == MEMCG_SWAP_MAX ||
> +            event == MEMCG_SWAP_FAIL;
> +}

Please inline this, it's not really worth the indirection.

>  static inline void memcg_memory_event(struct mem_cgroup *memcg,
>                                     enum memcg_memory_event event)
>  {
> +     bool swap_event = is_swap_memory_event(event);
> +     struct cgroup_file *cfile;
> +
>       atomic_long_inc(&memcg->memory_events_local[event]);
> -     cgroup_file_notify(&memcg->events_local_file);
> +     if (swap_event) {
> +             cfile = &memcg->swap_events_file;
> +     } else {
> +             cfile = &memcg->events_file;
> +             cgroup_file_notify(&memcg->events_local_file);
> +     }
>  
>       do {
>               atomic_long_inc(&memcg->memory_events[event]);
> -             cgroup_file_notify(&memcg->events_file);
> +             cgroup_file_notify(cfile);

This loop is a walk up the hierarchy and memcg keeps changing, so you
cannot cache cfile up front.

                if (swap_event)
                        cgroup_file_notify(&memcg->swap_events_file);
                else
                        cgroup_file_notify(&memcg->events_file);