Re: [PATCH] mm, memcg: fix inconsistent oom event behavior

[Shakeel Butt <shakeelb@google.com>]

On Sun, Apr 12, 2020 at 7:04 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> A recent commit 9852ae3fe529 ("mm, memcg: consider subtrees in
> memory.events") changes the behavior of memcg events, which will
> consider subtrees in memory.events. But oom_kill event is a special one
> as it is used in both cgroup1 and cgroup2. In cgroup1, it is displayed
> in memory.oom_control. The file memory.oom_control is in both root memcg
> and non root memcg, that is different with memory.event as it only in
> non-root memcg. That commit is okay for cgroup2, but it is not okay for
> cgroup1 as it will cause inconsistent behavior between root memcg and
> non-root memcg.

I still couldn't understand the cgroup v1's root vs non_root behavior
change. The behavior change I see is the hierarchical one i.e.
MEMCG_OOM_KILL event in the descendant will cause the notification and
count increment in the ancestors even in the cgroup v1. I suppose we
don't want that behavior change in v1.

> Let's recover the original behavior for cgroup1.
>
> Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
> Cc: Chris Down <chris@chrisdown.name>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  include/linux/memcontrol.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 8c340e6b347f..a0ae080a67d1 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -798,7 +798,8 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
>                 atomic_long_inc(&memcg->memory_events[event]);
>                 cgroup_file_notify(&memcg->events_file);
>
> -               if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
> +               if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS ||
> +                   !cgroup_subsys_on_dfl(memory_cgrp_subsys))
>                         break;
>         } while ((memcg = parent_mem_cgroup(memcg)) &&
>                  !mem_cgroup_is_root(memcg));
> --
> 2.18.2
>