Re: [PATCH v2] mm, memcg: fix inconsistent oom event behavior

[Michal Hocko <mhocko@kernel.org>]

On Mon 13-04-20 21:59:52, Yafang Shao 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.
> 
> Here's an example on why this behavior is inconsistent in cgroup1.
>      root memcg
>      /
>   memcg foo
>    /
> memcg bar
> 
> Suppose there's an oom_kill in memcg bar, then the oon_kill will be
> 
>      root memcg : memory.oom_control(oom_kill)  0
>      /
>   memcg foo : memory.oom_control(oom_kill)  1
>    /
> memcg bar : memory.oom_control(oom_kill)  1
> 
> For the non-root memcg, its memory.oom_control(oom_kill) includes its
> descendants' oom_kill, but for root memcg, it doesn't include its
> descendants' oom_kill. That means, memory.oom_control(oom_kill) has
> different meanings in different memcgs. That is inconsistent. Then the user
> has to know whether the memcg is root or not.
> 
> If we can't fully support it in cgroup1, for example by adding
> memory.events.local into cgroup1 as well, then let's don't touch
> its original behavior. So let's recover the original behavior for cgroup1.

Wthe localevents was mostly cgroup v2 feature. I do not think there was
an intention to have side effects on the legacy hierarchy. I thought
this would be the case but it is not apparently. Would it make more
sense to have CGRP_ROOT_MEMORY_LOCAL_EVENTS for legacy hierarchy by
default rather than special casing it somewhere quite deep in the code?

> Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
> Cc: Chris Down <chris@chrisdown.name>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: stable@vger.kernel.org
> Reviewed-by: Shakeel Butt <shakeelb@google.com>
> 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

-- 
Michal Hocko
SUSE Labs