Re: [PATCH] mm: memcg: make memory.oom.group tolerable to task migration

[Michal Hocko <mhocko@kernel.org>]

On Mon 16-03-20 15:35:10, Roman Gushchin wrote:
> If a task is getting moved out of the OOMing cgroup, it might
> result in unexpected OOM killings if memory.oom.group is used
> anywhere in the cgroup tree.
> 
> Imagine the following example:
> 
>           A (oom.group = 1)
>          / \
>   (OOM) B   C
> 
> Let's say B's memory.max is exceeded and it's OOMing. The OOM killer
> selects a task in B as a victim, but someone asynchronously moves
> the task into C.

I can see Reported-by here, does that mean that the race really happened
in real workloads? If yes, I would be really curious. Mostly because
moving tasks outside of the oom domain is quite questionable without
charge migration.

> mem_cgroup_get_oom_group() will iterate over all
> ancestors of C up to the root cgroup. In theory it had to stop
> at the oom_domain level - the memory cgroup which is OOMing.
> But because B is not an ancestor of C, it's not happening.
> Instead it chooses A (because it's oom.group is set), and kills
> all tasks in A. This behavior is wrong because the OOM happened in B,
> so there is no reason to kill anything outside.
> 
> Fix this by checking it the memory cgroup to which the task belongs
> is a descendant of the oom_domain. If not, memory.oom.group should
> be ignored, and the OOM killer should kill only the victim task.

I was about to suggest storing the memcg in oom_evaluate_task but then I
have realized that this would be both more complex and I am not yet
sure it would be better so much better after all.

The thing is that killing the selected task makes a lot of sense
because it was the largest consumer. No matter it has run away. On the
other hand if your B was oom.group = 1 then one could expect that any
OOM killer event in that group will result in the whole group tear
down. This is however a gray zone because we do emit MEMCG_OOM event but
MEMCG_OOM_KILL event will go to the victim's at-the-time memcg. So the
observer B could think that the oom was resolved without killing while
observer C would see a kill event without oom.

That being said, please try to think about the above. I will give it
some more time as well. Killing the selected victim is the obviously
correct thing and your patch does that so it is correct in that regard
but I believe that the group oom behavior in the original oom domain
remains an open question.

Fixes: 3d8b38eb81ca ("mm, oom: introduce memory.oom.group")
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Reported-by: Dan Schatzberg <dschatzberg@fb.com>
> ---
>  mm/memcontrol.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index daa399be4688..d8c4b7aa4e73 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1930,6 +1930,14 @@ struct mem_cgroup *mem_cgroup_get_oom_group(struct task_struct *victim,
>  	if (memcg == root_mem_cgroup)
>  		goto out;
>  
> +	/*
> +	 * If the victim task has been asynchronously moved to a different
> +	 * memory cgroup, we might end up killing tasks outside oom_domain.
> +	 * In this case it's better to ignore memory.group.oom.
> +	 */
> +	if (unlikely(!mem_cgroup_is_descendant(memcg, oom_domain)))
> +		goto out;
> +
>  	/*
>  	 * Traverse the memory cgroup hierarchy from the victim task's
>  	 * cgroup up to the OOMing cgroup (or root) to find the
> -- 
> 2.24.1

-- 
Michal Hocko
SUSE Labs