linux-kernel - Re: [PATCH] memcg: oom: ignore oom warnings from memory.max

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Date:   Mon, 4 May 2020 09:03:01 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: oom: ignore oom warnings from memory.max

On Fri 01-05-20 09:39:24, Yafang Shao wrote:
> On Fri, May 1, 2020 at 2:27 AM Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > Lowering memory.max can trigger an oom-kill if the reclaim does not
> > succeed. However if oom-killer does not find a process for killing, it
> > dumps a lot of warnings.
> >
> 
> I have been confused by this behavior for several months and I think
> it will confuse more memcg users.

Could you be more specific what has caused the confusion?

> We should keep the memcg oom behavior consistent with system oom - no
> oom kill if no process.

This is not the global mmemcg behavior. We do complain loud on no
eligible tasks and actually panic the system. Memcg cannot simply
do the same by default for obvious reasons.

> What about bellow change ?
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e28098e13f1c..25fbc37a747f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6086,6 +6086,9 @@ static ssize_t memory_max_write(struct
> kernfs_open_file *of,
>                         continue;
>                 }
> 
> +               if (!cgroup_is_populated(memcg->css.cgroup))
> +                       break;
> +
>                 memcg_memory_event(memcg, MEMCG_OOM);
>                 if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
>                         break;

I am not a great fan to be honest. The warning might be useful for other
usecases when it is not clear that the memcg is empty.

-- 
Michal Hocko
SUSE Labs