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Date:   Wed, 30 Nov 2022 17:27:54 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     =?utf-8?B?56iL5Z6y5rab?= Chengkaitao Cheng 
        <chengkaitao@didiglobal.com>
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Subject: Re: [PATCH] mm: memcontrol: protect the memory in cgroup from being
 oom killed
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On Wed 30-11-22 15:46:19, 程垲涛 Chengkaitao Cheng wrote:
> On 2022-11-30 21:15:06, "Michal Hocko" <mhocko@suse.com> wrote:
> > On Wed 30-11-22 15:01:58, chengkaitao wrote:
> > > From: chengkaitao <pilgrimtao@gmail.com>
> > >
> > > We created a new interface <memory.oom.protect> for memory, If there is
> > > the OOM killer under parent memory cgroup, and the memory usage of a
> > > child cgroup is within its effective oom.protect boundary, the cgroup's
> > > tasks won't be OOM killed unless there is no unprotected tasks in other
> > > children cgroups. It draws on the logic of <memory.min/low> in the
> > > inheritance relationship.
> >
> > Could you be more specific about usecases?

This is a very important question to answer.

> > How do you tune oom.protect
> > wrt to other tunables? How does this interact with the oom_score_adj
> > tunining (e.g. a first hand oom victim with the score_adj 1000 sitting
> > in a oom protected memcg)?
> 
> We prefer users to use score_adj and oom.protect independently. Score_adj is 
> a parameter applicable to host, and oom.protect is a parameter applicable to cgroup. 
> When the physical machine's memory size is particularly large, the score_adj 
> granularity is also very large. However, oom.protect can achieve more fine-grained 
> adjustment.

Let me clarify a bit. I am not trying to defend oom_score_adj. It has
it's well known limitations and it is is essentially unusable for many
situations other than - hide or auto-select potential oom victim.

> When the score_adj of the processes are the same, I list the following cases 
> for explanation,
> 
>           root
>            |
>         cgroup A
>        /        \
>  cgroup B      cgroup C
> (task m,n)     (task x,y)
> 
> score_adj(all task) = 0;
> oom.protect(cgroup A) = 0;
> oom.protect(cgroup B) = 0;
> oom.protect(cgroup C) = 3G;

How can you enforce protection at C level without any protection at A
level? This would easily allow arbitrary cgroup to hide from the oom
killer and spill over to other cgroups.

> usage(task m) = 1G
> usage(task n) = 2G
> usage(task x) = 1G
> usage(task y) = 2G
> 
> oom killer order of cgroup A: n > m > y > x
> oom killer order of host:     y = n > x = m
> 
> If cgroup A is a directory maintained by users, users can use oom.protect 
> to protect relatively important tasks x and y.
> 
> However, when score_adj and oom.protect are used at the same time, we 
> will also consider the impact of both, as expressed in the following formula. 
> but I have to admit that it is an unstable result.
> score = task_usage + score_adj * totalpage - eoom.protect * task_usage / local_memcg_usage

I hope I am not misreading but this has some rather unexpected
properties. First off, bigger memory consumers in a protected memcg are
protected more. Also I would expect the protection discount would
be capped by the actual usage otherwise excessive protection
configuration could skew the results considerably.
 
> > I haven't really read through the whole patch but this struck me odd.
> 
> > > @@ -552,8 +552,19 @@ static int proc_oom_score(struct seq_file *m, struct pid_namespace *ns,
> > > 	unsigned long totalpages = totalram_pages() + total_swap_pages;
> > > 	unsigned long points = 0;
> > > 	long badness;
> > > +#ifdef CONFIG_MEMCG
> > > +	struct mem_cgroup *memcg;
> > > 
> > > -	badness = oom_badness(task, totalpages);
> > > +	rcu_read_lock();
> > > +	memcg = mem_cgroup_from_task(task);
> > > +	if (memcg && !css_tryget(&memcg->css))
> > > +		memcg = NULL;
> > > +	rcu_read_unlock();
> > > +
> > > +	update_parent_oom_protection(root_mem_cgroup, memcg);
> > > +	css_put(&memcg->css);
> > > +#endif
> > > +	badness = oom_badness(task, totalpages, MEMCG_OOM_PROTECT);
> >
> > the badness means different thing depending on which memcg hierarchy
> > subtree you look at. Scaling based on the global oom could get really
> > misleading.
> 
> I also took it into consideration. I planned to change "/proc/pid/oom_score" 
> to a writable node. When writing to different cgroup paths, different values 
> will be output. The default output is root cgroup. Do you think this idea is 
> feasible?

I do not follow. Care to elaborate?
-- 
Michal Hocko
SUSE Labs