Re: [PATCH] mm: memcontrol: protect the memory in cgroup from being oom killed

[程垲涛 Chengkaitao Cheng]

On 2022-11-30 21:15:06, "Michal Hocko" <mhocko@xxxxxxxx> wrote:
> On Wed 30-11-22 15:01:58, chengkaitao wrote:
> > From: chengkaitao <pilgrimtao@xxxxxxxxx>
> >
> > 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? 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.

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;
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 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?
-- 
Chengkaitao
Best wishes