Re: [RFC PATCH v2 6/7] mm, oom: cgroup-aware OOM killer

[Vladimir Davydov]

On Thu, Jun 01, 2017 at 07:35:14PM +0100, Roman Gushchin wrote:
> Traditionally, the OOM killer is operating on a process level.
> Under oom conditions, it finds a process with the highest oom score
> and kills it.
> 
> This behavior doesn't suit well the system with many running
> containers. There are two main issues:
> 
> 1) There is no fairness between containers. A small container with
> few large processes will be chosen over a large one with huge
> number of small processes.
> 
> 2) Containers often do not expect that some random process inside
> will be killed. In many cases much more safer behavior is to kill
> all tasks in the container. Traditionally, this was implemented
> in userspace, but doing it in the kernel has some advantages,
> especially in a case of a system-wide OOM.
> 
> 3) Per-process oom_score_adj affects global OOM, so it's a breache
> in the isolation.
> 
> To address these issues, cgroup-aware OOM killer is introduced.
> 
> Under OOM conditions, it tries to find the biggest memory consumer,
> and free memory by killing corresponding task(s). The difference
> the "traditional" OOM killer is that it can treat memory cgroups
> as memory consumers as well as single processes.
> 
> By default, it will look for the biggest leaf cgroup, and kill
> the largest task inside.
> 
> But a user can change this behavior by enabling the per-cgroup
> oom_kill_all_tasks option. If set, it causes the OOM killer treat
> the whole cgroup as an indivisible memory consumer. In case if it's
> selected as on OOM victim, all belonging tasks will be killed.
> 
> Tasks in the root cgroup are treated as independent memory consumers,
> and are compared with other memory consumers (e.g. leaf cgroups).
> The root cgroup doesn't support the oom_kill_all_tasks feature.
> 
...
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f979ac7..855d335 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2625,6 +2625,184 @@ static inline bool memcg_has_children(struct mem_cgroup *memcg)
>  	return ret;
>  }
>  
> +static long mem_cgroup_oom_badness(struct mem_cgroup *memcg,
> +				   const nodemask_t *nodemask)
> +{
> +	long points = 0;
> +	int nid;
> +	struct mem_cgroup *iter;
> +
> +	for_each_mem_cgroup_tree(iter, memcg) {

AFAIU this function is called on every iteration over the cgroup tree,
which might be costly in case of a deep hierarchy, as it has quadratic
complexity at worst. We could eliminate the nested loop by computing
badness of all eligible cgroups before starting looking for a victim and
saving the values in struct mem_cgroup. Not sure if it's worth it, as
OOM is a pretty cold path.

> +		for_each_node_state(nid, N_MEMORY) {
> +			if (nodemask && !node_isset(nid, *nodemask))
> +				continue;
> +
> +			points += mem_cgroup_node_nr_lru_pages(iter, nid,
> +					LRU_ALL_ANON | BIT(LRU_UNEVICTABLE));

Hmm, is there a reason why we shouldn't take into account file pages?

> +		}
> +
> +		points += mem_cgroup_get_nr_swap_pages(iter);

AFAICS mem_cgroup_get_nr_swap_pages() returns the number of pages that
can still be charged to the cgroup. IIUC we want to account pages that
have already been charged to the cgroup, i.e. the value of the 'swap'
page counter or MEMCG_SWAP stat counter.

> +		points += memcg_page_state(iter, MEMCG_KERNEL_STACK_KB) /
> +			(PAGE_SIZE / 1024);
> +		points += memcg_page_state(iter, MEMCG_SLAB_UNRECLAIMABLE);
> +		points += memcg_page_state(iter, MEMCG_SOCK);
> +	}
> +
> +	return points;
> +}
> +
> +bool mem_cgroup_select_oom_victim(struct oom_control *oc)
> +{
> +	struct cgroup_subsys_state *css = NULL;
> +	struct mem_cgroup *iter = NULL;
> +	struct mem_cgroup *chosen_memcg = NULL;
> +	struct mem_cgroup *parent = root_mem_cgroup;
> +	unsigned long totalpages = oc->totalpages;
> +	long chosen_memcg_points = 0;
> +	long points = 0;
> +
> +	oc->chosen = NULL;
> +	oc->chosen_memcg = NULL;
> +
> +	if (mem_cgroup_disabled())
> +		return false;
> +
> +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +		return false;
> +
> +	pr_info("Choosing a victim memcg because of the %s",
> +		oc->memcg ?
> +		"memory limit reached of cgroup " :
> +		"system-wide OOM\n");
> +	if (oc->memcg) {
> +		pr_cont_cgroup_path(oc->memcg->css.cgroup);
> +		pr_cont("\n");
> +
> +		chosen_memcg = oc->memcg;
> +		parent = oc->memcg;
> +	}
> +
> +	rcu_read_lock();
> +
> +	for (;;) {
> +		css = css_next_child(css, &parent->css);
> +		if (css) {
> +			iter = mem_cgroup_from_css(css);
> +
> +			points = mem_cgroup_oom_badness(iter, oc->nodemask);
> +			points += iter->oom_score_adj * (totalpages / 1000);
> +
> +			pr_info("Cgroup ");
> +			pr_cont_cgroup_path(iter->css.cgroup);
> +			pr_cont(": %ld\n", points);

Not sure if everyone wants to see these messages in the log.

> +
> +			if (points > chosen_memcg_points) {
> +				chosen_memcg = iter;
> +				chosen_memcg_points = points;
> +				oc->chosen_points = points;
> +			}
> +
> +			continue;
> +		}
> +
> +		if (chosen_memcg && !chosen_memcg->oom_kill_all_tasks) {
> +			/* Go deeper in the cgroup hierarchy */
> +			totalpages = chosen_memcg_points;

We set 'totalpages' to the target cgroup limit (or the total RAM
size) when computing a victim score. Why do you prefer to use
chosen_memcg_points here instead? Why not the limit of the chosen
cgroup?

> +			chosen_memcg_points = 0;
> +
> +			parent = chosen_memcg;
> +			chosen_memcg = NULL;
> +
> +			continue;
> +		}
> +
> +		if (!chosen_memcg && parent != root_mem_cgroup)
> +			chosen_memcg = parent;
> +
> +		break;
> +	}
> +

> +	if (!oc->memcg) {
> +		/*
> +		 * We should also consider tasks in the root cgroup
> +		 * with badness larger than oc->chosen_points
> +		 */
> +
> +		struct css_task_iter it;
> +		struct task_struct *task;
> +		int ret = 0;
> +
> +		css_task_iter_start(&root_mem_cgroup->css, &it);
> +		while (!ret && (task = css_task_iter_next(&it)))
> +			ret = oom_evaluate_task(task, oc);
> +		css_task_iter_end(&it);
> +	}

IMHO it isn't quite correct to compare tasks from the root cgroup with
leaf cgroups, because they are at different levels. Shouldn't we compare
their scores only with the top level cgroups?

As an alternative approach, may be, we could remove this branch
altogether and ignore root tasks here (i.e. have any root task a higher
priority a priori)? Perhaps, it could be acceptable, because normally
the root cgroup only hosts kernel processes and init (at least this is
the default systemd setup IIRC).

> +
> +	if (!oc->chosen && chosen_memcg) {
> +		pr_info("Chosen cgroup ");
> +		pr_cont_cgroup_path(chosen_memcg->css.cgroup);
> +		pr_cont(": %ld\n", oc->chosen_points);
> +
> +		if (chosen_memcg->oom_kill_all_tasks) {
> +			css_get(&chosen_memcg->css);
> +			oc->chosen_memcg = chosen_memcg;
> +		} else {
> +			/*
> +			 * If we don't need to kill all tasks in the cgroup,
> +			 * let's select the biggest task.
> +			 */
> +			oc->chosen_points = 0;

> +			select_bad_process(oc, chosen_memcg);

I think we'd better use mem_cgroup_scan_task() here directly, without
exporting select_bad_process() from oom_kill.c. IMHO it would be more
straightforward, because select_bad_process() has a branch handling the
global OOM, which isn't used in this case. Come to think of it, wouldn't
it be better to return the chosen cgroup in @oc and let out_of_memory()
select a process within it or kill it as a whole depending on the value
of the oom_kill_all_tasks flag?

Also, if the chosen cgroup has no tasks (which is perfectly possible if
all memory within the cgroup is consumed by shmem e.g.), shouldn't we
retry the cgroup selection?

> +		}
> +	} else if (oc->chosen)
> +		pr_info("Chosen task %s (%d) in root cgroup: %ld\n",
> +			oc->chosen->comm, oc->chosen->pid, oc->chosen_points);
> +
> +	rcu_read_unlock();
> +
> +	oc->chosen_points = 0;
> +	return !!oc->chosen || !!oc->chosen_memcg;
> +}
> +
> +static int __oom_kill_task(struct task_struct *tsk, void *arg)
> +{
> +	if (!is_global_init(tsk) && !(tsk->flags & PF_KTHREAD)) {
> +		get_task_struct(tsk);
> +		__oom_kill_process(tsk);
> +	}
> +	return 0;
> +}
> +
> +bool mem_cgroup_kill_oom_victim(struct oom_control *oc)

I think it'd be OK to define this function in oom_kill.c - we
have everything we need for that. We wouldn't have to export
__oom_kill_process without oom_kill_process then, which is kinda
ugly IMHO.

> +{
> +	if (oc->chosen_memcg) {
> +		/*
> +		 * Kill all tasks in the cgroup hierarchy
> +		 */
> +		mem_cgroup_scan_tasks(oc->chosen_memcg,
> +				      __oom_kill_task, NULL);
> +
> +		/*
> +		 * Release oc->chosen_memcg
> +		 */
> +		css_put(&oc->chosen_memcg->css);
> +		oc->chosen_memcg = NULL;
> +	}
> +
> +	if (oc->chosen && oc->chosen != (void *)-1UL) {

> +		__oom_kill_process(oc->chosen);

Why don't you use oom_kill_process (without leading underscores) here?

> +		return true;
> +	}
> +
> +	/*
> +	 * Reset points before falling back to an old
> +	 * per-process OOM victim selection logic
> +	 */
> +	oc->chosen_points = 0;
> +
> +	return !!oc->chosen;
> +}